A heat pump is sized from the cooling load, then checked against your county’s winter design temperature. See how much of the heat loss it covers, where the balance point falls, and how much backup heat you need.
Pick your county to fill the 99% heating design temperature from EPA’s ENERGY STAR reference guide, or type your own. Picking your county also sets its 2021 IECC climate zone (see the climate zone map).
Filled from the same whole-home model as the whole-house calculator; replace it with a Manual J figure if you have one.
| Cooling load | — |
| Heat loss at design temp | — |
| Heat pump output at design | — |
| Balance point | — |
| Backup heat at design | — |
| Outdoor temp | Home’s heat loss | Heat pump output | Backup needed |
|---|
Heat pump output is a straight line between its 47°F capacity and its low-temperature capacity. Real equipment data (the manufacturer’s extended performance tables) should replace these figures before you buy.
The size starts from the cooling load, within ENERGY STAR’s cooling sizing limits: recommended 90–115% of the cooling load for single-speed, 90–120% for two-speed and 90–130% for variable-speed compressors (allowed up to 130%, 140% and 160%). The calculator picks the smallest size in the recommended range that covers your design-day heat loss. If none does and your county is heating-dominated, it steps up to the allowed limit to reduce backup heat; otherwise it stays in the recommended range and shows the backup needed.
In heating-dominated dry climates — a heating-to-cooling degree-day ratio of 2.0 or more (shown for your county) and a cooling load that is at least 95% sensible — ENERGY STAR allows heat pump cooling capacity of 90–100% of the load plus 15,000 Btu/h, which lets you size up for winter. The calculator shows when your county may qualify.
The balance point is the outdoor temperature where the heat pump’s output equals the home’s heat loss. Below it, backup heat (electric strips or a furnace) makes up the difference. DOE’s Building America guidance describes sizing choices from “size to cooling” to “size to the full heating load”; bigger sizes cut backup use but risk short-cycling in summer.
Cold-climate heat pumps: ENERGY STAR requires their capacity at 5°F to be at least 70% of their capacity at 47°F, which is the default here. The “standard heat pump” option assumes 60% at 17°F — an assumption, so enter your unit’s real figures when you have them. Heating capacity at 47°F is assumed equal to the nominal cooling size unless you enter it.
Heat loss at the design temperature = (sum of each surface’s area × its U-factor + air leakage) × (70°F indoors − the outdoor design temperature). The 70°F indoor and the 99% outdoor design temperature are the conditions ENERGY STAR requires for HVAC design.
Not included: duct losses, internal gains and room-by-room effects. This is a whole-house planning estimate, not an ACCA Manual J calculation, which a contractor should run before buying.
Start from the cooling load, within ENERGY STAR’s cooling limits, then check how much of the winter heat loss it covers. In cold, heating-dominated places a larger or cold-climate unit covers more of the winter, and DOE’s Building America guidance describes sizing all the way to the heating load with variable-speed equipment.
The outdoor temperature where the heat pump’s output equals the house’s heat loss. Colder than that, backup heat runs to make up the gap. A lower balance point means less backup heat.
Under ENERGY STAR, its heating capacity at 5°F must be at least 70% of its capacity at 47°F, with a COP of at least 1.75 at 5°F, plus an HSPF2 of 8.1 or more (8.5 for ductless).
Enough to cover the gap between the heat loss and the heat pump’s output at your design temperature. The calculator shows it in Btu/h and kW of electric strip heat. Many installers size strips to carry the whole house in case the heat pump fails, which is a separate decision.
From EPA’s ENERGY STAR Design Temperature Limit Reference Guide, which lists a 99% heating design temperature for every US county.
Official sources only, checked October 2026.
actonnagecalc.com is not affiliated with, or endorsed by, the U.S. EPA, ENERGY STAR®, DOE or the ICC. Their publications are cited as sources.