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Comment by Faaak | Hacker News Reader

Heat pumps _are_ amazing! The power of physics at play: spend 1 unit of energy to gain 4.

Some naysayers will say that it doesn't work the 3 days of the year where it's -15°C outside, without talking about the other 100+ days where it's not that cold and where the heat pump is amazing.

Disclaimer: I self installed one for my house (13kWth) and I'm very glad I did

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So, i have a house in Haliburton Ontario. Historically we drop a cool $3k a year on propane.

Last year we installed a Mitsubishi hyper heat ductless system.

We used zero gas this year. Read it again, zero.

It’s lakefront, very remote, and the largest electricity bill i got was around $450 for a month and then they dropped back to something more like $250. The savings are huge, I’m no longer stressed about running out of gas…and the heat pump performed well beyond its advertised specifications. We had a few -25C days and it was humming hard, but the house stayed a comfortable 20C inside. The house is around 3000sqft and we didn’t even get the largest unit, i can’t stress enough that they actually operate better than advertised.

We would run a fire from time to time but we did that with propane too, it’s mostly ornamental.

https://photos.app.goo.gl/FCwLJQAtoG67g9y86

https://photos.app.goo.gl/TwiMaSAj9hGxYqby6

https://photos.app.goo.gl/gYVYvVEB3whLCv1dA

https://photos.app.goo.gl/49RcBdBuUZ2jVN3J7

That's a lot of lines going to the outdoor unit.

Is there two per indoor head?

It looks like that unit may have 3 indoor heads. Two lines per head. Power to each head. Power to the outdoor unit.

Looks like it. I'd suggest anyone looking at a system like that install at least 2 separate outdoor units. That way if one breaks the other can still work - and most of the year it will be powerful enough to handle the whole house (some rooms will be a little uncomfortable but bearable)

This is correct, 3 heads. One on each level of the house.

It's worth running the numbers based on your particular utility costs, though. In our case, with somewhat expensive gas and very expensive electricity, the heat pump would have cost quite a bit more to run: https://www.jefftk.com/p/running-the-numbers-on-a-heat-pump

With an air-to-water system like a Vaillant aroTHERM plus you could possibly achieve your break-even COP of 4.5 (the 12kW unit costs about US$7,200 in Europe, although from what I hear about the US heat pump market you'd probably pay like $20K for the unit alone for some reason).

(Also, how do you guys function with those strange units? Therms, BTU/hr, etc. - all so confusing. Surely electrification and the shift to heat pumps could be a convenient excuse to start using watts (kW in this magnitude) for heat and joules (usually MJ) for gas!)

I do think an air-to-water could make sense for us, though it wouldn't be able to handle the coldest days because our radiators aren't sized to keep the house warm at the lower water temperatures it puts out. The main problem is figuring out who can install one, since it's a pretty unusual product here.

(You get used to whatever units you're using, and the US units make some calculations easier and others harder. If I could switch it all over to the SI system without massive transition costs I would, though!)

You might be surprised at how well cast iron rads can provide comfort at low outside air temps and low flow temps. (I'm in neighboring Cambridge in an old, poorly/non-insulated house.)

https://news.ycombinator.com/item?id=39144329 has a bit of details on the experiment I ran back in 2022 to prove 135°F flow would work for us. (If you have a condensing boiler, you can run this experiment safely; if you have a non-condensing boiler, you can run it, but not for very long as you'll be damaging the flue and boiler with condensation at these lower temps.)

My outdoor reset curve (sadly, on a gas combi boiler because of the "pretty unusual product" factors) is now set to 105°F at 55°F OAT and 154°F at 0°F OAT (which is lower than the design temperature here, but it gave me more resolution to tweak the line to fit the loss just right; it's spot-on on the lower end, with the system running 22-24 hours per day when it's cold out and stays that way up until around freezing, where the utilization falls off).

Matching the gain to the loss quite closely has resulted in a house that's the most comfortable since we moved in in 2007 and gas bills with the combi went down about 46% (versus a 1990s oil-to-gas conversion of a 1950s boiler, so not a realistic comparison for anything that wasn't built by General Motors [not a typo]).

We have one loop with cast iron radiators, but the other two loops are modern baseboard. When we installed a condensing boiler in 2015 I needed to adjust the outdoor reset curve up so the loop that serves the first floor wouldn't leave it under temp on cold days.

Even our cast iron radiators are smaller than you might expect for the age of the house, because they were designed for water above its normal boiling point (using mercury pressure: https://www.jefftk.com/p/mercury-spill).

