US companies are producing heat pumps that work below -20F | Hacker News Reader

US companies are producing heat pumps that work below -20F

electrek.co

265 pointsjbrins1330 comments

I wrote up a brief discussion here on the boiling points of refrigerants https://www.moderndescartes.com/essays/refrigerants/

The long and short of it is that if the heat pump works below -20F, then the boiling point of the refrigerant must be below -20F. This, in turn, implies a higher pressurization (as per the Clausius-Clapeyron eq) required in order to achieve a T_hot of 80F (or whatever output temperature you want. The higher pressurizations require more expensive components and compressors.

Maybe I’m dumb, but aren’t compressors pressure-agnostic? They just add pressure, right?

Other than needing to be a bit stronger to keep from bursting against the higher delta with the ambient atmospheric pressure like the rest of the components.

In this case there is a higher delta between the system and the atmosphere as well as a higher delta between the hot and the cold side, which is what requires a stronger compressor.

I had a HVAC guy mention that seals are problematic always with non hermetic compressors. Everything larger than a fractional HP system has a shaft seal between the compressor and the motor. Small systems being a pain because there is less refrigerant and no way to monitor the amount.

There is a way to monitor it, but you'd need to put in T-Junctions and add a pressure sensor on the high and low pressure sides. You'd then need to rig up some system to read those pressures. But _you_ would be unfortunately the one that has to do it because I don't think there are any off the shelf solutions.

The ideal model of everything is everything-agnostic.

Refrigeration stuff is generally soft copper, and modern refrigerants are already working up in the hundreds of PSI. So I can see that getting expensive or requiring a sea change in materials.

If they’re using copper, changing materials would probably save money. Copper is comparatively expensive. I suspect your premise that they are using mostly copper is incorrect, but then I write software for a living, what do I know.

Copper is (very) effective for conducting heat. Aluminum is also pretty effective but obviously has different material properties than copper. It might be a good fit here; I don't work in the space.

Aluminum generally has fatigue limits that mean it will fail eventually, releasing refrigerant (which is normally an environmental disaster)

what qualifies as a disaster? the amount of refrigerant in a consumer unit just doesn't seem like a lot even if 100% of it was released. if it was a continuous supply, then maybe it could get disaster level, but these are closed systems with a finite amount. at these levels, it seems to me that just driving one's car would qualify as an environmental disaster.

The issue isn’t just the 1st one that breaks the issue is the other 100,000+ that do.

If a typical large split system has about 10Kg of R410A (GWP = 2000x CO2) that's about 20 metric tonnes of CO2. Compare to 5 tonnes for an average passenger car per year. So a leak is like four years of driving a car.

Is it a disaster when an invisible odorless gas escapes and no-one notices? Or when a tree falls in forest? Not on its own, but our entire crisis is a pile of sand grains each too small to count.

My real point was accident vs disaster. Whether anyone knows about it or not (someone will know when the AC doesn't work and the repair tech sees it being empty). If one house looses coolant, then I'd call it an accident. If a vendor has a major problem during manufacturing so that the majority of units fail, then maybe we can use bigger words.

The newer refrigeratants that require higher pressures are much better for ozone and have a lower global warming potential.

Pentane and isopentane, R601 being one of them (of course flammability is a problem) and R744 which is just CO2.

aluminium also has the problem that it is a dissimaler enough metal from most other plumbing hardware (being either made of brass, copper or steel) that it will result in the buildup of a lot of gunk because the aluminium disolves.

That’s galvanic corrosion, but it’s usually an issue in water. I don’t know that it would be a problem with refrigerants, likely not.

Copper has long been the choice for piping because it's soft enough to bend without cracking. The obvious #2 choice, stainless steel, is often even more expensive.

(I don't like the standard HN analysis technique of assuming that another field has made a really basic error that can be spotted by an autodidact from outside)

While I generally agree with your response to the perceived arrogance - shouldn’t we all ask if we can do better than assuming someone else is an expert and has fully optimized a solution? As long as we approach new domains with a sense of humility and respect I think it’s desirable to always be looking for better solutions.

> assuming someone else is an expert and has fully optimized a solution?

I wasn't offering my own singular expert [0] opinion, but rather pointing out what I've observed the entire industry has seemingly converged on. Despite being subject to the hyper-cost-optimization of the consumer market, every refrigeration appliance I've seen still uses copper heat exchanger tubes and copper piping.

I would welcome someone chiming in saying something like "actually I work in HVAC and things are moving towards aluminum heat exchanger tubes and stainless piping" or "that's already the case for most new building-scale systems", because I'd learn something new. But "just asking questions" based on one single material property isn't particularly helpful.

And yeah, copper has gotten expensive. That has helped some new technologies displace it (eg PEX), but copper is also still used where it's needed (eg PEX and PEX fittings over 1" nominal (which is equivalent to 3/4 copper) are prohibitively expensive for some reason).

[0] In fact, I'm not a refrigeration professional, and have yet to pretend to be one. I just tend to look at how things are built.

First off, I was upfront about my ignorance. Secondly, copper is way more expensive than stainless steel, unless I’m missing something. More than 8x according to random websites on Google.

I’m sure there is a reason copper is used, but if the industry has to find an alternative, it would likely be cheaper.

Stainless sucks at heat transfer, and has to be welded. Copper can be soldered (low temperature), or I believe refrigeration lines are commonly brazed (higher temperature).

Raw material cost doesn't tell the whole story. Stainless is harder to work with, therefore costs more to manufacture. And even if it ended up being less expensive in the long run, there would a lot of capital investment to recover.

Stainless steel not conducting heat would be an advantage for linesets - you don't really want the heat/cold to be released in the lineset... it's released in the condenser/evap coils. Those coils are already often made out of aluminum, so dissimilar metals aren't really an issue as far as I can tell...

Flexible stainless tubing is more expensive than copper flexible tubing.

Also, copper flexible tubing is super easy to source.

FWIW there does seem to be a certain blind spot about PEX as an alternative in the US.

A friend redid his whole house with it himself to great success. He mentioned one of the contractors he got a bid from while deciding his plan, when asked about PEX, said "Well that just saves you time and money" in tone of dismissal.

I think there's something similar going on with ductless heat pumps in the US. They're a fantastic option in my area but still uncommon. Part of that seems to be the installers make more from other options.

I very much agree with you about the tendency of HN's community to assume other industries are simply obvious idiots, rather than realizing they're just ignorant of that industry's reality.

Personally, having lived in places with ductless heat pumps they suck.

Lots more maintenance (they need regular cleaning or they build up mold), they don’t circulate air through the house so rooms get their own weird microclimates and get stuffier, and it’s a constant hassle tracking down remotes.

Central is way more comfortable and lower maintenance, IMO.

My house with central forced air already has rooms with weird microclimates due to differing amounts of exterior walls and/or windows. I’ve had the thought that I could use ductless heat pumps to reduce the temperature differences between different rooms in my house.

Power for compressible fluids is approximately proportional to the relation of pressures not difference.

Even if you do need to make thicker pipes, strength in a metal generally increases with the square of the thickness, so it doesn't cost twice as much in materials to double the strength.

This isn't true. For pipes, double the pressure requires double the wall thickness, which requires double the material.

However, I don't think raw material cost is a big part of the cost of refrigeration systems anyway.

The “soft” copper tubing they use in refrigerant lines is not only very cheap (it’s not 100% copper) but is very much over built for pressure it is likely going to be subject to. It’s way cheaper, in this instance, to simply sell a line set that works with 99% of systems instead of having dedicated factory lines and consumer confusion from having different grades of registrant tubing.

> For pipes, double the pressure requires double the wall thickness, which requires double the material.

Double the wall thickness requires more than double the material.

Gearing on the motor driving the compressor, compression ratio, etc. all have a huge impact on wear and tear, power requirements, etc.

These all change based on the pressure delta.

> The higher pressurizations require more expensive components and compressors

Sure, that sounds like an acceptable compromise for those who need the lower operating temperatures

And maybe technology can get those compressors at the same price point of current compressors

But then current compressors would end up cheaper and a difference would still exist.

Or not produced anymore and prices go up for legacy system maintenance. Like try to buy 4MB of EDO RAM

Your point should stand, but it did make me discover that people make new 30pin 4MB simms, which were very difficult to get for years because they were commonly used in ATMs and only available used.

I don't think that will happen. The price difference will be substantial due to the changes required and volume will probably not be enough to offset those.

Scroll compressors have come down in cost a bunch in the last few years. There's really very few reasons not to get a heat pump, now, for new construction.

Even in areas where natural gas is much cheaper than electricity?

yes. Because Global warming/climate change is real and installing new furnaces bakes in decades of additional natural gas usage. Also, that natural gas price may change.

If you want to spend the money you can get systems that have heat pump, gas furnace/boiler, and electric. Often used for larger installations that can take advantage of differing prices - and can work best with water heat.

A heat pump is likely to have an average efficiency of near 300% - call it 250 if you want to hedge. A high efficiency condensing gas furnace gets about 95%. So on a per-watt-hour basis (equiv., BTUs), your gas must cost less than roughly 1/3 what your electric costs.

There are additional savings if your gas furnace is the only gas appliance in your house and you can remove gas service entirely, saving the monthly customer fee.

And to get to 250% COP in heating on average you'd expect to have, at least, 8.5 HSPF2 rating on your heat pump (it's still just the heating performance w/o de-frosting cycles as far as I understand so the real world COP of a 8.5 HSPF2 will be lower than 2.5)

Unfortunately that is the case in some places. Last year I did the math to see if a ground source heat pump would be feasible (Hudson valley, NY), but even accounting for 400% efficiency vs 95% for gas, gas was still ~30% cheaper per month than electric. And that’s ignoring the extra $50k in equipment cost for the heat pump.

Thanks - this is exactly the kind of data point I’m looking for. I have a big, drafty loft in Brooklyn that is heated with gas forced air. I’ve been curious about the relative operating cost of an air source heat pump, and I assume the sky high cost of electricity in nyc makes it a non-starter.

Also in NYC. I pay roughly 35cents per kWH (air-source heat pump) and $3 per therm natural gas (radiant heat, 95% efficient). I think 1 therm == 100k BTU, 1kWH == 3412 BTU. To equal 1 therm is takes 29kWH of electric heat costing $10.15. Then divide by the efficiency of heat pumps: $3.40 at 300% efficiency, $5 at 200%. I’ve read air-source heat pumps become far less efficient as the temps fall, so < 2x may be more realistic in winter.

Hey, thanks for responding. Am I understanding you correctly that 1 unit of gas heat costs $3 and each unit of heat pump heat costs between $3.40 and $5.00 depending on efficiency of the heat pump?

If my math checks out (which is doubtful) then yes, heat pump could be more expensive than gas heat on the worst cold days like last week (10F). However, a heat pump handles heating and cooling. So it’s cheaper to install 1 system that does everything rather than 2 systems to save money on a few cold days. Add a few solar panels and the heat pump cost goes to 0.

However, we put in a gas combi-boiler for hot water. Since we were renovating a townhouse, it was easy to throw in some radiant heaters on all floors. It’s a quiet, comfortable heat. I have the heat pump set to assist if the temps drop below 68F inside. All that said, insulation and air sealing is by far the most important thing to do first.

FYI: I checked the specs on my unit. It claims a 3.3X COP. So that’s $3 per therm, same as gas.

NYC’s design temp is like 17F or something, so the seasonal average COP is probably above 3 for good cold climate heat pumps.

The 300% is closer to an average. When it's over 40F you'll reach over 400% efficiency.

