What size heat pump do I need?

A heat pump is sized in two steps. First, a load calculation finds how much heat the house loses on a design day and how much heat it gains on a hot one. Then a unit is chosen whose capacity at those outdoor conditions covers the loads without overshooting them by much. Neither step uses the floor area alone, and neither uses the size of the system being replaced.

Why can’t I size a heat pump by square footage?

Because two houses of the same size can need very different amounts of heat. A U.S. Department of Energy fact sheet from January 2002 warns against “casual sizing estimates based on home floor area or contractor ‘experience’” and illustrates the point with a hypothetical 2,000-square-foot home in Raleigh, North Carolina, built two ways.

	Conventional construction	With efficiency upgrades
Wall / ceiling insulation	R-11 / R-19	R-19 / R-38
Windows	Single pane	Double-glazed, low-e
Ducts	Average leakage, in the unconditioned attic	No leakage, inside the conditioned space
House air leakage	8 air changes per hour at 50 Pa	6 air changes per hour at 50 Pa
Manual J design heating load	46,100 Btu/h (13.5 kW)	21,300 Btu/h (6.2 kW)
Manual J design cooling load	52,100 Btu/h (15.3 kW)	23,300 Btu/h (6.8 kW)
Heat pump size	4.0 to 4.5 tons	2 tons (24,000 Btu/h)

Same floor area, same city, and the heating load differs by a factor of 2.16 (46,100 ÷ 21,300). The kilowatt figures are our conversion at 1 kW = 3,412 Btu/h. The building details in the example date from 2002, but the lesson has not changed: insulation, windows, air leakage and where the ducts run decide the load, and a rule of so many square feet per ton cannot see any of them.

NYSERDA, New York’s energy agency, gives contractors the same advice in its heat pump program: do not size by rules of thumb, and do not replace a system with a like-sized one without your own calculation.

What does a Manual J load calculation measure?

ACCA Manual J, published by the Air Conditioning Contractors of America, is the procedure DOE recommends for residential heating and cooling loads in the United States, with Manual S for choosing the equipment. The NEEP sizing guide lists what any load calculation must account for:

• the areas and insulation values of walls, roofs, windows, floors and foundations;
• air leakage, including the moisture it brings in for cooling;
• duct losses, but only if the new system will use ducts;
• solar gains through roofs and windows, and heat and moisture from people and appliances, for cooling.

The calculation uses the local design temperature: the 99% heating value, which the outdoor air stays above for 99% of the hours in a year, not the record low. The DOE fact sheet adds that substituting annual extreme temperatures leads to oversizing, and that exaggerated indoor set points have a large effect on the result. It notes Manual J generally recommends 70 °F (21 °C) indoors for heating and 75 °F (24 °C) for cooling.

Two cautions from the NEEP guide matter most for homeowners:

• No padding. Load calculations already include safety factors, so the procedure should be followed as written “without ‘padding’ the estimates with additional safety factors”. DOE’s Building America guidance says the same: following Manual J, “additional upsizing or rounding up is not needed”.
• Air leakage is often overstated. NEEP recommends a blower door test rather than an estimate. Reasonably weatherized existing homes tend to have natural air change rates below 0.4 per hour at design conditions, and efficient new homes are often well under 0.1.

In some places the calculation is also a legal requirement. Washington’s residential energy code, based on the 2021 International Energy Conservation Code, requires equipment to be sized with ACCA Manual S using loads from Manual J or another approved method. It also caps the output at the smallest available size that exceeds the calculated loads, within the allowed oversizing limits. Whether a rule like this applies to your home depends on the code your state or city has adopted.

Should the heat pump be sized for heating or for cooling?

In a mild climate the cooling load usually decides. In a cold climate the heating load is often the larger one, and the choice becomes how much of the winter the heat pump should carry alone. DOE’s Building America guide for climate zones 4 and colder describes four approaches, each with a typical balance point, the outdoor temperature below which the heat pump needs help.

Approach	Sized to	Typical balance point
1	The design cooling load; a cold-climate unit is not needed	40 °F (4 °C)
2	Cooling, with a variable-capacity unit chosen to maximize heating	30 °F (−1 °C)
3	Most of the heating load, with a backup system or significant auxiliary heat available	0 °F (−18 °C)
4	All or nearly all of the heating load	−15 °F (−26 °C)

The guide notes that ACCA released the third edition of Manual S in 2023, which allows heating-focused sizing with cold-climate heat pumps. The second edition still applies when the heat pump is sized around the cooling load.

The NEEP guide gives numbers for the heating-focused case. For a full replacement or a new home, it suggests matching the system’s capacity at the design temperature to 100–115% of the heating load without auxiliary heat. The alternative is 75–85% of the load plus just enough supplemental heat to make up the difference, kept off unless needed.