I also have one loop of modern baseboard. Fortunately, it's in the attic conversion where they did insulate the rafters while doing the conversion, so it works even at that lower temp. I did do something slightly unconventional in plumbing that zone in a primary/secondary and it gets the water from the boiler "first" and returns it to the primary loop ahead of the main zone which is all cast iron rads. That means the baseboard gets the hottest water possible and the full potential flow from the boiler if it "needs" it. In practice, that zone tends to only run 4-5 hours per day while the main zone is running 22-24 hours, so either what I did works really well and/or I didn't need to do it in the first place.

But, you've already discovered your reset curve with modern equipment, so you know the right answer for your place.

Thanks for the story on mercury pressurization! Fascinating. I learned a lot about our old house (originally gravity circulated as well, but near as I can tell, pressurized only to the typical 12-15 psi and with an in-ceiling green steel expansion tank: https://structuretech.com/wp-content/uploads/2020/03/Old-sch... )

(And of course, sorry to hear about your contamination inconvenience and expense!)

Jefftk I'm in the same boat -- what I'm going to have to do if I want to do the air to water heating is bring in a booster/combo unit that will boost the water temp to make the house the right temp. So I'm still tied to gas but much lower amounts - heat pump does the majority of the work and I still have protection on very low temp days.

Hmm. I wonder if adding an air-to-water pump to the existing loop, before the boiler, can be made to work? Perhaps with a microcontroller intermediating the thermostats on both to turn each on at the ideal time?

That might work - an interesting take. I haven't really thought about the pre loop to the older boiler.

I think the challenge is determining how much energy you save. I guess we could run the calculations and back out nat gas savings.

Managing the microcontrollers might be a bit of a pain though.

There are products that exist for that, though as always: maybe not in your market.

Over here they're called hybrid heat pumps and are quite popular. (At least with those that offer to sell them.) They're used in improving existing gas powered solutions. The heat pump takes most (or usually all) of the heating, while the existing gas heater provides hot water and can add peak heat if needed.

Can you point to some of those. I am curious at other options for my own heating operations but the landscape is difficult to navigate as a consumer and then you need to find people capable of helping out.

Sure.

Popular brands here are:

- Remeha, with the types Elga Ace, Mercuria, and Mercuria Ace - Nefit/Bosch with Nefit EnviLine hybrid and Bosch Compress 7400i AW - Vaillant aroTHERM plus/pro/split

There's also the Quatt Hybrid, which I haven't heard of before, but it looks nice. And according to their website it has a large market share.

We got a heat pump installed alongside the gas system, since it's still working.

I would love it if there were a service or some code to look at 1) gas prices 2) electricity prices 3) how efficient the two systems are and switch back and forth depending. Like... if it's -10C out, run the gas. As it gets closer to 0C, switch over at some point.

If gas is flat out always cheaper, you could still put a cutoff point where you're willing to spend a bit more because it's better for the environment.

Depends where you are. Gas is certainly not cheaper for us. I did the math based on cost per KWH of fuel consumed v's the efficiency of the unit producing it. Natural gas would be slightly more expensive (marginal though...2% or so) but propane is around 3x more expensive, and with many people on oil the math is even worse.

In many regions electricity isn't as cheap as ours though, so that changes the game.

Right - it feels like a pretty dynamic calculation and it'd be cool if there were a service to do it for you.

The number of times I've almost started making that service...maybe when I get a free moment, I really want it to exist too.

100% agree. Our electricity costs are fairly low, and its a very low carbon source (mostly hydro electric + nuclear), so for us it makes a huge amount of sense. If you're in Alberta, where most of the electricity comes from coal...theres no logic in switching to electricity.

I'd argue that's politically motivated and very deliberate however...

Almost all coal plants in Alberta have been shut down and it's a small minority of net generation now. Natural gas has taken up most of the slack, but there's actually quite a lot of solar and wind generation in Alberta considering the politics (though that's likely to slow down now).

http://ets.aeso.ca/ets_web/ip/Market/Reports/CSDReportServle...

Ah i didn't realize the coal plants were retired! That's good to see. I was looking at the carbon impact per kwh in Alberta earlier in the year for a project I was working on, and was surprised how high it was. I guess other areas of the country just have a lot of other sources diluting the impact.

Yeah Alberta has very little hydro, while Ontario/Quebec/BC have lots, so our electricity generates a lot more carbon.

>Historically we drop a cool $3k a year on propane.

Similar to our situation, also in Canada; we cut our fuel bill by 3/4 after getting a heat pump. I still run the furnace on the coldest days, because it's hard to beat. But 9 days out of 10 the heat pump is all we need. The fact that it doubles as an AC unit (and is even more efficient) is gravy.

I also bought a heat pump hot water tank, and so far so good.

Can I ask what model did you get and what part of Canada are you in?

More to the point, I'm looking for a recommendation for a smaller unit to heat 750 sq ft. in Quebec.