Now a question to address is whether your radiant heat is actually getting 95% efficiency. If you have an old radiator design system that was converted to using a condensing boiler, it may not be designed to keep return temperatures low enough for the boiler to operate at maximal efficiency. (old boiler systems were designed to keep the return water supply hot to prevent acidic condensate from eating the heat exchanger. This is the exact opposite of what you want with a modern system, where the return water needs to be cold enough that the flu gases condense into it to recover heat.)

If you have a way to monitor the return water temperature into your boiler, you could check. You need the return water to be below about 100 Fahrenheit in order to have 95% efficiency.

Would fixing the draft and adding insulation be the most effective first step?

It’s a 630 square foot surface with a 56 degree temperature at the moment according to FLIR. It creates a cascading cool breeze that just neutralizes the forced air furnace. When the furnace stops I can feel the waves of cool air sliding off the external walls.

> gas was still ~30% cheaper per month than electric

this sucks, because gas _should not_ be that much cheaper than electricity, but for subsidies or other market altering shenanigans.

Namely externalities. Natural gas is almost certainly more expensive for home heating than electricity, it’s just that a portion of that expense is paid by a large number of third parties.

Seeing as how natural gas is a required input for making synthetic fertilizer, without which billions would starve, it's hard to think of a product with more positive externalities than natural gas. Every time you go to the supermarket, you should be thankful that natural gas allows synthetic fertilizers to feed the world.

It's easy to think of negative externalities and ignore positive externalities (or vice versa), but accurately summing these up is very difficult.

I didn’t say that all natural gas usage has negative externalities. But also, don’t the natural gas costs you’re talking about show up in the price of food?

No, because prices are set by supply and demand, not the total social cost/benefit that obtains when all effects, direct and indirect, are taken into account.

It is the discrepancy between market equilibria and total social cost/benefit that creates the theory of externalities. But my point is that actually determining social cost/benefit requires very extensive knowledge of all the social benefits and costs provided by something. For example, if you were to ask me what the positive externalities of clean drinking water are, I'd say they are enormous. But I couldn't really give you a number, and if anyone tries to give you a number, they are lying. But that is why I support subsidizing access to clean drinking water, rather than letting the market sort it out. It's just too valuable.

When we are talking about things like energy or transportation, they generally have enormous positive externalities, which is why they are historically subsidized. For example, even if you never use a road, you benefit from the stores you buy from having access to roads so that goods can be delivered to them more cheaply. But it's even more subtle, because cheaper transportation means bigger markets. One of the reasons why local businesses historically opposed the construction of roads is that it would force them to compete with other businesses along the same road. If most people had to walk, then they get a captive market. As soon as you introduce roads, prices come down, and quality increases because lots of little monopolies are destroyed.

So even if you never leave your house and only shop locally, you are getting huge financial benefits from cheap transportation. Thus there is a positive externality. This is why we don't mind that trucks cause most of the damage to roads but don't "pay their fair share" of road maintenance costs. It's because those trucks create enormous positive externalities by making all the goods we buy much cheaper, even if we ourselves never hire a truck. This is also why many nations subsidize railroads, bridges, roads, etc. The internet also has this aspect of enormous positive externalities. And when you are talking about energy -- it's the basis of modern civilization. Again, if you never use any energy yourself except wood that you chop down, you benefit from all the others who provide you with services having access to energy.

Now I'm not saying that natural gas only has positive externalities and that there are no negative externalities. What I'm saying is that no one can tell you what the net is, because these are things foundational to our economy and have profound effects on everything else.

So I would not try to use the logic of externalities when talking about these foundational technologies. Externalities makes sense if you are using a lawnmower and it annoys your neighbors, so you let them bid on how much you need to pay them in order to keep using the lawnmower. In very simple models, it makes sense. In the real world, it almost never makes sense, and ends up being a type of economic smokescreen for what is basically politics choosing winners and losers based on arbitrary or cherry picked criteria.

So my recommendation is to not try to use "externalities" as a justification. Here's what I would argue, for example, when I am dictator. We want to transition to nuclear power and away from natural gas. So let's tax natural gas and use the money to pay for nuclear power so that it becomes cost competitive with natural gas, and then let's reserve natural gas for things nuclear can't do for us -- such as creating fertilizers. See what I did there? I made my actual motivation the main cause and didn't invoke any economic theory of externalities. Because overall goals should not be slaves to economic models, but rather the economic models should be servants of the overall social goals. The economic model can tell you how much to tax natural gas so that nuclear becomes cost competitive. But whether to decide to switch away from one source to another requires wisdom rather than brute force calculation of all outcomes -- there is no economic model that can spit out the answer. We should not give economists that type of power.

I understand the theory of externalities. I certainly understand that it’s difficult or impossible to know all costs to all parties for some good. But that doesn’t mean there aren’t pretty clear examples, like burning fossil fuels. It certainly doesn’t mean you can dismiss any mention of externalities simply because they are complicated. Some people are bald.

You talk about how certain goods are immensely beneficial to society, but that’s not sufficient to claim there are positive externalities. Your example of a road being used by a store that you buy from is a poor example. Those transportation costs would simply show up in the cost of the stuff you buy (disregarding government subsidies for roads, of course, but that’s a separate issue).

Well, wouldn't it then be better to use the gas for that, and use other means for heating homes?

Yes, using natural gas for the things that only natural gas can do would be an excellent public policy.

However that's not the policy we are pursuing. We are pursuing very blunt instruments to try to discourage investment in developing natural gas fields full stop. Which is bad public policy.

The other issue is by raising the price of natural gas, we are raising its price for all uses. And the price of natural gas tends to determine the price of fertilizer, which tends to determine the price of food. So this policy makes food more expensive.

Even worse, natural gas and things like coal and oil are in some sense substitutes. Not perfect substitutes, but what happens when we make oil more expensive is that people shift to coal and natural gas to generate energy, and so these two become more expensive as well. Similarly, if we were take all coal offline, that would increase demand for natural gas and oil as substitutes, and again, the price of these commodities would rise, which would increase the price of food.

What would be nice would be to see if we could de-couple these prices somehow -- e.g. tax usages of natural gas downstream of the price of the commodity itself. Other ideas are also possible.

Gas is a defacto waste product from oil extraction, and very energy dense.

It’s shocking it’s not a bigger delta the other way.

$50k in equipment? At what scale are we talking. I got a Mitsubishi heat pump with three heads installed for less than $15k and it works well into the negatives.

Ground source requires drilling and is not the more common type of heat pump found at most homes.

Note that you are unlikely to achieve 95% efficiency on the gas. That requires you to run your plumbing at about 20 degrees (celcius), since burning gas means simple thermodynamics apply. At 40C you're already down to 90%, and virtually every installation I've seen actually is set to 60-80 degrees, at which point you're nearing an efficiency of only half.

Hotter water means a more reactive system and smaller radiators (you'll need five times the volume for a given power output when you compare plumbing water of ~30 to 70+. You are almost certainly not installing that. Maybe you are installing floor heating, in which case you are likely OK.

But most people I speak to don't realize that 1. Gas efficiency goes up as temperature differential goes down, and 2. energy transfered to water also needs to leave the system at your radiators. So size them for the effiency you want, or be disappointed with a significantly higher gas bill.

I don't have any hard data at hand, but I can say that running my AC (which is just a one-way heat pump) for a few days out of the month in the summer makes my combined gas+electric bill about the same as in the middle of winter when my gas furnace is running all the time.

Id love to see a combined heat and power system that burns natural gas to drive a heat pump compressor, and recovers the combustion heat. I wonder what multiplier it would get on a therm.

I've been contemplating the same thing. Something like the WhisperGen stirling engine hot water heater should be available to everyone. Hell even a water cooled honda generator that runs off natural gas, and pumps the radiator fluid into your home for heating purposes would be nice. It seems weird that we aren't producing electricity when ever you burn natural gas for heating.

The issue is maintenance and capital expense due to complexity.

Gas furnaces are dead simple, and still break and require maintenance. Heat pumps, more so. Generators? By far the most.

Combining the three sounds like a huge headache for marginal efficiency gains.

This is why I started off referencing external combustion engines like Stirling engines. These tend to have very low maintenance needs as the combustion occurs outside of the pistons, so there is no fouling of the moving parts. One could use self lubricating materials like graphite rods as your piston heads and have quite high reliable components. The stirling engines that NASA utilizes are considered maintenance free.

As far as efficiency gains, we're talking 25-40% of the energy being produced as electricity and the majority of the rest being turned into hot water for residential use, either by one household or a whole city block. This doesn't sound like a terribly marginal gain. Specially if the energy production is done during the hours when traditional solar installations are not producing which are generally when folks are running their heating.

Generators with heat recovery exist, search on micro CHP.

Yes thanks, I'm aware. I have actually worked in this field in the past building wood gasification micro CHP units.

There are refrigerators that run directly off of propane. https://www.ferrellgas.com/tank-talk/blog-articles/how-propa...

The working temperatures of ammonia are probably wrong for a heat pump, but perhaps something similar could be devised.

The main reason in my area is that a.) these fancy new efficient ones aren't available easily and b.) no contractors in the area to install/service them. I just got a new gas furnace and really wanted to add a heat pump to the mix but after several quotes all of them either said no or strongly avoided the question. I couldn't get a straight answer.

> then the boiling point of the refrigerant must be below -20F

This isn't the limiting factor for choice of refrigerant... There is always a low enough pressure that anything boils.

The problem is that at very low pressures (think a few millibars), gasses need huge diameter pipes and huge pumps to move even a small number of kilowatts of heat.

Another issue is that at pressures below one atmosphere, the system will suck in ambient air, causing condensation to collect inside your refrigeration circuit, which creates ice crystals, etc… nobody does that.

Plenty of systems work below atmospheric pressure. For example, Fridge freezers most commonly use isobutane (R600a), and when operating with the freezer at -20C, the pressure will be half an atmosphere (ie. -0.5 bar below atmospheric).

The systems are filled using a vacuum pump to ensure there is very little atmospheric air in them, and they also use a 'filter dryer', which is a chemical compound chosen to trap any remaining water and various other common contaminants.

On top of that, the maximum theoretical COP for a cold source at -30c and a hot side at 35c is 4.7. Real world heat pumps usually get half of that, possibly even less for this extreme delta t, so around 2. Natural gas tends to cost less than half per unit energy compare to electricity, even less typically, so you'll never make back the capital costs. Of course if this technology matures to the extend it's much, much cheaper and you tend to have milder winters and hot summers it's good enough and possibly worth it, but we haven't gotten to maintenance and installation costs yet, as the 35c hot temp assumes underfloor heating or tons of fancoils which have their own issues, the fact that the heat pump must be massively overspectd to output enough heat in peak conditions etc.

> “additional efforts are needed to address common technical and market barriers to wider adoption by consumers – which include performance at temperatures of 5F and below, installation challenges, and electricity grid impacts during peak demand periods.”

There are definite market barriers at play. In my house in New England, I tried to replace my aged boiler with an air-to-water heat pump (after carefully verifying, via experiments during a cold week in February, that my heat distribution would indeed work fine at a supply of 130°F). Only one company was even willing to come out and provide a quote and their quote was around 2.5x the costs of "put another boiler in", such that the payback period would be "literally never".

If, after doing the research to find out about them and specifically seeking one out, I couldn't manage to make an air-to-water heat pump make sense, I doubt that very many of them are being sold. I suspect it's one of those items that, if more were sold, more firms would sell/install them, bringing the costs into the realm of economically reasonable (and lowering the risk of having a difficult-to-support heating plant in the decades to come).

My house uses a heat pump + oil-burning furnace. It gives a pretty good combination of efficiency, and ability to handle really cold weather.

I think this is most of the way to what the future looks like: a high efficiency heat pump with cogeneration, so you can still burn propane or natural gas for heat and limited power to drive the heating system in an emergency (and if you don't have gas service to the premises, you have either a propane tank [1] or an exterior outlet to hook up outside). You must have a solution for when utility power is down for substantial periods of time.