Situation (NEEP)	Sizing strategy
Existing system stays; heat pump cuts heating costs in the main living area	Each zone sized to the load of the area it serves; the total is smaller than the whole-house load, and capacity at design temperature is a secondary concern
Full heating system replacement	100–115% of the heating load at design temperature, or 75–85% plus minimal supplemental heat; full Manual J
One isolated room or addition	The heating and cooling loads of that space only
New construction or gut rehab	As for full replacement; take particular care not to oversize in very tight, low-load homes

What happens below the balance point, and how the backup is controlled, is covered in our explainers on auxiliary heat and on the dual-fuel switchover temperature.

How do I check whether a specific model is big enough?

Compare the load with the unit’s capacity at the same outdoor temperature. DOE’s Building America guide is direct: “Equipment should not be selected based on nameplate capacity. Instead, the capacity at design conditions must be determined.” The nameplate heating figure is measured at 47 °F (8.3 °C), and an air-source heat pump delivers less heat as the outdoor air gets colder.

The capacity at your design temperature is in the manufacturer’s extended performance tables for the exact outdoor and indoor unit pair. The NEEP guide warns that not all published data is consistent; some tables do not show maximum capacity at colder temperatures. It suggests cross-checking against the cold-climate product list, which reports maximum heating capacity at 5 °F (−15 °C). How those ratings work is explained in our guide Do heat pumps work in cold climates?

Why does the minimum capacity matter?

A variable-speed heat pump can turn down, but only so far. On a mild day the house may need less heat than the unit’s lowest output, and then it cycles on and off. The NEEP guide calls the minimum capacity “as important as the maximum”, and gives three checks:

• Cooling. If the unit’s minimum-speed cooling capacity is over 115% of the design cooling load, look for a model with a higher ratio of heating to cooling capacity, a lower minimum, or both.
• Multi-zone systems. Avoid outdoor units sized at more than 120% of the heating design load where possible.
• Small rooms. The outdoor unit’s steady minimum output at 47 °F should not exceed the heating capacity of the smallest indoor unit connected to it. If that figure is not available, NEEP suggests avoiding an outdoor unit rated at more than three times the smallest indoor unit, and installing more, smaller outdoor units rather than larger indoor ones.

NYSERDA puts numbers on the cost of getting this wrong. Inverter heat pumps are most efficient in their modulating zone, typically 40% to 80% of full capacity. Effects on energy use and comfort begin when a system is sized at 120% of the building load and can become severe above 150%. Low-load cycling carries an efficiency penalty of as much as 25%.

Oversizing is common. The 2002 DOE fact sheet cites a survey in which nearly 40 percent of contractors said they oversized equipment on purpose, giving reasons such as reducing callbacks or customer demand. It also cites a Florida study in which units oversized by 50 percent or more showed a typical 9 percent increase in annual cooling electricity use.

Can I go by the size of my old system?

Only as an upper limit. For a whole-house replacement, the NEEP guide says the heat pump’s capacity at design conditions should not exceed the existing heating equipment’s capacity, and will often be smaller. Two things usually shrink the load: insulation and air sealing done since the old system went in, and duct losses that disappear when ducts are replaced or abandoned. The guide adds that measuring how long the existing equipment runs in very cold weather can help estimate the real load.

NEEP recommends fixing insulation, air leaks and duct leaks before installing new equipment, and the DOE fact sheet makes the same point: as a home’s efficiency improves, the recommended system size drops.

What do tons, Btu/h and kilowatts mean on a quote?

Residential heat pumps are sold in tons, a cooling measure equal to 12,000 Btu per hour. The nominal size is the cooling rating; heating capacity at your design temperature is a separate figure in the performance tables.

Nominal size	Btu/h	kW
1.5 tons	18,000	5.3
2 tons	24,000	7.0
2.5 tons	30,000	8.8
3 tons	36,000	10.6
3.5 tons	42,000	12.3
4 tons	48,000	14.1
5 tons	60,000	17.6

Kilowatts are converted at 1 kW = 3,412 Btu/h. More on the units in How BTUs, tons and kilowatts convert.

What should I ask a contractor?

• Was a Manual J calculation done for this house, and can I see the report with the heating and cooling loads?
• What design temperature and indoor set points did it use?
• Was air leakage measured with a blower door, or estimated?
• What is this unit’s heating capacity at our design temperature, from the extended performance tables for the exact indoor and outdoor models?
• What is its minimum output, and how does it compare with the load on a mild day?
• Where is the balance point, and what supplies heat below it?
• Is the system sized for heating or for cooling, and which edition of Manual S was used?

A quote that answers these with numbers from a load calculation and the manufacturer’s tables is sized for your house. A quote that gives only tons per square foot is not.