Yeah! The AC bonus is amazing. The summer gets so sticky, not anymore. We got it in around July I think, made for an amazingly comfortable August.

I live a few 100 km east of there and just had a heat pump installed last fall. I still have to spend $800 a year on wood but now my home is 20 C all day every day and I no longer have to break the ice on the dog's water bowl in the mornings. I still fire up the stoves to keep hydro costs down and so far the hydro bills haven't been particularly different from previous years when I had to use supplementary electric resistive heat. Then again, it's been a particularly mild winter with zero days below -30.

I had a Moovair with three heads installed.

This winter was really mild. I was curious if we'd spend more or less, and honestly i wasn't super concerned. My napkin math suggested it would be comparable, but come with the benefit of reducing the risk we'd run out of gas. The propane truck cant make it up our road from late November early April, and last year we got down to around 20% remaining (which is around the point where the tank stops working due to there not being enough pressure).

Huge bonus that its been cheaper, and substantially...but I just love relaxing about the reduced risk of the house running out of fuel.

Did you consider putting up solar to counteract the additional electrical load? Sure it would take 7-10 years to pay off the solar asset but after that its free electricity and heating. Something I'm playing with but its a bit more difficult in the city.

This needs far more attention. Combi boiler installers tend to massively oversize the CV. But with heat-pumps you need to be far more accurate. And it does not make sense to design for a temperature that occurs only a couple of days in the year. You can just have the backup heater kick in which is far more efficient for a couple of days than having a heavier heatpump for the rest of the year that can not modulate back as much as a smaller heatpump.

I recommend this technology connections video which dives into this in wonderful detail https://youtu.be/DTsQjiPlksA?si=gx18FGOcjsv5PpNg

> than having a heavier heatpump for the rest of the year that can not modulate back as much as a smaller heatpump.

Aren't basically all modern heatpump variable speed, and thus can modulate back?

That said, I totally agree with your overall point about right-sizing your heat pump, but it is more about saving money on the unit rather than worrying about cycle times.

Modern heatpumps can modulate, though it isn't 100%. I recommend people consider two smaller heat pumps in many cases - it costs more upfront but can modulate down more and if one system breaks the other can handle everything most days of the year (some rooms will be a bit uncomfortable)

In fairness, the days when it's -15°C outside are exactly the days when you really want your heating to work well...

Most heat pumps fall back to electric heating when it's too cold. So, on these few days you will need the same amount of electricity a typical electric heating will need.

which spikes the electricity demand from the grid and puts it under the most strain when its most critical for the grid to stay up, which means the grid has to be overdesigned, which means that the air based heat pumps are a poor choice for a reliable grid.

It depends on where you are. In many places worldwide, extreme cold is also generally quite clear from clouds unless you're literally in a storm, in which case, the high winds are just as likely to cause problems. Rooftop solar should be more than enough to offset your own usage.

In nearly all places extreme code means dark! Sure there are less clouds, but the latitude is high and so there are not only few hours of daylight, the earth's angle is also working against solar.

The demand comes at the coldest part of the day, which is typically overnight, which is far off peek demand times.

I don't think this conclusion follows. It is true that variability is a challenge for grids, but it is not clear that it is better to trade more total energy usage (per useful unit of work) for less variability. Variability is certainly a challenge, but not an insurmountable one, and also one that must be faced regardless nearly everywhere, as solar power has become too cost-effective to be ignored.

Isn’t electricity demand always going to be highest on the coldest days?

The UK is rolling out smart meters to every property. In the past 2 winters (following on the Russian gas crisis), they have run programs that pay £3 per kWh reduced demand. This is a nice way to balance the grid during extreme demand.

> programs that pay £3 per kWh reduced demand.

That much? How do I get in on that?

My energy supplier's demand reduction scheme only pays £10/month total.

octopus energy seems to be the best for energy i have seen so far in the uk alongside their smart tariffs https://share.octopus.energy/umber-squid-619

It's called the National Grid ESO scheme and you sign up through your supplier when it's open. However, some suppliers take a cut of the ESO payment. Alternatively, you can sign up to Uswitch Utrack (https://www.uswitch.com/mobile-app/), which passes 100% onto the customer. (Disclaimer: I used to work at Uswitch.)

This is untrue. Mitsubishi hyper heat units do not have this feature, and they’re the gold standard. I wish they did, though

Hyper Heat is a tech that is on multiple different models of Mitsubishi. All of the SVZ (ducted air handlers attachable to Hyper Heat or H2i mini-splits) do support (optional) electric resistance heater kits.

That’s correct, air handlers for ducted systems can absolutely have heat strips. That’s actually pretty standard. I have never seen one for a mini split though — can you share a link?

https://ces.mitsubishielectric.com/wp-content/themes/melco/a... documents the SVZ indoor unit's ability to have electric resistance heat and be connected to Hyper Heat SUZ outdoor units.