During the winter storm that just passed, a friend in the Midwest called that their house had no power and was rapidly cooling. The utility could provide no ETA to resolution. I walked them through (over the phone) safely enough backfeeding enough power from a gasoline generator (outside, with the extension cord run through a basement escape window) into their furnace circuit to bootstrap the furnace (and run the blower fan) to keep the house warm so that the pipes didn't freeze and burst. If the HVAC system had had a small battery and some way to generate power from the heat it was burning, the gasoline generator would've been unnecessary. Perhaps an integrated thermoelectric generator [2]? A standby generator isn't financially practical for most folks ($6k + install).

(EDIT: to the safety folks out there possibly concerned, the furnace breaker and main breaker were tripped, and the meter was pulled to prevent any chance of harm to electrical linemen from inadvertently energizing the utility line; take no chances with safety, do not attempt this at home)

[1] https://www.amerigas.com/about-propane/propane-tank-sizes

[2] https://patents.google.com/patent/US5427086A/en

For others interested in preparing for a situation like this, here's a video laying out in reasonable detail what to do.*

https://www.youtube.com/watch?v=1JNuovFpCpQ

* I don't like that they skipped putting a bushing or an NM clamp on the back of the box into the furnace, but otherwise the video looked sound when I watched it a couple weeks ago.

I've been waiting to do this in the event of a power outage; that said, I'm really curious what the current draw of my furnace blower will wind up being. There is a label that suggests it has a peak draw of 8.5A, but that seems like a lot. Still, it seems like a nice backup plan in the event of an outage, even if I only have a couple kWh worth of battery storage.

Many furnaces use third-party controller boards with well-known suppliers. You might find in the furnace manual that there are different wire-to-board permutations to allow different fan (interior recirculating blower) speeds. From home testing a few years ago, I recall RMS currents 50%-80% of peak rated draw, but could be mistaken.

What's the model number of your furnace / blower? I've been doing a ton of research in this area as I'm remodeling my place and have several furnaces and duct loops to deal with.

From what I've seen so far, unless your unit is super old, the 8.5A is almost certainly just a 'peak' startup current that's overstated as well. Your blower should say something like "1/3 HP" and from there it's straightforward conversion of 1HP = ~750W so a 1/3HP would be 250W. On a 120v service, that's 2 Amp. With maybe 15% efficiency loss, at peak speed, the blower motor wouldn't be drawing more than 2.5A. It could/will draw more than that to start, but literally just for a second or two.

Thanks, that's useful! That was my thought w.r.t. startup current, since I assume it is an inductive load. The model is a Carrier 58STA090-14 and it is indeed 1/3 HP. Seems likely I could run that load off my car in a pinch, which is definitely a good option to have available.

So per the spec sheet -- the "Full Load Amps" for that model could actually be up to 5.2A per the bottom portion of the 90-14 column second from the right on page 5:

https://d1049ui2fjityy.cloudfront.net/userfiles/inriver/docu...

That motor has a switch so it could be in 3 different settings -- As shipped, the blower motor speed is set to be faster for AC (more draw in the summer) to prevent the coils from icing over, and then it's shipped to be the slowest fan speed while in heating mode, but it is possible that your installer changed that. If indeed it is in the "slow" speed for heat, then 2.5A would be about right.

Also anyone with a high efficiency furnace should be away there is a second smaller blower on the exhaust.

I don't know what's up with that, but the standard fix is to let the faucets trickle.

Challenging when you're on a well and your well pump requires power. Will your generator support 220V and the surge start a well pump requires? That surge is going to occur every X minutes to replenish the pressure tank as it depletes due to trickling. In this scenario, driving the 15A furnace circuit was a more simple fix.

Something else to consider: electric pipe heat cable to keep pipes from freezing draws ~7 watts per foot, is quick to install, and easy to power if you've planned ahead.

I guess I must plead ignorance once again. Maybe flushing the pipes would be possible? I just know I would electrocute myself screwing with anything more powerful than a laptop battery.

I happen to have valves that would allow me to drain almost all of my house to prevent freezing, but that's not universally the case (and even in my case, I couldn't drain the hot water tank fully without power without letting it just flood the basement).

Freezing pipes is one concern, though having heat for the humans is another pretty desirable thing, so if I could spend effort on something that would result in drained pipes and no heat or on something that would result in enough heat in the house to make that unnecessary, I'm going for option #2 every time.

> (and even in my case, I couldn't drain the hot water tank fully without power without letting it just flood the basement).

That doesn’t sound like it should be up to code? I’d expect building codes to require a floor drain where your water tank is.

Houses designed to be vacant in freezing temperatures will have a drain port at the main shutoff valve so the pipes can be fully emptied.

Unfortunately, this is not a universal feature in residential construction.

In an emergency you can often turn off the water main, and then disconnect the union that connects to the meter. Most of the water in the house will drain out.

Most houses, the curb is downhill but not all - and there is always some branch run somewhere with a loop in it under a floor or something. :s

Problem is high efficiency furnaces/boilers require a powered vent, so they need quite a bit of power to operate. And then you’d need to circulate “cold” air/water against the heat exchangers.

This usually adds up to several hundred watts.

New homes should be built to better insulation standards. You have to solve so much and use so much energy to solve for a drafty house or lack of insulation on the outside walls. Newer homes are factoring in the R-value of 2x6s and finding ways to insulate those now.

In this scenario, high efficiency heat pump with a backup gas fireplace. Those typically don't require any electricity to run as they can ignite off AA battery or a push button. My PC battery backups barely last 30 minutes on a wifi router because of conversion loss. Furnace fans also account for up to 1/3 of energy usage of a unit. Cogneration would add $5-10k to a home build versus a 50 gallon propane tank with a single emergency gas fireplace unit.

If you've got wood available, you could also have a wood furnace as backup, and / or an actual fireplace (an efficient one ideally, possibly a masonry heater if you have the space and foundations to handle that load).

This is a great solution too. I have a wood 'heatilator' (or fireplace insert) and during the recent cold snap in the Midwest I went through about 1/4 face cord of kiln-dried oak over 3 or 4 days (that's a lot by my standards). It kept the gas furnace mostly idling and the home warm (in the 80's by the fireplace actually, lol). Probably not exactly cost efficient with the cost of wood these days but I like being able to take some of the load off the gas furnace.

Most of the heat from a fireplace goes up the flue unfortunately. It’s one of the least efficient heating sources per the dept of energy (~15% efficient). A low tech solution is more thermal mass perhaps in high performance dwellings.

Wood burning furnaces are a challenge because homeowners insurance providers don’t want to insure dwellings that use them (but will if certain conditions are met, such as it not being the primary hearing source, professionally installed, etc).

Wood stoves are really efficient these days, hitting 70% plus, and it's Carbon Neutralish™ as a fuel source (obviously uses fossil fuels for harvest and transport and such, but that's only some percentage of the total)

I'm always amazed at the ability of our questionable 1990s wood stove to happily heat the entire house. It's a great backup heat source on days when it gets proper cold out, which is thankfully rare here.

> Most of the heat from a fireplace goes up the flue unfortunately.

Depends on the fireplace. Direct-vent gas fireplaces take outside air for combustion and then exhaust it, with no inside air being used in the process. There are also direct-vent wood stoves that can do the same.

Wood stoves can get about 85% combustion efficiency, but that's not heat-delivered efficiency (often 10-20% lower):

* https://www.epa.gov/burnwise/energy-efficiency-and-your-wood...

This is why very old homes have absolutely massive fireplaces in the center of the house - the huge size led to a massive thermal mass that would extract most of the heat from the circuitous smoke path inside.

The advantage of wood is you can burn it without any outside source of anything.

I was also without power last week for 6 hours.

I have a SEER 18 heat pump for cooling which can reverse for heating in the winter but it also has a natural gas burner. Based on the installer's advice we set the system to use the heat pump if the outside air is 40F or above and switch to natural gas below 40F.

The problem was that when the power was out the electronics in the system could not communicate with the thermostat in the house. My Nest thermostat literally said there was no system connected. I wish there was some kind of UPS to power those electronics and the blower fan.

Can't you just hook up a sizeable computer UPS for that purpose? That should run it for long enough to get you through most outages.

The heat pump will be a 240VAC load. Consumer-level computer UPSes in the US are 120VAC.

The 24VAC transformer for the control circuitry will be on one of the 120VAC legs, but it's likely the installer didn't specifically wire it to allow partial powering of just the transformer and electronics board.

Hm... you might be able to get away with ordering a EU UPS then, those are 240V, you'll have to add a center tap transformer and maybe mess around a bit with it to get it to output 60 Hz (that will only improve efficiency).

I think if you’re just looking to run the fossil fuel equipment from the UPS, you can do it by just ensuring that and all the transformers and controls are on that phase.

That will take an HVAC person who knows who things work rather than just following colors on a wiring diagram. Probably easier and more supportable than trying to import and mod an out of region unit.

Probably quite a bit more $ though. Anyway, I saw your message and figured it has to be solvable in a cheap and quick way, never mind me :)

There is, an inverter sized for the load with an auto transfer switch attached to a set of 100ah batteries will do what you're looking for. And that is what a UPS is on the inside. They just tend to use terribly small batteries.

Such setups are easy to build if you want it and running a 100-200w blower doesn't require too many batteries either. However, if you need to run a compressor, the number of batteries required to power that for 6hours would start to get cost prohibitive.

https://www.amazon.com/s?k=2000w+pure+sine+wave+inverter+aut...

https://www.amazon.com/s?k=220v+automatic+transfer+switch+50...

https://www.amazon.com/s?k=100ah+lifepo4+battery

This guy has accumulated essentially everything you could want to know about such setups, at any voltage, any reasonable power range, in various applications: backup systems, full off-grid, mobile power.

https://www.youtube.com/@WillProwse

https://www.mobile-solarpower.com/

For a 240v load, there are inverters that output 220v or you can use one inverter to power each hot line, just buy the batteries at the same time from the same manufacturer and ensure they will stay balanced. In your case you could also use a cheaper manual transfer switch if you don't mind going into the closet to flip it when the power goes out.

What model do you have? I currently have a boiler for hydronic baseboards and would like to move to a more all in-one-solution for heating/cooling that is predominantly electric. My goal is to move all my gas appliances to electric with solar but it would be great to have gas backup and gas for those cooler days.

> A standby generator isn't financially practical for most folks ($6k + install).

An Honda EU2200i generator costs US$ 1400 (and less expensive generators can probably be found):

* https://powerequipment.honda.com/generators/models/eu2200i

You also need an external plug, a cable going from that plug to a sub-panel, and a line going from the sub-panel to the main panel with an interlock device for safety, if you wish to avoid running extension cords.

There are dual-fuel generators that can also use either gas/petrol or propane, and some tri-fuel generators that could also be connect to a natgas pipes (and conversion kits that can be added to propane-capable generators).

The problem is that while all that electrical work is a weekend project for a DIYer, it's quite expensive to have a professional do it. So then you start bundling things (since I'm already paying thousands, maybe I'll just splurge for that Generac with the auto tranfer switch...)

(FWIW, I believe there is an aftermarket propane conversion kit for the EU2200i. Converting a gasoline engine to run on propane is straightforward)

The Honda EU2200i can be aftermarket modded to run tri-fuel. I did it to mine just in case, and currently have it set to run propane. A simple swap of a small part will let it burn natural gas instead.

I also like that I don't have gasoline to deal with - I can stock up a 20lb propane tanks and generally forget about it. Rotating gas is a giant pain if you keep enough on-hand for an outage of any appreciable amount of time.

Would you mind sharing a link to the part you mentioned?

https://www.amazon.com/Grenergy-Gasoline-Conversion-Generato...

https://www.hutchmountain.com/products/honda-eu2200i-propane...

