Scope. Sizing method for ducted and ductless air-source heat pumps in US heating climates. The capacity percentages below are typical values for illustration; for a real job, use the specific unit’s published capacities. Nothing here is affiliated with or endorsed by ACCA, AHRI, NEEP or ENERGY STAR.
The nameplate is a 47 °F rating
Heat pump heating capacity is rated at 47 °F outdoor. That is the “3 tons” on the quote, and it is the least useful number on the spec sheet for a heating-dominated house, because almost nobody’s design day is 47 °F.
As the outdoor temperature falls, an air-source heat pump moves less heat: the refrigerant has less to pull from, and the compressor can only pump so much mass. So its capacity slopes down as the house’s heat loss slopes up. A furnace does not have this problem, which is why the furnace habit of “match the heating load, done” fails on heat pumps.
Every listed unit publishes at least two heating points: capacity at 47 °F and at 17 °F (the AHRI rating conditions). Cold-climate units also publish 5 °F, and many go lower. Those are the numbers to size from.
Standard versus cold-climate
A typical standard single-stage heat pump keeps about 60% of its rated capacity at 17 °F, and keeps falling below that. A cold-climate unit — usually inverter-driven, often with vapor injection — holds far more: 80% or better at 17 °F is common, and the good ones are near 100% at 5 °F at maximum speed.
Two yardsticks decide whether a unit is “cold-climate” in the US:
- ENERGY STAR cold-climate criteria: a COP of at least 1.75 at 5 °F, and heating capacity at 5 °F of at least 70% of the 47 °F rated capacity.
- The NEEP cold-climate ASHP specification and product list: NEEP publishes minimum, rated and maximum capacity and COP at 47, 17 and 5 °F for each listed system. For sizing, this list is the most useful public data there is.
Both are revised from time to time, so check the current version before you write a spec around them.
Worked example: a 5 °F design day
Take a house with a calculated heating load of 42,000 Btuh at a 5 °F design temperature (typical of much of the upper Midwest and the inland Northeast), 70 °F indoors, and a cooling load that points to a 3-ton unit. Compare a standard heat pump with a cold-climate one of the same nominal size, using straight-line capacity between the 47 °F and 17 °F points:
| Standard (60% at 17 °F) | Cold-climate (80% at 17 °F) | |
|---|---|---|
| Capacity at 47 °F | 36,000 Btuh | 36,000 Btuh |
| Capacity at 17 °F | 21,600 Btuh | 28,800 Btuh |
| Capacity at 5 °F (design) | about 15,800 Btuh | about 25,900 Btuh |
| Share of the 42,000 Btuh load | 38% | 62% |
| Backup heat needed at design | 26,200 Btuh (7.7 kW) | 16,100 Btuh (4.7 kW) |
| Balance point | about 28 °F | about 23 °F |
Same nominal size, same house, and the cold-climate unit carries the house about 5 °F colder and needs 3 kW less backup on the coldest night. The real difference is often larger than this straight-line model shows: an inverter unit runs above its rated speed when it is cold, and NEEP lists that maximum capacity separately.
Balance point and backup heat
The balance point is where the two lines cross. Above it the heat pump carries the house by itself. Below it something else covers the shortfall:
- Electric strips in the air handler. Cheap to install, expensive to run. Size them from the gap at the design temperature: (heating load − heat pump capacity at design) ÷ 3,412 = kW, rounded up to the next strip size. Many installers go larger so the strips can carry the house through defrost or a compressor failure; that is a comfort and code call, not arithmetic.
- A furnace in a dual-fuel setup. The furnace takes over below a changeover temperature set from the balance point and local fuel prices. Size the furnace to the full design load, because it may be running alone.
A lower balance point means fewer hours on backup and a lower winter bill. You buy it with either a bigger heat pump or a better cold-weather unit. Usually the second is the better deal: a bigger standard unit drags its oversizing into every shoulder-season and summer hour.
So how big should it be?
Start from a real load calculation — Manual J, with the 99% heating design temperature for the actual ZIP. Then:
- Size the heat pump from the cooling load first. Oversizing for cooling costs comfort and humidity control all summer.
- Read the candidate unit’s capacity at your design temperature from its AHRI or NEEP data, not its nameplate.
- In a heating-dominated climate, step up a size or step up to a cold-climate unit if the balance point lands uncomfortably warm. ACCA Manual S allows some extra heat pump capacity beyond the cooling load where heating dominates; check the limits in the edition you work to.
- Size backup heat to the gap that remains at the design temperature.
- Put the capacity at design, the balance point and the backup size on the proposal. A homeowner comparing three quotes cannot judge compressors, but they can see which contractor showed the numbers.
Running the numbers
The free online Manual J calculator gives a whole-house heating and cooling load from a ZIP code in the browser. The HVAC Load Calculator app goes further for heat pump jobs: pick Heat Pump as the system type in any mode, choose standard or cold-climate, and the results and PDF show the heat pump’s capacity at the design temperature, the balance point and the backup heat needed, using the same straight-line model as the example above. Use it to check a quote or frame a proposal; for a specific unit, confirm against that unit’s published capacities.
Working in Canada? The Canadian heat pump sizing guide covers CSA F280 and the rebate programs.
FAQ
How much heat does a 3-ton heat pump put out at 5 °F?
Depends on the unit. Tonnage is a 47 °F rating. A typical standard unit might give around 16,000 Btuh at 5 °F; a cold-climate unit 25,000 Btuh or more. Read the 17 °F and 5 °F capacities from the AHRI or NEEP listing.
What makes a heat pump cold-climate?
ENERGY STAR’s cold-climate tier asks for a COP of at least 1.75 and at least 70% of rated capacity at 5 °F. NEEP lists capacity and COP at 5 °F for qualifying units. Check the current criteria.
Should a heat pump be sized to the full heating load?
Not by default. That usually means a unit far bigger than the cooling load, which short-cycles in mild weather. Size from cooling and the mild part of winter, find the balance point, and cover the rest with backup heat or a better cold-weather unit.
How do I size backup heat strips?
Heating load at design minus heat pump capacity at design, divided by 3,412, gives kW. 42,000 minus 26,000 Btuh leaves 16,000 Btuh, about 4.7 kW, so a 5 kW strip. Going larger to cover defrost or a failure is a judgement call.
What is the balance point?
The outdoor temperature where the heat pump’s falling capacity meets the house’s rising heat loss. A cold-climate unit pushes it colder, so fewer hours run on resistance heat.
