Heat Pumps Take on Cold Climates
spectrum.ieee.org
spectrum.ieee.org
OTOH if you’re heating with propane or fuel oil, heat pumps are probably a good alternative.
The best whole house air-source heat pumps had a SEER2 of 18.5, or a COP of just under 5.0. I need a COP of over 5 to just break even in terms of cost efficiency. So a heat pump would actually cost me more on an ongoing basis. And the price for a heat pump installation was $10k more than a furnace/AC.
I just couldn't do it. To pay more up front and then ongoing was just too much. If you showed me even a super long range projection of saving money, I would have been down for it, but it just was not there.
If I can get solar on my house it may make sense, I will cross that bridge in a few years as my roof is almost end of life, but that has its own headaches as I have limited space (brownstone, flat roof), and am in a historic district.
Next go around, as heat pump tech improves and hopefully gets cheaper, it will probably make sense, but for me right now, it just didn't work.
Gas cost has varied a bunch over the years I’ve had it, electricity less so. But the furnace has only run a few days per year and only when when it’s been (depending on gas cost) below 10 to -5 degrees Fahrenheit.
My heat pump (trane xv19) kicks ass and if I were doing the hvac upgrade again I’d skip the furnace entirely and let the heat pump work a little harder/longer on the coldest days.
It’s damned near always more cost-efficient to run the heat pump. And my shiny new high efficiency furnace will never, ever pay itself off saving me a few bucks a day the three or four days per year that the math works in its favor.
Electricity can range from 11¢/kWh to 29¢/kWh in California or higher: https://www.energybot.com/electricity-rates/
Natural gas ranges from (converted) ~ 2¢/kWh to 8¢/kWh in Florida: https://www.eia.gov/energyexplained/natural-gas/prices.php (using conversion factors from https://www.nrg.com/resources/energy-tools/energy-conversion..., which I think assumes 1000 ft^3 ~= 1 therm ~= 29.3 kWh)
So your heat pump's COP has to exceed the ratio between natural gas and electricity in your area to make it cost-effective (or you can get solar + battery backup).
Electricity prices can vary quite a bit, too... in Cali, for example, it's gone up from 14¢ to 24¢ over 16 years, which is nearly double the rate of inflation.
What is the make and model of the thermostat you are using?
Is it from the heat pump OEM (Trane?) or a third party?
It’s a rebranded Nexia
* https://www.bayareaservice.com/wp-content/uploads/2020/02/18...
The process to update the costs (which has to be done a few times per year when my utility changes their rates) is not very user friendly - have to get into the technician menus and change scary looking stuff) but it works quite well.
It makes financial sense in a lot of places to switch to solar plus battery for nearly all energy use. Especially if you can charge your EV directly from solar. Heat pumps at night and in the evening need the batteries though.
As page where you can enter some numbers:
* https://www.heatpumpswork.com/cop-target
Also, given a US zip code, how often the temperature may drop below 15F / -10C (which is a cut-off for many units):
* https://www.heatpumpswork.com/cold-days
A good HVAC contractor will create a design based on your building and location, and it will cover 99% of average days efficiently for heating and cooling:
* https://www.greenbuildingadvisor.com/article/design-temperat...
* https://www.airequipmentcompany.com/2021/what-does-design-da...
For the remaining 1% it will basically have to run all the time to keep up. (In any given year you may not even get below/above the average outlier 1%.)
When did you last see a kerosene-powered fridge? It's about 40 years since I last saw one. Hence my 'obsolete technology' label.
Reverse-cycle (heating) heat-pumps rely on fans to extract the heat away from the machine and send warm air into the room. If you have electricity for the fans, you might as well use that electricity to power the heat-pump too.
The fans are a tiny part of the energy used by a heat pump, most of the energy is for running the compressor. Electric heat pumps are more efficient and economical than electric heat, but still generally more expensive than gas heat, and impractical compared to gas if that electricity itself is made from gas in the first place.