If you are at all handy this is probably a 30 minute job, it involves cutting a single hole for the propane tank connection and otherwise can be done with a screwdriver.

It's been reliable for me going on 4 years now - although I only really have run the generator for maybe 100 total hours since then.

I haven't looked for some time, but there are also other kits on the market. I those these guys due to reputation on forums, and how clean it is to install.

Up in the northeast power goes out every winter. It also gets restored a lot faster than the midwest, personal experience, but the reliability of it going out at some point during the winter means that everyone has a wood stove and a backup generator. Portable ones are practical enough and wiring a couple of circuits and a plug for the generator is not cost prohibitive for most.

What I've found in traveling the whole country for a few years is that local power generation or storage always forces you to consider just how little power you actually need. You can pretty much never just power the whole house, or even a whole RV. Even 50amps is ALOT of power and trying to provide that from solar/generator/batteries is incredibly expensive.

So your real question is pretty much never going to be, "how do I power my whole house in an outage?" but rather, "What is absolutely essential to spend up to 3000w on?" and even then knowing that a generator putting out 3000w burns 20lbs of propane in about 4hrs and 3000w for 4hr on batteries would require 10 to 12 100ah lithium batteries at $350+ each.

I still agree with you that super efficient heat pumps are the future without fossil fuels but that probably also has to come with greatly improved insulation values in almost all homes regardless of region. Think double the current DOE recommendations. If you're running on electricity, you will feel the cost of every bit of heat you lose through the walls, roof, and windows. More insulation is a one time cost versus monthly, it always makes sense in the long run.

However, we'll need regulation to force home builders to invest in that instead of just adding unnecessary sqft that people never use but have to heat and cool anyway. For some reason people don't want to pay extra for a house that is well built and well insulated, they'd rather go with bigger is better even when it's already absurd, and builders have noticed this.

Mine is the same with a gas furnace/heat pump combo. It's great.

My house uses a NatGas boiler and in-floor radiant heat. It also has wood stoves for those -20F (or worse) nights when I need a little supplemental heat (or the power is out and the main heating system is offline).

A heat pump would be a waste up here in Alaska esp given I don't need A/C. Just opening the windows and running some fans in the summer tends to do the trick for cooling.

> Just opening the windows and running some fans in the summer tends to do the trick for cooling.

The problem with 'natural' ventilation is you also get things humidity, pollen, dust, etc coming in as well.

One of the major advantages of mechanical ventilation (like HRV/ERVs, which cycle the air) is that things are tempered and filtered beforehand. Sometimes you also want to deal with individual variables: the temperature is fine, but the humidity is off.

There's the expense of maintaining a central A/C, ductwork, etc., that I'd rather not deal with (not to mention electricity is expensive here). Yes, if the air outside is dirty (forest fires, volcanic eruption, etc.) you've got a point. HEPA filters can help some with this, I suppose.

Humidity isn't usually a big issue here most days. If anything, it can be a bit dry (but not as dry as running an A/C would make it). Though, I do have a portable dehumidifier I use when I need to :)

Is filtering incoming air (for a mix) necessarily the norm, though? Even in the rare high-performance homes built on spec[ulation], my sense is that filtering incoming air might be omitted, in favor of a single filter acting on recirculation.

And forced air can be quite dirty (dust/pollen/etc.) if you don't clean your ducts regularly.

It wasn’t the norm, but now that more people have pollen allergies and widespread summer fires are common again, it’s become more important.

My use case is similar. The oil furnace feeds hot water into a network of radiators. My design goal is to maximize the electricity outage that we can make it thru without frozen pipes, and it has to be on a budget (so no generator or giant UPS).

The house is mostly unoccupied (a second home). Operation in an outage has to be fully automatic - it needs an automatic transfer switch I guess. The typical indoor temperature is just 5.5C (42F) - this does not leave much margin for the house to cool down during an outage when outside is like -5C (or -10 or -20).

But it turn out that an oil pump and a water circulation pump do not draw THAT much power, so if I can run them off backup power (say, a new car battery plus an inverter), it should last for some time before ice has any chance to form.

I’ve got the same thing. I did a bunch of tests last year to figure out relative performance and costs of heating my house at various outside temperatures. This included doing fine grained electricity monitoring with an iotawatt and rough fuel oil usage using an ultrasonic sensor.

Of course, now I’ve got a ton of data that I haven’t finished building my automation with. And, unfortunately, I’ve got an annoyingly bad Lennox iComfort for my heat pump thermostat and may have troubles building the automation anyway.

I recently spoke to a well driller that also does geothermal well drilling. He was telling me that the systems are more or less obsolete at this point, as the air-to-air units have such a high SEER rating that the ground loops really will never pay off.

Is geothermal any quieter? I was assuming the ground loop pump would be quieter than the air-to-air fan.

There's no outside noise in a ground-source system (for all practical values of noise: it's just flow in insulated pipes). The pumps are nearly inaudible inside the mechanical room as well.

Inverter-based air-source outdoor units are nearly inaudible during any time when your neighbors are likely to have their windows open. When the weather is uncomfortable enough to have the units need to run at high-speeds, they are no longer inaudible, but the neighbors are likely to have their windows closed.

I'm looking at a system now that is 45 decibels - 10db quieter than my robovacuum which is one of the quietest in the market.

What attracted me to geothermal was the year round availability of 50F water - easier to extract winter heat at that temperature, and you could create a simple fan / radiator ducted air cooling system for the summer. The latter would actually store summer heat in the ground, and could be powered purely by solar.

I have been doing a bunch of research into geothermal recently and this doesn't seem to match with what I have seen. But it probably also varies a lot by location and such.

They appear to still be about twice as efficient as an air to air one, and have less parts and maintenance (no outside unit to deal with). Yes the initial install is a lot higher because of the drilling but that should last for decades. In the US there is also 30% rebate at tax time which helps cut the costs down a bunch too.

You're looking to spend at least $30k if you need a vertical system, probably a lot more. And from what I understand, they ground loops inevitably don't reach their expected lifespan, so you'll be drilling new holes sooner than later.

If you live in a very cold climate, I suppose with tax credits it could make some sense. For most people, AC + Natural gas is the cheapest way to go.

I watch the PBS show This Old House which takes place in the Boston area. The plumber / HVAC guy Richard Trethewey is a fan both ground ("geothermal") and air source heat pumps including the air source heat pumps that work at 0F and heat water for radiant floor heating. I'm surprised that more companies aren't doing it there.

The majority of their projects are pretty high end (and they tend to want to demonstrate new technology, because it's more interesting).

For the 'project homes', perhaps, but if you're going to go through the effort of building new or re-doing an entire structure, you might as well use the latest and greatest stuff/practices/code.

They also have many segments of smaller DIY jobs for simpler fixes/maintenance, e.g.:

* https://www.thisoldhouse.com/plumbing

They most likely get funding (in the form of steep discounts or free equipment) from the equipment manufacturers to do those installs. They’re called demo homes in the trades.

It’s a good way to buy a bunch of headaches if you do it on your own without manufacturer support.

The 'cast' of 'This Old House' is mostly MIT MechE grads who figured out they could make more money doing renovations for Route 128 techies than burning themselves out working at their companies.

Oh yeah, Richard Trethewey and Tom Silva are definitely MIT guys:

* https://www.thisoldhouse.com/21071273/richard-trethewey

* https://www.thisoldhouse.com/21071309/tom-silva

* https://www.thisoldhouse.com/cast

Well, they've done episodes like this: https://www.thisoldhouse.com/season-20-ask-toh/23197052/s20-...

Advanced technology being developed at advanced research labs: who would have thought?

But that doesn't change the fact that hosts are mostly 'regular folks' in the trades that hook up a bunch of wiring and pipes (or glue/nail/screw together a bunch of wood).

I had 'cast' in quotes for a reason- I was half thinking of Norm (though I don't think he went to MIT) and more of the various sub contractors that filter through the show (some of which seemed to be thinly disguised product reps). Still, you can't deny they were higher quality outfits than anything most of us are likely to run into.

I'm not seeing anything about mit... were you guys being sarcastic?

I haven't watched since a kid w/ the Bob Villa days....

I mean that did describe Car Talk

No they are mostly second and third generation trades people.

> their quote was around 2.5x the costs of "put another boiler in", such that the payback period would be "literally never".

Doesn't that depend on the costs of both energy sources?

Last year I made a similar choice, albeit at smaller scale, just for one water heater. Picked an electric heater with heat pump, also cost 2.5x more than plain electric, but 1/3 the energy cost. It will take a few years to pay back ...

> Doesn't that depend on the costs of both energy sources?

Yes, it depends on the costs and efficiencies of the competing energy sources, the difference in capex, the annual building heat load, the projected lifespan of each source, annual maintenance costs, and the interest rate.

Was it a company that also did gas boilers?

If it's not a company that solely does heat pumps, I have heard a lot of contractors will give outrageous estimates because gas is simpler for them and they don't want to do it without the huge markup.

I had ground source heat pump installed with vertical wells in a city by a dedicated geo installer. The cost with tax credits came out not much more than a high end gas furnace and water heater. Going airsource would have been even more cost competitive, especially with the federal tax credits in place starting in 2023.

They did. To (only partially) guard against that, I made it clear that I wouldn’t be hiring them to hang a gas boiler. (That still allows them to prefer someone else’s gas boiler job over my air-source job, but at least prevents me bidding against myself.)

And there definitely would be more labor, more piping, and more electrical work to switch to an ASHP; that’s part of the market forces problem that is hard to overcome with anything other than large price increases for gas or larger direct subsidies for switching.

Was it the company mentioned in the second half of this show:

https://youtu.be/TlX5z32T1J4

Air to water is pretty new technology for the U.S. As you mentioned most oil boiler hydronic systems are spec’d for a much higher temp (like 160 to 180F). I’ve been curious myself if you can salvage any of the existing baseboards with a 130F supply.

I have a 5 head mitsubishi cold weather mini split installed last year but I am still keeping my oil boiler for now for domestic hot water and supplemental heat. If I could switch that to an air to water heat pump for a reasonable cost that would be nice.

We need heat pump that work below -20F like we need an electric car that can go 1000 miles in a single charge - which is we really don't for the 99% of the use case. What's needed is a heat pump that's cheaper to install than gas furnace or oil boiler for the 80% of the population. On few days of the year when it's -20F or below, it's ok to use resistive heater as a back up.

Isn't the primary issue not the cost of installation, but rather the cost of the fuel? Natural gas in most of the US is far cheaper than electricity, and even if heat pumps are theoretically more efficient than natural gas (energy in:heat out), if the fuel is two or three times cheaper none of that matters.

In most places in the US, in normal working conditions, Heat Pumps are 30%+ cheaper than a gas furnace to run.

However, in some really cold places like Chicago, Minneapolis, etc - the days where current heat pumps are inefficient might be enough to make it cheaper to always run gas.

OP is proposing to have both systems and only run the gas furnace on extreme days - which would lead to a ~30% reduction in running costs.

I suspect the CapEx of having two heaters wouldn't make sense, though.

It'd be better to just have a hear pump that can run efficiently at colder temps.

I think it’s the opex of 2 grid connections that messes with the numbers.

Depends on how your utility charges, but I feel like many/most sell you the gas roughly “at cost” and then some fixed monthly delivery/connection charge that covers their regulated rate of return on their network equity.

Oil or propane supplemental heat might be cheaper than Natgas supplemental heat. My money is on wood pellet stoves for supp heat for those that don’t require full automation. Or maybe even dirty coal…

Also, the capex of gas delivery. Not needing infra to deliver gas to each and every house would likely be a pretty big win.

For new construction sure - for existing it’s already there and apparently needs little maintenance.

