BTUFit › Heat pump size
Heat pump size calculator
A heat pump heats and cools with one machine, so it has to be sized to the bigger of the two loads. In most of the US that is heating. An 1,800 sq ft house in a mixed climate wants about 3 tons for cooling but around 77,400 BTU of heat, so the winter number, not the AC number, sets the size.
Why heating usually picks the size
The cooling engine here is the same one behind the central AC calculator (about 15 to 24 BTU per square foot by climate); the heating engine matches the heating BTU calculator (33 to 55 BTU per square foot by zone). Outside the Gulf Coast the heating number is larger, and since one heat pump serves both seasons, that larger number governs. Size to the AC load and you buy a unit that cools fine and struggles in January.
Capacity falls as it gets colder
A heat pump is rated at 47°F. Its heating output drops with the outdoor temperature, so the honest question is not the nameplate but what the unit still delivers at your coldest design temperature. Rough capacity retention, from Building America Solution Center and PNNL field norms:
| Outdoor temp | Standard heat pump | Cold-climate heat pump |
|---|---|---|
| 47°F (rated) | 100% | 100% |
| 17°F | 60–70% | 80–90% |
| 5°F | 45–55% | 70–80% |
Read it against your heating load. If the house needs 77,400 BTU at design temperature and a standard unit holds half its rating at 5°F, the nameplate has to be large or the backup heat has to carry the rest. Cold-climate models hold more, which is why they are the default recommendation north of the mild-winter zones.
The design load and backup heat need a Manual J
This tool gives you the two loads and the larger of them, enough to sanity-check a quote and to see that heating governs. It does not set your final equipment. The 99% design heating load, the balance point where the heat pump hands off, and the size of the backup all depend on your actual windows, air-tightness and local design temperature, which is a Manual J and a contractor's call. Refrigerant and 240V work are licensed trades regardless.
Frequently asked questions
Do I size a heat pump for heating or cooling?
The larger load, and outside the Deep South that is heating. A heat pump does both jobs with one piece of equipment, so it has to cover whichever season demands more. This tool runs both loads and reports the bigger one; in most of the country the winter number wins.
Why is my heat pump short on the coldest days?
Two things stack up. It was probably sized to the cooling (AC) tonnage, which is the smaller load, and a heat pump's heating output falls as the outdoor temperature drops. A unit rated at 47°F can deliver roughly half that at 5°F if it is a standard model, so an AC-sized heat pump ends up well under the heating load in a cold snap.
What is a cold-climate heat pump?
A model built to hold capacity in the cold. Where a standard heat pump keeps about 45% to 55% of its rated output at 5°F, a cold-climate (variable-speed, often "hyper-heat") unit holds roughly 70% to 80%, per Building America and PNNL field data. It is the difference between leaning hard on backup heat and barely needing it.
Do I still need backup heat?
Often yes, and how much is a Manual J decision, not a rule of thumb. The sizing turns on your 99% design temperature, the balance point where the heat pump can no longer keep up, and the backup (electric strips or a furnace) that covers the gap below it. Have a contractor run the numbers before you pick equipment.