Whether you call that mini-split or not is up to you, but it's definitely a heat pump system that is Hyper Heat and supplemental electric heat capable, and getting down to one-ton units seems "mini" to me.

Mini-split means "smaller than conventional system ["mini"], condensor and evaporator are connected by long refrigerant lines ["split"]". It doesn't necessarily mean "wall/floor/ceiling indoor unit that has no ducts", though a "ductless mini-split" is the most common configuration of mini-split (because of the cheapness and ease of installation). https://zeroenergyproject.com/2022/03/09/what-is-a-mini-spli...

We have a air-to-air heat pump (minisplit). We also have heat foil in the floors. We've had weeks around -20C and below most winters.

I do hear the minisplit working hard those weeks, but we just needed a bit of extra help from the floor heating to have a comfortable 20C indoor.

Depends on how many of those days there are. If it's only half a dozen or so, I can deal with putting on an extra layer of clothing in the house for a handful of days per year. As long as it's warm enough inside to prevent damage like pipes freezing, a small amount of personal discomfort for a few days is acceptable.

Get a few space heaters, they're cheap.

Many new homes that are heat pump only will also require some sort of resistive heating as a backup.

>Some naysayers will say that it doesn't work the 3 days of the year where it's -15°C outside,

That used to be true but modern air-source heat pumps are better. But even so the efficiency drops it's just physics. Even if a heat pump can grab heat at -15C it will need to run longer when it's very cold, reducing lifespan of the unit. At some point it will just switch over to pure electric so your power to heat 1:4 is now 1:1.

Ground source heat pumps are far better and even more efficient that air source but quite expensive to install.

-18°F here (-28°C) here this winter. The heat pump (Mitsubishi) definitely worked hard and burned through our net-metering credit, but it did what we needed it to do. -15°C is just not even remotely a problem.

> Disclaimer: I self installed one for my house (13kWth) and I'm very glad I did

My HVAC guy keeps telling me to install a couple of heat pumps (he doesn't like driving out to me), solar panels, and an in-house battery; what sort of complexity was this job? Are there online sources you used?

Installing heat pumps doesn't require a ton of domain knowledge (assuming you're already a handy person), but it's a lot of work. It took 3 guys who do it for a living 160+ man hours to install our mini-splits. They had to drill through walls, attach channels to the siding, crawl into a tight crawlspace, do some plumbing when they hit a water pipe with their drill, wire up electrical, and add breakers to the panel.

The only thing that might catch you up is designing the system and ensuring you right size it for your heat load requirements. I'm sure you could research this pretty well, but your HVAC guy might also be happy to consult on that portion.

If you have a good spot (ie exterior wall), installing a mini split can be an easy DIY. It probably took me a max of 6 hours to do it all.

With this sort of work let the HVAC guys do the research for you. Call 3 of them get quotes. Then research what they are offering. Pick the one you like. Last time I did this around 2008 it was about 5k-15k I was quoted. I asked all 3 for a small, medium and large systems. For me it was mostly an in place replacement. The ducts were already there. The refrigerant lines were all ready in place. The biggest cost was the unit themselves and some change out of the controllers. The labor was about 1 days worth of work for 4 guys.

You can also gain quite a bit by just fixing drafts and putting in proper insulation. Which can be much cheaper to do. I also had the guys go thru and fix an leaks in the duct system. That way the air was coming out where it should. It is amazing how badly that is installed many times. I also had them put in an attic fan which vented the attic when it got to about 110F. Insulation would have helped more there and I screwed up and put it off. If your house is older than 2000. I say go thru and review the existing insulation and look for drafts first.

It trimmed my bill from about 350 a month to 200. My new house has excellent insulation the house is slightly bigger and the power bill is in the 80-150 range (less because I got solar, but I figured out the actual cost anyway). It has one unit and an air valve to switch between the floors. So the total cost is lower but the one unit will run longer. That savings I am getting is mostly because of better insulation.

Just to be clear, it works perfectly at -14c and above? Or is there some disappointing performance in an intermediate range?

I'm in Northern England and it's not uncommon for weeks of -5c to 5c in winter, some snowy days, plus serious damp making it feel even colder. So I'm curious if a similar system would be similarly amazing here.

I've read many people say they work perfectly because it won't hit -20c (a nice Strawman...)

At low temps COP is lower, meaning you'll not get 4 units of heat for 1 unit of energy spent, but COP is still higher than 1 meaning using heat pump is still cheaper than using basic electric resistive heating.

I think Northern England climate is ideal for heat pumps -- it never gets too cold, never gets too hot.

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