You generally have an air conditioner though as well as a furnace and you can get those that can also work as a heat pump. Mine supposedly works as both but they only set it up for cooling and my gas furnace does all the heating :(

Maybe the gas is cheap, but the monthly and up-front connection costs mess with the economics.

And that cost will only go up as people cut the gas cord.

Depends on how your utility bills out it’s infrastructure: some charge minimal monthly connection fees, others a lot.

I honestly wouldn’t want to own a residential-focussed gas distribution company unless someone revolutionizes stirling engines or micro cogen systems and people start cutting their electric cord.

I live in a pretty standard suburban subdivision in the US. All the houses are 25 years old. I have about 60 coworkers in very similar situations. I think everyone of us has natural gas that was installed when the original company built all the homes in the subdivision over the course of a year. Stuff gets a lot cheaper when everything is put into the ground at the same time. My monthly gas connection fee is $10 so that in the summer my bill is around $19 which is $9 for the water heater and $10 for the connection fee.

The thing with heat pumps is that they can be as much as 300% efficient (effectively) or more. This is because they can get "free" heat from the environment.

So even if per unit of energy gas is way cheaper, heat pumps can still come out ahead.

Modern heat pumps can be up to 500% efficient (using 1kWh of energy gives 5 kWh of heat) at optimal temperature.

Yes, I went with "300 or more" because that's a more typical number for a run-of-the-mill unit.

And even the low-temperature-capable heat pumps reduce toward 100% at the low temperature end, IIRC.

Yep, here's a spec sheet for a unit I'm looking at as a drop-in ducted replacement for my existing system -- it has all of the models listed, but I'm looking at the 4-ton unit (12k btu per ton, so 4-ton = 48k BTU, hence the MDU18048): https://mrcool.com/wp-content/dox_repo/mc-uni-perf-ss-en-01....

It takes 7kw at 17ºF to provide 48k BTU. There are 0.293W/hr per BTU so 7,000W = 25k BTU "in" and 48k BTU "out" or a COP of 1.9. At -15ºF, it's using 5.9KW but can only produce 28,500. So a COP of ~1.4. Still better than electric resistive heating but not by much. At 40ºF, the COP is more like 3.3 which is in line with the very efficient numbers.

Fortunately my 99% design temp is ~8ºF and my area only sees 20hrs below zero per year, so this will work just fine for me.

This is true when it’s warm enough outside that you don’t need the heat pump anyway. When the temperature becomes cold, it’s a different story.

Heat pumps might make sense when you have a well-insulated house, like the kinds in Northern Europe, to trap heat and reduce the power required to heat the room. American houses in most places are far too drafty.

> Heat pumps might make sense when you have a well-insulated house, like the kinds in Northern Europe, to trap heat and reduce the power required to heat the room. American houses in most places are far too drafty.

If you need significant heating or cooling in a home, the first thing you need is to deal with is insulation, otherwise nothing you do is going to work well.

That’s true, but unless you are doing some kind of industrial work (including running very hot computers), you shouldn’t need significant heating or cooling in most of America.

The idea that indoors needs to be between 72° (F) and 75° at all times is unsustainable.

The other problem is that if you do decide to insulate the house, then you may end up needing to use A/C because the house can no longer be wind-cooled to an acceptable temperature, and you will need to update the heating system as traditional methods of heating (fireplace, stove, simple furnaces) depend on external air circulation.

> That’s true, but unless you are doing some kind of industrial work (including running very hot computers), you shouldn’t need significant heating or cooling in most of America.

> The idea that indoors needs to be between 72° (F) and 75° at all times is unsustainable.

The WHO recommends a general minimum household temperature of 64°F for all populations for health reasons, with a higher minimum for sensitive groups including children and the elderly; maximums are more regionally variant because acclimitization (which takes years) plays more of a factor in high-heat health risks, but even in extreme regions seems to top out at about 90°F for the general population.

Lots of the US spends lots of time significantly out of at least one end of that range (and, especially given that the top gets lower in colder regions, lots of the US spends lots of time significantly out on both ends of the scale.)

> The other problem is that if you do decide to insulate the house, then you may end up needing to use A/C because the house can no longer be wind-cooled to an acceptable temperature

If you have a house that is insulated well and also lacks doors and windows that can be opened, sure.

But... that presents other problems, too.

I’m talking about significant cooling and heating, which is what you mentioned. Set your thermostats to 66° F heating and 88° F cooling and compare your electricity gas usage to 72° F/75°F for the year.

Unless gas is subsidised I don't really understand how gas can be cheaper. A modern heat pump can give 3-5 kWh of heat using 1 kWh of electricity. And a modern gas generator is roughly 50% effecient at creating electricity (according to the Internet). So by using electric and a heat pump you should be able to get 1.5-2.5 times more heat from the same gas by making electricity of it first compared to burning it directly.

At least in the US, electricity rates and natural gas prices are highly regulated for retail customers. Thus the wholesale price of gas does not generally correlate to the retail price of electricity. In places like California electricity prices are very high due to renewable generation requirements. You would definitely want to have your own solar installation before converting over to all electricity in that state.

We’re being forced to. After 2030 no more gas water heaters or furnaces can be sold. The cost of heating your house and water here is going to probably triple or more, and will fail every time there’s a blackout.

You used to be able to buy water heaters that would last forever. My house had one from the 1950's with a copper tank. Still going strong. Just don't be convinced b y the repair man to get a new one when you have to replace the thermocouple every 8 years or so. The furnace is from the late 1980's, still fine.

I'm not sure if such well made devices are still available for purchase however.

Modern residential gas furnace is 90% or more efficient.

Makes sense (I would have guessed closer to 99% actually). But 99% is still less than both 150% and 250%:-P

What is the efficiency of the power generation system? The whole system costs matter not just the individual HVAC unit

You seem to assume that energy cost is the same no matter how it's delivered. Here in California, electricity is much more expensive than natural gas for the same amount of potential energy.

Yeah delivery cost might be a factor, maybe it costs more to deliver 1 kWh of electricity through a wire than (the equivalent of) 1 kWh of gas with a truck (that's what you use right?). Gas, if its stored in a big tank under your house, also had the advantage of beeing "locally cached", so you don't have to dimension the system for the max load in the same way.

Many urbanized areas in the US have natural gas distribution pipelines.

Rural areas rely on propane delivery, which is roughly 2x the cost of the piped natural gas (per unit of energy).

> A modern heat pump can give 3-5 kWh of heat using 1 kWh of electricity.

At lower temperatures the modern heat pumps cannot attain this COP, which is why they are working on heat pumps that work at lower temperatures.

I imagine there are more costs in running a power plant than the cost of fuel - hydro, solar and wind power is not free despite not using any fuel.

Interesting thing here is that natural gas is about 1/3 the cost of electricity per energy unit, so when the COP is above 3, it's cheaper to heat home using heat pump.

The reason why natural gas is about 1/3 of the cost of electriciy is because most natural gas power plants run at thermal efficiency of about 30%.

Combined cycle natural gas plants have an efficiency of about 60% (lower heating value). Even simple cycle NG plants are about 40% efficient (again LHV).

Yup. And at least here in Finland the pumps come with the resistive heater included so it automatically switches over to it once it gets too cold.

Which where most people live here averages to less then 1 day per year so not a big deal.

Way bigger issue here is some fall storm destroying the power lines and being without electricity for multiple days when it already is cold enough they one needs heating.

That's an issue even with gas furnaces here in the US, as they won't run without electricity.

Yes but it only requires a couple hundred watts to power the blower motor and electronics of a gas furnace. They're easy to power off of a small portable generator or even a cheap inverter hooked up to your car. The same cannot be said of a heat pump or resistive heater.

They are, and I have a contingency plan for this myself, but few people have the tools and knowledge required to accomplish this.

Anyone with $200-500 can go buy a kerosene heater and some sealed kerosene which will keep one ventilated room warm for awhile in an absolute emergency.

With the common skill set people have now, half the people trying it would kill themselves. :s

True, but furnaces have a much lower power draw. They are easy to run off a generator, which are common in areas that frequently lose power.

Millions of Canadians would disagree with that assessment.

I'm Canadian myself. Heat Pump is actually very popular in the eastern maritime provinces and is gaining in popularity elsewhere also.

Sort of, but really we need both. Yes cheaper for most people. But as someone who lives in Minnesota, we definitely get below those temps. By code we require full backup resistive heaters for a heat pump at such a rating which increases the full cost and installation cost. Plus, it is much less efficient than a heat pump (though efficiency lowers as the temp gets colder due to defrost cycles).

AFAICT one of the main difficulties with heat pumps is that they want to use low temperature heat emitters, similar to condensing boilers. This is a general thermodynamic rule, but hits sources aiming for high efficiency extra hard, since they've been designed around exploiting it.

So you can't just take a decades old system with oil/gas using finned radiators, just replace the boiler, and have it supply enough heat on the coldest day ("design day"). Rather you'd at least need to add some additional emitters, greatly increasing the scope of the project for a professional installer.

What I haven't been able to find an answer to is that everybody says hydronic heat pumps need low delta T of 5-10 degree F (implying high flow rate for given heat transfer). But I would think the real constraint would be just on their leaving water temperature, and a heat pump (load side) that took in 100F and put out 120F (at say 5GPM) would be happier and more efficient than one that took in 110F and put out 120F (at 10GPM). But I've yet to find anything that confirms this.

> What I haven't been able to find an answer to is that everybody says heat pumps need low delta T of 5-10 degree F (implying high flow rate for given heat transfer)

I don’t see why this would matter at all. Maybe the heat exchanger would need to be sized differently for a different flow rate, but in general a lower entering water temperature on the hot side seems preferable.

Your intuition is correct on the last point. Where the "everybody says" side is coming from is the average water temperature in the emitters (and therefore heat flux from emitters to the building) will be higher if the flow (supply) is 120°F and return is 110°F than if the flow is 120°F and return is 100°F.

That's why designers are often specifying lower delta-Ts for low-temperature emitters: to allow the flow temperatures to remain as low as possible [for efficiency] at a given average water temperature [for effective heating].

It's definitely in line with general thermodynamics. One would expect lower entering water temperature to be more efficient (especially with the condenser being counterflow). I'm just not sure if there's something specific about the design of real world heat pumps that makes it so that higher delta T across the pump is problematic. I thought I would have come across some hydronic professional explaining this a bit more in depth while focusing on each variable, but so far I have not.

Maybe it never comes up in practice because heat pump systems inevitably need some kind of buffer tank. If you're designing a system from scratch then you design for lower delta-T in the emitters to keep the max water temperature down. And if you're using existing emitters then you just live with the inefficiency due to higher max water temp, but still keep the flow from the heat pump to buffer tank high regardless (to keep the max water temp from being even higher).

There is a YT video with Matt Risinger discussing/promoting "Thermafloor"(IIRC) which uses 25mil aluminum over the full surface of the flooring panels. They discuss some of the benefits of lower water temperature, but not all of the variables.

I'm not an expert on the topic, but I imagine that a heat pump that can handle -20F is also much more energy efficient when the temp is 0F (in comparison to a heat pump that was rated only for -5F, operating at 0F).

I think a cheap heat pump and a cheap wood stove for emergencies would be ideal. Depending on resistive heat can be dangerous if power goes out.

Eh, for the first time I got an alert message on my cell phone asking people to reduce their electricity usage during a blizzard here in rural Minnesota.

If everyone is suddenly using electric heat when it’s -20 in an area there might be load issues.

The number of electric devices added to the grid is likely a predictable percent increase every day/month/year, and likewise, the acceleration of that change is probably somewhat predictable, too.

This is not to say we shouldn't be concerned, but denying yourself the most energy efficient technologies available (EVs, heat pumps, etc) because you're afraid of power outages 5-10 years from now seems like overkill.

