COP & sizing question

oreo123

Last winter there were disappointed homeowners when their heat pumps were not producing enough heat at 0 F. And then they got their elect bill.. If the COP falls off to say 60 percent output - wouldn't it make sense to spec a heat pump for more btus? A larger btu system would consume more power and cost more to buy but it would keep one warm in the coldest nights.

Mass. homeowners say state ignoring cost in push toward all electric heat

My state of Mass has some cities that will not allow new construction to have fossil fuel. Heat pumps are the only option. Elect rates are 32 cents per kilo - amongst the highest in the nation. Assuming the new construction is stretch code & passes blower test I wonder if their elect bill will still soar in comparison to a high efficiency nat gas boiler or furnace.

mattmia2

the cop will still fall off but you need the capacity at design conditions with the degradation in performance so you need to size it for that.

if i read the chart for a particular heat pump i looked at correctly, somewhere around 0f the efficiency became such that it was essentially just an electric resistance heater so electric resistance heat may become a better option below some outdoor temp.

both the electric grid and the gas distribution network need significant rebuilding. part of the intent of the rules is to avoid having to rebuild both. the other part is that there are a lot more ways to generate electricity than there are to produce natural gas so it is a lot more flexible. the problem here is that there is little regulation requiring you get a quality product when you buy a heat pump.

DCContrarian

Modulating heat pumps typically have highest efficiency between 25% and 50% of rated output. So yes, sizing larger does give higher efficiency. The real win is in avoiding insufficient capacity where you have to rely on resistance backup heat. Heat pumps with vapor injection don't see higher efficiency at lower outputs, but they keep their capacity better which means less need for backup heat.

Something like the Mitsubishi M-Series has a COP of 1.47 at -13F and 1.93 at 5F ( See https://ashp.neep.org/#!/product/34583/7/25000/95/7500/0///0 ). . So you're much better off avoiding resistance heat.

Jamie Hall

To answer the implied question in @oreo123 's post… there are really three questions. First, will going to a higher BTUh output heat pump help keep your clients from shivering on cold winter nights? Yes, but only if it is a heat pump designed for cold temperatures. Some are, some aren't. Second, the some of the ones that aren't can be saved by adding electric resistance heat.

Third, will the client save money in relation to what gas or oil would have cost? No. Emphatically no. The electric rates vs. oil or natural gas prices in New England (not just MA') are high enough that oil or gas fired heat is going to be cheaper, maybe a lot cheaper. The Mitsubishi @DCContrarian mentioned is a good cold climate heat pump, and should only cost about twice as much to run as a modern oil or gas boiler or furnace.

DCContrarian

@Jamie Hall : "The Mitsubishi @DCContrarian mentioned is a good cold climate heat pump , and should only cost about twice as much to run as a modern oil or gas boiler or furnace."

Your bias is showing.

As fate would have it, I just finished an analysis of that particular heat pump vs oil for someone in Massachusetts, the difference was $5.09 a year, in favor of oil. You can read it here:

https://forum.heatinghelp.com/discussion/201837/help-sizing-a-multi-zone-mini-split-system-minimum-capacity

If you add the cost of annual cleaning of an oil burner the heat pump is cheaper.

I think the point is that people are sold heat pumps with the claim they'll save lots of money. In New England that's generally not true.

GGross

I think you are confusing COP with capacity and while they are related it doesn't tell the full story. IF your COP fell to 60% (0.60) you would be worse off than using the least efficient gas heating, COP with electric heat should NEVER fall below 1.0 (1.0 is the COP of electric resistance heat, think electric baseboard, or electric coils in an air handler), and most new inverter mini splits have a low COP of around 1.4 at the fringes of their ranges.

What you are looking for is both a high COP (coefficient of performance, higher number smaller bills for heat delivered to the space) and a higher capacity. If your primary goal is to just meet the capacity requirements of the space, then step 1 is a load calc. Let's say your load calc says you need 35,000 btu/hr at 5f, then you would look for a heat pump that can output more than 35,000 at 5f (be careful to make sure you have the correct indoor delivery to accommodate those btu's as well!) need heat at a lower temp? then base your load calc on that lower temp! or plan to use to auxiliary heat of some kind. There are currently on the market mini splits heat pumps that will output 100% of their rated capacity at -13f and 70% at -31f (albeit only a 1:1 system currently) the tech for high capacity at extremely low ambient temps is here and will be seen in more products very soon.

https://media.us.lg.com/m/26d8d4d82299980f/original/SB_SZ_Art_Cool_Deluxe_R32_KSSAL121A_2025_05_19_121743.pdf

COP is a bit more tricky. From a basic perspective, higher number, lower energy bills (more to it than that of course) some things that are important to know if trying to lower your bills the most, you want to know what COP will break even with your gas heating alternative for your area. Then you figure out what temperature your heat pump functions at that break even COP, that would be your temp (or slightly above) that you would want to switch to gas heat. Of course if you are choosing a new heat pump you can look at different models and brands and find one that has a her COP at a lower temperature

Jamie Hall

Can't argue with your analysis, @DCContrarian — on a whole year basis. And I agree that, on a whole year basis in MA a high quality cold climate heat pump will cost about the same to run as an oil fired boiler. What I object to — and perhaps it is a bias, but if it is it's not against heat pumps (I have one, and would strongly recommend them in more southern climates) but against people who claim that they are cheaper to run (they aren't, in New England) and cheaper to maintain (they aren't) and reasonable in cost to purchase (they aren't) and will keep you warm on the coldest nights (they won't, in New England). Sell what you've got — it's an excellent system — but don't oversell it.