If you power a heat pump with gas directly, you are effectively getting over 100% efficiency: you keep all of the heat generated from the gas, plus are 'pumping' heat from outside. It's going to be even more economical than regular gas heat, which is already more economical than anything else.
I live in a rented apartment with a mini-split, and it worked great for 363 of the last 365 days. Those two days, we had a sudden cold spell and temperatures went down to -6 F (which isn't that cold), but we found out the hard way that our unit was only rated to -4 F.
Once it went down to -5, the heat pump didn't just work less efficiently, it stopped working altogether. It was the only real heat source in the house. We tried turning on the kitchen oven for a while, but it couldn't keep up. We had to leave the apt for those days and crash with a friend.
Other than that, though, it's been wonderful! SO much nicer than other heat sources (furnaces, space heaters, etc.) It's comfortable, cheap, quiet, and has its own thermostat. We just leave it on and it does its thing and it's magical.
I added an efficient wood stove instead. There's a good tax rebate that covers installation as well - I took advantage of that last year.
But even if it's because the defroster stopped working, or it's just an auto-shutoff feature to keep the equipment safe... the end effect is the same: There is no more heat when you most need it, and you'll freeze. We were wrapped up in multiple layers that day and tried to warm the house however we could, but ended up just needing to stay with a friend cuz it was so cold.
In some areas, "two sources of home heating" is mandated by law, but not where I live now. It still seems like good advice, specially if you can do that with two separate fuel sources (like electricity vs natural gas vs propane vs heating oil), in case one infrastructure goes down, as they often do during the worst storms.
Some heat pumps have a resistive heater built into them which kicks in when it's too cold for the heat pump to have a CoP of greater than 1.
This claim defies thermodynamics (and I expect better from IEEE).
Heat pumps are bound to be less efficient as the spread between inside and outside temperature grows. (They might be just as effective or they might be just as efficient as current heat pump tech, but they will not be as efficient at -20°C outside air temp as they are at +10°C.)
> At about 4 °C, most air-source heat pumps currently on the market start operating at less than their full capacity, and at some point (usually around -15 °C), they can no longer do the job at all. At that point, an auxiliary heat source kicks on, which is less efficient.
> But advancements in compressor technology over the last five years have addressed that issue. By controlling the compressor motor’s speed, and improving the timing of when vapor is injected into the compressor, engineers have made heat pumps efficient in colder temperatures.
The way I read it was "just as efficiently as a conventional furnace". IOW, a COP of 1 at bitter cold temperatures.
Do a google image search for "heat pump cop curve" and glance at the results. All of them are significantly downward sloping because the underlying thermodynamic cycle effect is very strong (agree not the only) limit.
It is a very old house that before had a oil heating system and I think the replacement worked quite well.
He added a ton of solar as well, which is a nice combination.
The claim can be literally true.
It may be they are at x% efficiency in both cases, where x is optimal for bitter cold but not for mild winter.
It can be. Would you take an even-money wager that the CoP curve will be flat [or increasing] with decreasing outside temp vs my wager that it will be downward sloping with decreasing outside temp?
What they wrote is " But heat pump manufacturers say they now have the technology to heat homes just as efficiently in bitter cold as they do in milder winter temperatures."
That is about whether they gained the technology to make the finished units as efficient in bitter cold as they are currently efficient in mild winter.
Given they were inoperative in bitter cold, and fairly inefficient in mild winter, this is almost certainly trivially true.
Here is a real curve: https://hvac-talk.com/vbb/attachment.php?s=f7eb5f83efc880440...
If we all mild winter 32F, and bitter cold -10F, they would only have to improve efficiency by 2x at bitter cold to make it as efficient as current heat pumps as mild winter.
That is easily possible.
As for whether they are strictly flat/decreasing, they are not.
Here is more data: https://connect.fujitsugeneral.com/3cfb13a7-a386-4697-aa3e-5...
You can see capacity, input power, and efficiency do not have a strictly linear (flat or decreasing) relationship with outdoor temperature. This is an artifact of it being impossible to good at all things at once.