If everyone uses resistive heat during that 1% of the time that it's -20F or below, the electric grid goes down and then no one gets heat. Consider what just happened in the southeast with TVA and rolling blackouts. That was precisely because it was too cold for heat pumps and so everyone's resistive heat engaged at the same time. I don't think your EV range comparison is a particularly good one. You can control your stops on a road trip, you can't control when it's colder than -20F outside.

If they can manage the grid granularly enough and isolate critical environments like hospitals, nursing homes, etc., I really don't see a good reason for us to overbuild to handle the third standard deviation of electricity demand. It makes a lot more sense to set the expectation that on the coldest days of the year, your house may spend a few hours disconnected from the grid in order to shed load. Not only does the avoid overbuilding, but it also contributes to less overall fragility in society, because black swan events that happen once every 50 or 100 years are the sorts of things we can't build for anyway, so it's better if people are prepared to endure the unexpected from time to time anyway.

Having heat pumps that can operate during arctic freezes is of course, and pardon my pun, pretty cool, but I wonder which percentage of the consumer market actually requires this? Especially keeping in mind that your pump not actively heating its internal storage for a few hours every day is not a huge issue: it only becomes problematic when the external heat exchange is unavailable for 6-8 hours or so.

Another comment in the article, regarding electricity grid impacts during peak demand periods, is more interesting to me. Currently, there is no mechanism whatsoever for heat pumps to automatically shift their grid draw (or re-delivery) to certain time slots, and/or to coordinate those slots with other units nearby. Both of these would greatly help to balance the grid, but won't be available until standardization gets off the ground and expensive retrofits are done. That's a shame, really...

It's usually about 35F to 22F daytime when I turn on the heater, so efficiency at low temperatures would be the main thing that matters. It doesn't get to -20F that often most years, but it does hover around -5F for days at a time on occasion.

Current generation heat pumps are probably still worth it but ultra low temp performance would be nice.

There may be no existing, comprehensive central management of electrical demand from heat pumps but that doesn't mean there are no mechanisms whatsoever, because there are systems already for regulating AC demand. In our region of the US, residential customers can opt in to a system where the electric utility can remotely disable their air conditioning compressor during times of peak load, in exchange for discounts. There are agreements on how long that can remain in effect, which would probably have to be stricter for heating systems, and probably have other safeguards added. There are also non-centralized AC mechanisms through Nest thermostats, where they can pre-cool buildings early in the morning to spread load out more evenly during the day.

Maybe just as important, whether or not there is a centralized mechanism for dealing with it, utilities already have exactly the same load problem with AC in the summer and have more or less figured out how to deal with that. It may be different in places where AC is less widespread, but in most of the US I wouldn't expect heat pump load issues to be much of a problem in the winter.

Winter is worse than summer, because Delta-t is much bigger. 130f to 70f is 60 degree difference, while -30f to 70f is 100 degrees difference.

Also deploying solar will solve the problem in summer

There's quite a big market for this, such as a lot of the Nordics, where -15C for a few days is common enough that not having heating then would rapidly become an issue as homes freeze quickly. At those temperatures you have heating on 24/7 to keep the cold at bay. Electricity costs on those cold days are indeed high due to the constant demand of heating.

House heating tends to have controls from thermostats or other devices; I'd tend to look to them for grid-related time-shifting.

I did run across a release on a new generation of a heat-pump hot-water heater, which does seem to have some kind of grid-shifting built in. A. O. Smith's Voltex AL, https://cleantechnica.com/2022/12/21/all-i-want-for-christma...

> House heating tends to have controls from thermostats or other devices; I'd tend to look to them for grid-related time-shifting

Not for heat pumps. In a typical configuration (large heat reservoirs combined with under-floor heating in a well-insulated house), temperature-setpoint changes take 12-24 hours to propagate.

So, governing the (in-room) target temperature settings is unreliable/unpredictable. Whereas the in-pump storage temperature is a whole lot more manageable.

The old advice "turn your thermostat down at night" therefore also doesn't apply to most heat pump installations -- in fact, it might be disadvantageous. "Select the average temperature you need and don't touch it" is much better advice. Need localized heat/cold? Use another solution for that...

> don't touch it" is much better advice

Saw one heat pump controller discussed which had a curve set for demand vs outside temperature, and the "thermostat" was to set a delta relative to this.

The standard large buffer tank is 120 gallons, which stores about 20kBTU at 20F dT. So to last 4 hours you'd need your design heating load to be 5kBTU/hr, which I think is out of the realm of even new construction.

You could raise the max water temp and install a few tanks to get in the ballpark, but that's an assumption that most systems won't have. Also I wouldn't be surprised if "operates down to -20F" includes reduced efficiency/output that already relies on the buffer/storage to compensate for.

I'd like to see an efficiency curve of this heat pump at -20F, 0F, +20F, +40F

I have a heat pump that can be used for both cooling an heat along with a natural gas burner. The installer has set the system to use the heat pump at 40F and above and switch to natural gas at below 40F below based on the efficiency of the heat pump dropping at low temperature.

My heat pump is a SEER 18 unit primarily for cooling in the US south so I'm sure a heat pump designed specifically for northern cold climates will be more efficient than mine at low temps but I'd like to see how much.

Well that installer cutoff was likely waaaaay too high. Your heat pump should be fine to 30F, maybe even 25F before needing gas.

Modern systems are 24 SEER and good to -5, and these research units take that to the next level.

> Well that installer cutoff was likely waaaaay too high. Your heat pump should be fine to 30F, maybe even 25F before needing gas

One possibility might be that the unit lacks de-icing circuitry. If that were cut for cost optimization, the unit would still work fine for cooling and for moderate-temperature heating, but anywhere near 30F it would ice up and stop working.

Unfortunately only the very top-of-the-line (very expensive) variable speed compressor units are rated for anything close to 24 SEER / -5F (unless that's what you meant by modern systems).

> Well that installer cutoff was likely waaaaay too high. Your heat pump should be fine to 30F, maybe even 25F before needing gas.

The parent post was saying it's cheaper to use natural gas at those temps, so that's why the installer did the cutoff there.

You can look at COP numbers for heat pumps here: https://ashp.neep.org/#!/product_list/

A nice explanation of the working of heat pumps from Technology Connections: https://www.youtube.com/watch?v=7J52mDjZzto

Great channel. Doesn't he lament that US installers are still charging "don't bother me" enormous premiums on heat-pump installs?

Yep. And he’s installed, I believe, multiple split units himself.

The needs of the homeowner and the needs of the grid are at odds with one another here.

The electricity grid wants the highest possible efficiency on the coldest days, so that they can serve as many users as possible without building more infrastructure.

The homeowner wants the average efficiency to be as high as possible over the whole season, to reduce heating/cooling costs. They don't care if one or two really cold days have bad efficiency, as long as the system has sufficient output to keep the house comfortable.

Someone needs to use laws or incentives to align those two - because if every home owner used one of todays heat pump systems, then the electricity grid would fail on the coldest days of the year.

If my power goes out (due to ice storms or hurricanes) and I need to rely on a local power supply (battery, solar panels, gas generator), I would like my AC to be as efficient as possible.

I didn't explain clearly... AC systems don't have a single efficiency number - they have different efficiencies depending on the indoor temperature, the outdoor temperature, and the number of kilowatts you want delivered.

You can generally design any AC system to work efficiently at any specific combination of those variables - but if any variable deviates far from the optimum design point, efficiency will drop.

So the real question is, not "I want an efficient system", but "I want an efficient system when it is 20F outdoors, because that's the temperature most of the year".

Are there even heat pumps where efficiency goes down when the delta-T decreases?

Yes. If the cold side gets too hot, the pressure on the compressor input gets too high, which means the compressor is doing far more work with every stroke - and input electrical power goes up massively. The motor overheats and the thermal cutout stops it. When the motor is cutting in and out efficiency goes way down.

This is an issue with fridges. When you buy a new fridge and first turn it on, it's called a 'pulldown'. The compressor gets far hotter than it ever normally gets in normal operation. Most fridges are only rated for 3 pulldowns in their lifespan - and if you do more than that and the fridge fails, they'll claim it isn't in warranty anymore. And in modern fridges, the software keeps track of how often so they can deny the warranty claim too...

Seems like that fancy software should be able to manage the pull down and prevent the compressor getting so hot.

I guess people would get mad if their refrigerator took longer to cool off.

Most heat pumps where I live are inverter based and have a very gradual ramp-up, they also control the compressor according to the demand. Some heat pumps even have several inverters to control the compressor and the fans.

One of my fridges has inverter based compressor too.

> Someone needs to use laws or incentives to align those two - because if every home owner used one of todays heat pump systems, then the electricity grid would fail on the coldest days of the year.

Or a technology where grid maintainers can tune down your heat pump, EV charger, hot water cylinder, etc all the way to minimums.

I believe this is already required in Australia for some tech.

I think the ultimate combo will be heat pumps + wood pellet stove as supplemental heat.

Then you can cutoff the gas grid connection and its associated standby/account/blah blah charges. It’s a big sunk cost in a lot of places that messes with the economics of switching to heat pump as primary heat.

Pellet stoves are semi-automated. Around 90% efficient. If you already have central heat pumps, you can install one and let your HVAC circulate the heat around. Can stockpile as much fuel as you want. Cheaper than oil or propane and not much more expensive than firewood once accounting for improved burn efficiency. Just need to empty the ash gray once a week or so, and dump a nice smelling bag in for every ~24h of operation.

Relatively straightforward install: just need a wall to punch through and a standard power outlet. Minimal clearance requirements. Fun to watch the fire tornado.

Big downside is they need some electricity (mainly for for the powered vent). Hit or miss when it comes to insurance companies that think explosive gas systems or high current electric devices are safer.

Pellet stoves require electricity to run the auger. Also- nobody gives away free pellets but it's pretty easy to come by free wood people are giving away.

Gas heaters also have around 90% efficiency, I think. It seems like gas is strictly better?

Depends on your constraints. The CO2 round trip is a lot longer for gas.

We just installed a Solstice Inverter Extreme[0] before the the storm hit. The low in our area was -24F. The heat pump is advertised to work at -22F, but was still heating the house at the low. Efficiency and capacity are reduced with as the temperature drops. We have backup resistive heat in the buffer tank of our hydronic setup. It is early but we are happy with the performance during this first cold snap.

[0] https://www.spacepak.com/solstice-inverter-extreme

What's the rough cost?

(obviously there will be lots of variation based on the complexity of the install, but the ballpark is still interesting)

About $10k for the heat pump. NPV is positive for 10 years compared to a propane boiler. This doesn't include installation, but it is doable for the advanced DIY homeowner.

The entire system cost $25k + installation. Not cheap, but in our climate we heat 7+ months and it is worth it. The extended federal tax credits help, but our state does not do anything in terms of rebates.

-29C for non-Americans. Its a prototype.

How is this news? The last air-air heat pump I looked at was guaranteed working down to -35°C/-31°F.(Mitsubishi Electric UWANO 8700)

Is this just the US being far behind when it comes to heat pumps or something?

At those temperatures that pump works at COP of 1. Basically it just switches on the resistive heating element it has for this case.

What is being talked about here is having one that actually works as a heat pump at those temperatures instead of a resistive element.

Though if you live in a place where such cold days only happen a few days per year tops this is just fine.

I recently bought some land in Minnesota on a lake. I really want to do a 'no compromise' off-grid setup. Water and septic wont be a problem but I am worried about the heat.

I plan to install a huge solar array with a battery house. I'd like to run everything off electric, including the heat.

