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If I have a furnace with some BTU rating, and given the outside temperature, is there a way, or a web site, or whatever, that would tell me what rating an air-source heat pump would be need, to supply the same amount of heat? And, if that can be answered, how many amps the heat pump would draw while providing that heat.

I'm just looking for a 'theoretical' answer, not designing a DIY project, not looking to pick a particular model, etc. I suspect the question can't be answered without more info; if so, it would help to hear why.

Edit, b/c comments aren't forever: The comments in ecnerwal's answer below answer my question. To wit, knowing the current furnace's BTU/hr energy use at some temperature, AND a heat pump's COP at that temperature, the heat pump's energy needs to provide equivalent heating can be estimated. With appropriate units, COP = heat_energy_out/electrical_energy_in.

Availability of COP vs temperature data is limited, but it approaches 1 at the (low) temperatures I'm interested in.

Feel free to correct, if this is in error.

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  • Most heating/cooling systems should be based on the size of the house/building, insulation values, and weather values. Once those figures are known, then the type of system can be sized. If BTUs are known then should be a chart showing the wattage needed to match BTUs. Heat pumps are are bit more difficult to match, since below certain temps they need to depend full(expensive) electric heating, since they are more of a hybrid of heating types.
    – crip659
    Jan 2, 2022 at 17:55
  • What is the coldest temperature normally seen where you live?
    – Ecnerwal
    Jan 2, 2022 at 18:22
  • Yes, what is your 99% heating design temperature, and have you considered having a competent contractor run an ACCA Manual J, Eighth Edition analysis of your house? Jan 5, 2022 at 3:30
  • From what I've seen in my own research the size of the heat pump is based off required cooling power and you get what you get for heating power. So, look at the size of air conditioner you'd need and pick a heat pump with the same cooling power. If your location is cold enough that you'd never use air conditioning then every HVAC installer will recommend not installing a heat pump. They will gladly take your money if you insist on a heat pump but they don't want to hear complaints if the heat pump fails to provide any actual heat or save any money on annual heating costs.
    – MacGuffin
    Aug 18, 2023 at 6:03

2 Answers 2

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If (unlikely) your gas furnace is correctly sized, X BTU/hr = X BTU/hr - slapping in something too big is a classic approach to upsell more furnace than you needed, and because the guy selling the furnace isn't paying for the standby losses when it's only running for 50% of the time on the coldest days, and 10-15% on typical days.

The power input required is trickier, since the COP (Coefficient of performance - how many Watts (electric) in for how many Watts (heat) out is variable, and depends on the specific heat pump and the outside temperature. The relationship to outside temperature is often poorly documented, but some data can be found.

For instance, many/most "central" heat pumps have a COP that hits 1.0 (might as well run resistance heat) very close to 32°F/0°C, and are useless below that point. Many (but not all) mini-split heat pumps are above 1.0 down to -15°F - which is why you find lots of Mini-Splits in colder areas, and very few central units, as they are worse than useless for most of heating season.

If your house is poorly insulated, improving the insulation to allow use of a smaller heat pump (or fewer separate heat pumps /heads if going to Mini-Splits) can pay off handsomely in new system cost (quickly) as well as operating cost over time (long-run.) I successfully heat a 2000 square foot well-insulated building with 2 12K BTU/Hr (nominal) heat pumps in a climate that goes to -15 (and that was tested last year) - haven't seen the increasingly rare -20°F since the system was up and running. Despite marketing as 12K BTU/Hr (which is the cooling capacity), claimed heating capacity is 13.5K or something like that. You'd be hard pressed to find a house in the area with less than a 100K BTU/Hr furnace/boiler, though - poorer insulation, few small boilers/furnaces made, or the boiler is also providing domestic hot water and is oversized for that purpose. But with serious insulation I'm just fine with perhaps 27K BTU/hr.

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  • Properly designed heat pumps / refrigeration systems can have a COP of 4 or more. Design is the issue here since the quality of simple things like measuring and construction quality are not met.
    – Solar Mike
    Jan 2, 2022 at 18:40
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    But the COP is not fixed with temperature - heck, mine go over 11 at small differentials, and usually operate between 3 and 4, based on what info I can find. But there are also plenty that have a hard time managing 3 and quickly drop to 1 near freezing. And even good ones drop off somewhat below freezing, due to the need to run defrost cycles. COP is a number that depends a great deal on what the temperature is - and the system design.
    – Ecnerwal
    Jan 2, 2022 at 18:46
  • @Ecnerwal - (after reading about CoP): If the COP at temperature was known, and also the furnace's therms (per hour or whatever) at the coldest time, couldn't the required max heat pump input power be easily calculated? (If this needs more than a simple answer, I can make it a separate question.)
    – George
    Jan 3, 2022 at 14:46
  • Sure, with all that data in hand, the calculations are straightforward. Guessing at them leads to "Garbage In, Garbage Out" calculations. Do you have any of that data?
    – Ecnerwal
    Jan 3, 2022 at 15:27
  • @Ecnerwal - Actually, yes. Because, spreadsheets need to be fed.
    – George
    Jan 3, 2022 at 18:55
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I would evaluate the actual heating requirement for the property.

Gas boiler installers were notorious for oversizing boilers.

And you can take into account any changes like extra rooms, retro-fitted insulation or change of purpose etc

Once you have that then the heat pump can be easily sized - manufacturers provide information for that.

The "theoretical" answer will need you to work out the R value for the walls, windows, floor and roof and then, using the annual ambient temperature and degree-days to work out the amount of heat required.

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  • 2
    System size (heating capacity) depends on the low temperature "design day" - operating cost goes with degree days, but you can have the same degree-days in climates with very different design days, and if you don't hit the design day, the system will be running 100% and the house will still be cold. Which is one insight to why folks who get sloppy with the thermal calculations like to oversize the furnace/boiler to cover their slop.
    – Ecnerwal
    Jan 2, 2022 at 18:27

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