It is also obvious to every reader you can not make a "maximally efficient in all" cases heat pump as efficient in bitter cold as in mild winter. So rather than assume that is what they are claiming by adding a lot of implicit things to it, i would just read it in a straightforward manner and when I do, it seems pretty easily true.
So if they can operate at the same percentage efficiency away from ideal, the extra difference from the actual source/sink temperature could be very minimal right?
I just found out my heat pump was configured incorrectly in the Nest Thermostat bc I have a Rheem and their heat cool valves are reversed from typical.
TLDR my January heating bill was $500 bc heat pump never came on!
The air from heat pump feels more comfortable than the dry air from the furnace. I got installed with a the furnace back-up option, that way if the heat-pump can't generate the heat, it switches to propane (it becomes very cold here in New England in Winters). From my experience, the heat pump is working well in 30F and above weather (-1.C)
Heat pumps burn a lot of electricity, luckily I have solar to off-set that cost, although Solar generation is low in Winter, it adds up well over the year.
Heat Pumps + Solar is the way to go.
Over the last 11 days or so (February with a combination of cold and unseasonably warm days) I've used 1047.15 kWh of electricity, of which I estimate half to two-thirds is used on the heat pump. Taking the new ultra-low overnight pricing into account I'm paying an average of 9¢ (CAD) per kWh. I've been trying to pre-heat my home at the tail end of the ultra-low overnight rates (2.8 ¢/kWh) leading up to the 7 a.m. cutoff time.
My gas furnace was my only gas appliance, so I can remove now remove it and save on the month connection charge which is about $30 a month, even in the summer when I don't use gas.
Canada also, at least for now, has a pigouvian carbon tax, so I'll be saving on that as well.
In terms of cost, it's a lot cheaper than gas + electricity (it evens-out on the coolest days, and is cheaper otherwise). It will not amortize the cost of the heat pump itself for a while, but that's a cost I'm happy to eat right now.
Or do you have existing ductwork (which a house with hydronic gas heat probably does not have, or else they'd probably not be asking the question)?
I understand the situation -- I had searched to see if hydronic gas heating used ducts, and it can but it seems that this is not common.
Their issue is not an issue of heat pumps themselves, but rather the lack of an air handler.
I also tried to switch in 2022 and found that it would literally never pay off due to the dramatically higher upfront project cost, and so a combi gas boiler now hangs on my basement wall.
* - LWT = "leaving water temp" aka "supply" aka "flow" depending on where you are in the world
On the savings side: I think we're about even compared to oil. But I'm okay with that: we didn't have any air conditioning at all in the house, so we had no duct work. (We bought the house at the end of 2022 like this.) Since we were already doing significant work to install the duct work and get a new air conditioning system, I wanted to make them heat pumps to get off of oil. I think in the long run, we'll save money. I eventually plan on installing solar panels. And even if it's a wash in terms of money, I'm care about climate change, and I want to do what I can to electrify my house.
edit: After looking at other comments, I should clarify what we have. We have two condensers outside, and three air handlers inside. It acts like "central air", for the most part. We do have one mini-split, for the main bedroom, as we could not get ducts to it. My point here is that "heat pumps" do not necessarily mean "mini splits." My house is almost entirely centrally heated and cooled through duct work.
It's literally the same physical design as an air conditioner. So... yes, unless you hate seeing AC units too, I guess.
Adding ducts to an existing house qualifies as a ton of demo in virtually every case.
The cost of a heat-exchanger is a compromise between cost and efficiency.
So far, 35% of households in my province have installed heat pumps, most keeping their EBBs or oil furnace as a backup. Every home that switches from EBBS is cutting their heating electricity usage by 50% (annually), freeing up electricity for the homes switching from oil furnaces to use for their heat pumps so our grid isn't being overloaded.
I haven't crunched all the numbers but so far my electricity bill has gone up by about as much as my gas bill has gone down, so there really isn't much savings there which is too bad. I mainly did this though for the environmental benefits, as I want to slowly phase out all natural gas in my home. Next step will be to replace my gas range with an induction cooker, and the last will be a heat pump water heater.