I am in early days of thinking about this and I have time to plan. Anyone have insights on electric heat in ultra cold environments? I assume I can simply scale up a solar array and battery capacity to meet needs (dead of winter, with spans of cloudy days). The only fossil fuels I want on property are for equipment and if I must have it, a backup NG generator.

I don't know anyone who runs heat pumps in MN, I'm sure there are some but most folks are burning NG or wood pellets. Electric heat seems relegated to secondary needs, like base boards or heated floors.

*edit cloudy

What about a wood burning stove? That would be more reliable if you are truly off the grid.

Keep in mind that you don't need a traditional electrical battery for heat - storing heat directly is extremely cheap, you just need insulation and thermal mass (i.e. sand/rocks, any old crap will do). There are a fair few systems that use easy summer-solar to generate heat, store it for months at a time and then release the heat in winter.

It scales up really well thanks to the cube-square law, which is a euphemistic way of saying it's hard to make a viable system that's really small. But if you're off-grid because you're in the middle of nowhere, then you can spare a few square meters anyway.

If you do some back-of-the-envelope math on this, you'll notice that this will be cost prohibitive to implement using batteries.

I have a Mitsubishi Hyper Heat, and it operates to -13f, which would be unheard of in Western Oregon (where I live). So long as your house is well insulated, you'd likely be fine with a heat pump. Pay attention to R-Value for insulation, but if you choose the right materials, that goes a long way for heat storage.

I know there exceptions in Minnesota where it can get considerably colder than -13f, but I don't know how long those super low temps are sustained. You would definitely need some sort of emergency heating system. An outdoor propane tank (the larger variety) and a propane heater would likely do the trick as an emergency backup. Not great for air quality in your house, but plenty of people use that as their normal heating.

Look up ground-sourced heat pumps. You store your heat in soil during the summer, and draw it out in the winter.

I want this someday too (also live in MN). Wood pellets or just a wood burner are your best bets for secondary heat. I grew up in WI and we only had electric base boards and a wood burner (then pellet stove). Get the house up to 85 before bed and by morning it's in the high 50's and start the fire again. You need to be ok with fluctuating temps to really make it work.

You mention cloudy days but the one positive about our brutally cold winters is that when it's below zero it tends to be sunny out. If you set up your solar to extract as much sun as possible during these times, as well as build your house with large south facing windows with stone or concrete flooring you will not need to heat the house much during even the coldest days (solar radiation will heat the house and the concrete flooring will release what heat it gained in the evening).

The issue with heat pumps is that most of the ones currently being installed in South East of the US don't work well below +20F let alone 0F or -20F. Most of them are being installed with an electrical resistive backup heat, which is incredibly inefficient.

The problem is when a cold spell like Christmas 2022, with temperatures down towards 0F. All the heat pump users switch to resistive backup heat and it overloads the electric grid and we get rolling blackouts.

In my opinion, heat pumps are amazingly efficient at moderate cold temps, but they really need propane or wood heat backup for the really cold temperatures instead of resistive heaters.

What is inefficient about resistive heat? Isn't electricity->heat basically 100% efficient?

Maybe you are saying that heat->electricity->heat is inefficient, since most electricity is produced from heat inefficiently.

I always get tripped up by this, since I live in an area where almost all electricity is hydro. In that case resistive heating seems fine.

That 100% efficient figure is correct - you get one Watt of heat for one Watt of electricity. It's just that heat pumps can deliver 3 or 4 Watts of heat for each Watt of electricity (usually quoted at around 300% or 400% efficiency!). Compared to that, resistance heaters aren't as efficient.

The explanation that made the most intuitive sense to me is that it takes less energy to move heat from one place to another (air at 273 Kelvin to air at 300 Kelvin, like a heat pump does) than it does to create heat from nothing (like a resistor does). That's why the heat pumps can get deliver more heat to you from the same amount of electricity.

That makes sense, but GP was calling resistive heating inefficient compared with propane or wood heat, which doesn't make sense to me.

The best natural gas power plants are 64% efficient. A modern furnace is around 95%. Sure electric restive heat is 100% efficient in your house, but the whole system us much worse.

Of course electric can come from many sources, if your is renewable at the time resistive is good. However you might also be using some old 1920s coal generator that is 10% efficient (these still exist, but are only used in the worst emergencies)

Natural gas (not sure about wood; in most areas it’s abundant) is about 1/3 of the cost of electricity in America, so it’s economically less efficient.

Resistive heat is 100% efficient, but heat pumps can operate at greater then 100% efficiency. That's because a heat pump doesn't actually generate heat, but just moves it around. Even when it's cold outside, there's still a lot of heat energy in the air, which can be moved inside to warm your home.

Due to the increased efficiency, heat pumps are better then electric resistive heat (when temperatures outside are within the heat pump's operating range, that is). This is regardless of the method of power generation.

Since heat pumps are moving heat around rather than actually producing it, they can be effectively better than 100% efficient so it's not so much that resistive heat is inefficient, but that it's less efficient than a heat pump.

Electricity offers resistive heating, and heat pumps. Heat pumps are much more efficient than resistive heating.

Otherwise, you have chemical fuel which burns, and a bit of electricity to pump it around (either by forced air or water pumps).

In terms of electrical input, resistive heating is the worst of the lot, even if it can be sourced in a carbon neutral way (unlike nat gas or fuel oil).

In the US, anyway, when people say electric resistance heat is inefficient they are comparing it to natural gas heat. It's the same story in water heaters; if you install a HPWH you are betting on not needing to resort to resistance heat because if you do that too much, you probably should've installed a cheaper gas unit instead. (I'm a happy HPWH owner weighing the timing of adding a heat pump for central heat.)

We’ve had heat pumps that work down to -5F for years. If you’re installing one that can only go to 20F in 2022(3?) something is very wrong.

Backup heat methods increase the complexity and cost.

I am just saying what the typical install is in the South East US. The heat pumps installed may produce some heat at 5F but they can't keep the temperature to the set value, so there are resistive elements (Aux heat) to make up the shortfall.

It's pretty common for people with heat pumps to have Aux Heat kick in during cold spells, which cause power grid overload issues.

I realize you can insulate a house well enough and have a good enough heat pump to avoid backup heat, but 5F or 0F days are rare enough that the codes do not enforce this.

When I researched this year on replacing my A/C / Propane Furnace system with a heat pump, I found that companies didn't seem to want to advertise what temperatures their heat pumps can operate effectively at. If I look at some marketing materials from Google it seems companies like Carrier and Trane are only willing to talk about their heat pumps working in low temperatures if it's regarding their top-of-the-line (very expensive) variable speed compressor units. No one talks about what temps the mid range units can handle, and I'm guessing it's because they don't work well below 20F.

My heat pump is advertised to operate as low as -13F. One thing to note is the efficiency of heat pumps is not optimal when they are operating close to their extremes.

Where I live, most people had electric baseboard heating before they installed their heat pump, so 99% of the time they are using less electricity and on those really cold days, they are using as much as they did before getting a heat pump, so we know our grid can handle that. The grid in the US is weak because Americans have traditionally depended more on fossil fuels or wood for heating.

If you want an (entertaining) deep dive into heat pumps, I can recommend Technology Connection's https://www.youtube.com/watch?v=43XKfuptnik and its followup, https://www.youtube.com/watch?v=MFEHFsO-XSI

Do we really need heat pumps that work that far into the negative? You can always turn on an internal resistive heater to compensate after all.

Not quite something you can do on the other end though, when trying to cool with exceptionally high ambient temperature. It's such bullshit that the physics of this universe does not allow for resistive cooling.

Resistance heat is extremely expensive to operate. Burning something is cheaper.

Well it is technically 100% efficient which is more than one can say for most things, but it's true that heat pumps can be far better than that.

The problem is that if everyone comes off of fossil fuel heat and switches to heat pumps, you need a grid that is capable of delivering enough eletricity to power simultaneous resistive heating to every house... because every house is going to need resistive heating at the most dangerous time to not have any heat at all.

These cold snaps tend to also coincide with extreme winds and other weather events that can take out power lines, so a stressed grid just compounds the issue.

On top of that, it's also the time when you need the most energy for heating because it's the coldest.

What no one tells you about heat pumps that work down to 10 degrees F, zero degrees, minus 10 degrees, is that you probably cannot get anywhere near enough BTUs out of your heat pump, at such an outside temperature, to keep your house above freezing. But it will draw a great deal of power getting what BTUs it can.

As outside temperature goes down, the amount of heat input, in BTUs per hour (or kilowatts, really) needed to maintain a comfortable temperature rises linearly with the difference. When the difference is greatest, the heat pump can deliver least.

So in practice, well above such temperature, your controller has turned it off, and is burning propane instead.

Of course, the better-insulated your house is, the fewer BTU/hr it takes to keep it warm. Spending a lot on a beefy heat pump without making sure your insulation is in good shape would be a mistake.

When the CoP goes to 1, the heat pump should shut itself off so it is not wasting power. That's what mine does. The electric baseboards I used before putting in the heat pump take over at that point.

It was down to -8F (not including wind chill) a couple days ago here. I have a small, old house but I'm considering replacing the old system I have now with a heat pump.

My concern is two-fold:

- my house is not well air sealed

- my house is not well insulated

So I worry that I'll need an extremely oversized heat pump in order to have enough capacacity for the coldest days.

I suppose having a backup heating source would prevent needing such a large unit. Plus it would provide some amount of redundancy of the heat pump were to fail in the winter.

The question then is, should I use electric backup heat, or stick with my existing gas connection? Electric is simpler and there's no exhaust fumes or CO risk to worry about, but gas is still cheaper here I think.

The best ROI is to insulate and air seal.

And if you can only do one, air sealing is almost always preferable.

We live in the Colorado Front Range in a 50 year old house that isn't particularly well insulated. We have a Mitsubishi high-efficiency air source heat pump distributing air through our existing duct work. We have electric backup inside the air handler. Last week it got down to -15F overnight. The heat pump ran throughout and there was only really activity from the electric backup during the occasional defrost cycle. We keep our house pretty warm too as we have a baby.

In winter the heat pump seems to have no issues meeting our heating needs albeit at quite a bit higher overall utility cost compared to our old gas furnace. In summer the heat pump is much more efficient than our old AC unit and our solar panels cover >100% of our usage from ~April-Oct.

Prior to installing the heat pump we did as much insulating as we could (mostly attic) and I think that was important for enabling the heat pump to keep up in the cold. The nature of the heating is lower level and continuous compared to the occasional blasting from a gas furnace, so you do want to prevent rapid leakage of heat from the house.

My 2 cents: Find your local "design heating temperature" and then determine your tolerance for the house falling a few degrees below the setpoint during the 1% of hours that the outside temperature will be below that. Most people would be fine if their house fell to 67°F or 68°F for a few hours, especially since some of those hours are bound to be at night (where other people are setting their stats back to those levels anyway and you might well be asleep and never notice).

If you have no other gas appliances and generally reliable electric supply, I'd be inclined towards going with electric backup. That avoids the $10-15/month gas meter charge, which pays for a fair amount of the more costly electric heat. If you have other gas appliances and cheap [at least for now] natural gas supply, going with a natural gas backup allows you to use the cheaper source of heat for both backup and/or in case electricity rates go up substantially versus gas.

CO risk is minimal and you should have a line-powered CO meter in the mechanical room and some CO meter on each level of the house anyway.

> have a line-powered CO meter

Thank you for stating this. If your furnace requires electricity to run, you don’t need a battery-powered or battery backup CO meter. Unless you have a wood stove or light fires indoors when it gets chilly. Otherwise you’re just wasting batteries.

What are your no-use natural gas costs like? Might make a wood stove, oil, wood pellets or maybe even propane cheaper. And then you’re not depending on two grids for standby/supplemental heat.