DCContrarian

"cheaper to maintain (they aren't) ."

In one house I have an oil-fired boiler, in the other an air-to-water heat pump. I just had the annual cleaning for the oil burner, it was $400+ with electrode replacement. I was chatting with the technician afterwards and mentioned that I have a heat pump in the other house. His comment, "do you just hose it off once a year?"

DCContrarian

"Then you figure out what temperature your heat pump functions at that break even COP, that would be your temp (or slightly above) that you would want to switch to gas heat. "

I know this advice is often given, but I think it's misguided.

If gas is much cheaper than electric, it never makes sense to switch to electric.

Similarly, if electric is much cheaper than gas, it never makes sense to switch to gas.

But if the two are close in cost, the amount of money you save by switching is trivial.

I cited above the analysis where in Massachusetts, at current prices, gas and oil are within $5/year of each other. As part of that analysis I calculated the savings from using electric when it's cheaper and oil when it's cheaper. It was less than $190 per year. Now that's not nothing, but there's no way it's going to cover the long term capitalized cost of having two systems. It doesn't even cover the cost of an annual cleaning on a second unit.

oreo123

Wow. Thanks all. This is a lot of reading for sure.

I just installed a combi boiler with enough btus to do a single fam house. Four rooms first floor, 4 bedrooms second floor. Bath on each floor. Aprox 1150 feet per floor. I see the combi running at 94-95 % efficiency at full throttle and 1 percent. Each floor is a separate zone.

I was wondering if when I install next a.c. system just for second floor - what is cheaper to run in the winter. I believe its nat gas heat instead of heat pump.

A couple of mos ago I read that to achieve better cop in very cold weather that the units had larger compressors. And higher h.p. Is that true?

In my state some communities require new builds to not have fossil fuel heating systems. Been reading that the highest efficiency units have bigger and thinner coils to achieve the best heat transfer. Wouldn't this be creating more weak spots / breakdowns?

IF this state wants to push heat pumps this state needs to lower rates somehow. Looking at a recent bill of 380 its broken down between generation of 153 and delivery 227. IF somehow the generation were zero we are still in for the delivery costs. 1158kw

Hot_water_fan

COP isn’t capacity. There is 1 easy way to have capacity at 0F - install a heat pump that’s correctly sized.

@Jamie Hall check out the new MA electricity rates for electric heating

DCContrarian

"I was wondering if when I install next a.c. system just for second floor - what is cheaper to run in the winter. I believe its nat gas heat instead of heat pump."

That is too complicated of a question for there to be any useful rules of thumb.

The cost of gas depends upon the efficiency of your burner and the local cost of gas. The cost of gas is highly variable, it varies by at least a factor of ten around the country.

The cost of a heat pump depends up the COP of the heat pump and the local cost of electricity. The cost of electricity also varies by at least a factor of ten. It's only loosely related to the cost of gas.

The efficiency of a burner is more or less constant, but the COP of a heat pump varies greatly, depending on the heating load of the house it's installed in, the climate of that house, and the design of the heat pump.

The actual calculations aren't hard — see https://www.greenbuildingadvisor.com/question/how-to-model-and-predict-electricity-usage-for-a-heat-pump

but there are no useful generalizations.

DCContrarian

"A couple of mos ago I read that to achieve better cop in very cold weather that the units had larger compressors. And higher h.p. Is that true?"

There are different types of heat pump. The only type I would consider installing in a cold climate would be variable speed with vapor injection like the Mitsubishi Hyper Heat.

Those units see lower COP at partial load than at full load. So you want to size them for the actual heating load, no bigger, no smaller.

Jamie Hall

As @DCContrarian says — and I have said, in different ways (we really do agree on a lot, @DCContrarian !), there are no simple answers. Indeed, each application will be different, and — unhappily for the consumers out there — really requires a well-informed analysis. This includes the local environment, the characteristics of the structure, the siting of the structure, the preferences of the client, and what is available in terms of hardware and energy sources. Unfortunately, in some jurisdictions it also includes legal mandates. How to comfort condition a structure is assuredly not a one-size-fits-all situation!.

To add to the fun, optimum operation of many systems — particularly hybrid ones — can be somewhere between mildly and fiendishly complex. A lot of that can be managed by good computing hardware and software with adequate inputs, but…

DCContrarian

If you focus on a small geographic area and one line of equipment it is possible to remove a lot of the variability. That's when you can start developing useful rules of thumb.