It may be worth insulating it first. It cost me just about $1500 to go from effectively no attic insulation to R60 for a thousand sq ft, and besides making a massive difference in comfort it reduced energy usage by about 50%.

I’m in a very similar predicament but the temperature where I’m at only go down to the 30s-40s mostly during the winter. I get some rebates where I’m at to replace a furnace so that brings replacement costs to be about equal between furnace and heat pump. I don’t have solar, kw per hour costs are around 13 cents. My AC is non op needing to be replaced (it regularly gets into the 100s for months on end) and my furnace is functioning at 25 years old and no problems, although when it turns on it sounds like a small bomb exploding in my ducts. I have 50% of my house with extremely drafty single pane windows, the other 50% have been replaced. House was made in the 1930s out of brick and appears to be well insulated otherwise though.

I am afraid that my overall utility expenses will go up drastically with a heat pump though. Can a heat pump even keep up with a house that loses heat like crazy out of the windows? I seem to get contradicting information from the salespeople, the internet, etc. if anyone has any anecdotal info it would be greatly appreciated

If your main heat problem is the single-pane windows, perhaps you could use the bubble-wrap trick?

https://www.builditsolar.com/Projects/Conservation/bubblewra...

Others have said it, but it bears repeating. Do the air sealing and then insulation ASAP. It increases comfort and the ROI is way higher than moving to a heat pump.

I started with my old Vermont home with minimal insulation, single pane metal frame windows that would ice up, and drafts everywhere. After our first winter, we had the local efficiency program recommend a contractor who came out and measured air exchange and identified the order of work. New windows, sealing attic, re-insulating attic. We got a considerable tax credit for the work, and the comfort improvement was astounding.

Just this last summer we made the jump to mini-split heat pumps. They are working well, but I doubt that would be the case without the air sealing. We keep them set at their lowest, 61F, and supplement with a wood stove in the living room, maybe burning 2 cords a year.

As an aside, this is our first winter, and our bill for December was 300 dollars, at $0.18/kWh. That's about 25% lower than our estimated oil charges last year for the same period. However, this year oil is ~$4.6, almost 50% higher, so that hedge against fossil fuel price is really paying off. Add in the AC comfort that is largely offset by our solar array, and they are a worthy investment (oh, and 0% financing).

None of this would be working without that critical investment in air sealing and insulation.

Thanks for your input. I am actually in the process of air sealing and reinsulating my attic.

I will have to reevaluate after that is done.

The furnace at my mother's house in Houston went out and I tried to get her HVAC guy to replace it with a heat pump. (The AC is 30 years old and also needs to be replaced) He clearly had no idea what a heat pump is and came up with the following excuses to not install a heatpump:

1. Heat pumps are only for extremely cold weather

2. The heat from heatpumps is too hot, they only install them at elder care facilities where they need extra heat

3. It's far more complicated than a furnace / AC setup, and will require a lot more maintenance.

Not sure what this all means for the industry if common HVAC guys don't even know about heat pumps yet.

-22F / -30C capable heat pumps already exist, e.g.:

* https://www.spacepak.com/solstice-inverter-extreme

Are these using resistive heating when it gets too cold for the heat pump to run, or are these heat pumps actually efficient down to -22F/-30C?

AFAICT, they're using the heat pumps all the way down.

Anyone know how these pumps manage to squeeze enough heat out of ambient air to make it worth the while? -20 is down there! I understand how heat pumps work, but what's the differentiator?

It helps if you look at it as temperatures in Kelvin, to bring something from 250K to 300K requires a 20% increase, which can be done by an ideal engine at an efficiency of 300/(300-250) = 6. Sure you can't ever reach that ideal but the upper limit is high enough to make it worthwhile.

I'm not fully sure at what point it stops being worth it, but anything above 150K can get an efficiency of more than 200%, in theory at least, and 150K is way above any reasonable ambient temperature. As far as I know the ideal is not even that hard to approach it's just how to do it in small scale and quickly that is the problem.

What does an efficiency above 100% mean?

It means you transfer more heat than work put into the system.

Instead of burning fuel to generate e.g. 1500W of heat, a heat pump uses 500W of electricity to transfer heat inside generating the same 1500W heating effect.

It's a multiplier for the electrical input - if your heat pump is 400% efficient, then it means the electricity is being 100% turned into heat, plus 3x as much heat is being sucked in from outside.

> -20 is down there!

That's 244 Kelvin. It may not seem like a lot of heat to the human body, or judging by the state of everyday elements like water. But in terms of energy, there's actually still quite a bit there.

My Senville mini split claims to work at temps down to -22F. During this recent winter storm, temps got down to about -10 where I am, and it managed to keep my garage around 50F. Nowhere near comfortable, but pretty damn impressive to be able to keep my relatively poorly insulated garage 60 degrees above the weather.

I was quite happy that this storm occurred while I was in Christmas PTO, since my garage is my office. I could have made it workable with a supplemental space heater but it was nice not to have to!

Meanwhile, most people in the US don't even know what a "heat pump" is, and think that "air conditioners" are inherently wasteful and bad for the environment.

It is kinda a concern with heat pump rollouts in Europe: now that people that didn’t have air conditioning before will have it and will adopt it.

Having said that, too much of my EU family still thinks air conditioning makes you sick… and a lot of the latest germ theory (aerosol transmission or respiratory viruses) is confirming they might be right. Ugh. At least the rental cars usually have AC nowadays.

That’s absurd, AC doesn’t “make you sick”. What basis could anyone have for such a claim?

AC evaporators and condensate pans can breed bacteria and mold. At best this can be smelly, at worst it can make you seriously sick. See the history of Legionnaires disease.

If you have individual room split units you just clean them yearly or so. Just open the interior unit and spray it with the cleaning solution. Smells nice too :)

Whole house a/c systems like the US has are an entirely different matter... don't know how you clean all those ducts.

I’ve never seen anyone clean their A/C system.

Put "air conditioning cleaning spray" in Amazon. You open the plastic cover of the indoor part and spray the internal radiator.

There's stuff for cars too.

I’m confident the market for this is 99% HVAC contractors and repairman rather than consumers. It’s good to know they should be cleaned.

You clean the outside condenser coil with a similar process (except you can/should flush it out with a garden hose as some of the coil cleaners are somewhat aggressive and should be rinsed thoroughly off the aluminum).

Change any filters. Leave the ducts alone.

You need to maintain these systems. Otherwise refrigerators, sinks and toilets also make you sick.

There are hazards to recirculating air (instead of opening a window), e.g. aerosol viruses, off-gasing and some of these people smoke like chimneys.

I am lost.

Are heat pumps the same thing as split air conditioners that can heat air?

Air conditioners are heat pumps. They move thermal energy from the inside to the outside to cool a space.

In this context, a heat pump refers to the same type of system, but in reverse. Instead of moving thermal energy from inside to outside, you're moving that energy from outside to inside. This can work even when it's cold out, because cold outside air still has a lot of thermal energy that can be moved.

The main benefits of heat pumps for heating are twofold:

1) It's significantly more efficient then electric resistive heat, because heat isn't being generated, simply moved around. 2) Heat pump systems can be configured to work to both heat and cool a space. There are very few changes needed to make this happen, meaning that if you need AC, you might as well get a heat pump to do both jobs.

Most air conditioners I encountered (as a person from a cold country) had this option. The Samsung one I have works from -15C (which is really not that cold, but still). Is this the same technology just for even colder tempreratures?

A mini split is usually a heat pump system although some of them use resistive heating. They have been getting much better in recent years.

Heat pumps are also known as "reverse cycle air-conditioners".

outside source(d) heater

Why not just use a natural gas or electric heating element to heat up the air coming into the heat exchanger when it's below the operating window? In most of the US below zero is a <10 day per year issue. Sure it's terribly inefficient on those days, but I'd bet it's cheaper than maintaining an entire secondary heating system.

The US grid isn't capable of handling everyone running on resistive heating because traditionally many Americans have used fossil fuels or wood for heating. The US grid needs to be improved for this reason and also for EVs.

Cool. But it hit -39c in my area a couple weeks ago. They still have some way to go.

If you run copper tubing underneath the frost line, a heat pump can still run at that temperature. Problem is, that's a whole lot of expensive construction for relatively minor energy savings. I hear they're doing it in Alaska, where it's used much more frequently once it's installed.

I’m notably south from Alaska and we had a frost depth of 3m a few winters ago. (Caused various problems with water pipes; city instructed many neighbourhoods to leave taps on 24/7.) Expensive to bury stuff below that depth.

Fort Mack? Ive spend some time in the prairies too.

Not even, Calgary. News article from 2019: https://www.cbc.ca/news/canada/calgary/frozen-pipes-homeowne...

As alluded to in the article, some people were still having to run their taps for weeks after the deep freeze was over due to the frost depth.

My AC runs just fine at -15C (+5F), the problem is when temperature swings between zero and sub-zero, it causes formation of the ice in the outer unit which can break the fan.

Some heat pumps designed for low temperatures have heaters to avoid the freezing of condensation.

It has this, but I'm talking about the area where the fans are rotating - it's not heated. Rarely, under conditions I described, ice forms there.

How expensive would it be to run source water to an entire neighborhood for heating and cooling? Have one heating/cooling plant and per building heat pumps.

This is called district heating and is not uncommon, some cities in the US have it (it’s used in Manhattan for example) but it’s more popular in Europe and Asia, especially in Scandinavia and former socialist countries.

https://en.wikipedia.org/wiki/District_heating

Lots of colleges have steam tunnels too. With individual heat pumps you probably lose some efficiency but the source doesn't have to get as hot or as cold. Doing a residential retrofit sounds like herding cats but in somewhere like Phoenix or Tucson the efficiency gains might be worth it.

This is frequently done when you have a (very hot) boiler in a power plant or chemical process nearby. A natural gas turbine can't generate electricity very efficiently on even 212f steam, so instead of building some gigantic turbine to get that last bit out, you route the waste heat into a nearby neighborhood.

You are likely interested in Drake Landing in Canada.

https://www.dlsc.ca/

https://en.wikipedia.org/wiki/Drake_Landing_Solar_Community

https://www.hpbmagazine.org/content/uploads/2020/04/15Su-Dra...

It uses solar in the summer to charge a thermal bore which is discharged in the winter. That provides nearly all of the winter heat from the seasonal thermal energy storage.

> In 2012 the installation achieved a world record solar fraction of 97%; that is, providing that amount of the community's heating requirements with solar energy over a one-year time span.

The third link (the pdf) is the case study from High Performing Buildings on the design of the community.

> At first glance, the tidy two-story houses lining two streets in a Canadian suburb look much like the thousands of other homes that surround them; but a district heating system that stores summer’s abundant solar energy to heat the homes during winter makes this community a global pioneer in heat storage technologies for residential space heating. Drake Landing Solar Community proves that such a system can deliver a large fraction (over 90%) of space heating with solar energy in a cold climate. The builder designed the 52 detached single-family houses to appeal to mainstream home buyers who want energy efficiency without sacrificing aesthetic appeal.

> Drake Landing provides a real-world example of how conventional heat- ing fuel consumption for space heating can be nearly eliminated, reducing greenhouse gas emissions by more than 5.5 tons per house annually. This is made possible in the harsh 9,027 heat- ing degree day climate by using solar heat collectors with seasonal heat storage, energy-efficient house design and construction, and a low temperature district heating network to distribute the heat to the homes.

Note that this isn't more cost efficient - it needs to scale up quite a bit more.

> The project development team understood from the outset that the Drake Landing system was too small to be economically competitive with the extremely low cost of natural gas. However, subsequent feasibility studies show that larger systems of similar design can deliver solar energy at about half of the cost compared to Drake Landing, and additional work is underway to improve cost performance further.

Can they work in pair with co2 capture? Assuming neg 20 deg is co2 liquid point ?

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