Heat Pump Size Calculator
Sizes an air-source or ground-source heat pump against both loads, and shows how much backup heat is needed on the coldest day.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
tons = cooling load ÷ 12,000, rounded to the nearest half ton delivered = rated × capacity fraction at design temperature backup = heating load − delivered
A heat pump moves heat rather than making it, and the colder the outside air the less there is to move — so its output falls exactly when the house needs most. Sizing on the cooling load and covering the winter shortfall with backup heat is the standard compromise, because a unit large enough for the coldest hour would be badly oversized for summer.
What to watch out for
Heat pumps are sized differently from furnaces, for one reason: their output falls as it gets colder.
- Size on the cooling load where the system does both. A unit sized for the coldest winter hour is heavily oversized in summer, and an oversized unit in cooling cannot dehumidify.
- Cover the winter gap with backup. Electric resistance strips or an existing boiler run only on the coldest days, which is a small share of the season's energy.
- Capacity at design temperature is the number that matters. A "3-ton" unit is rated at 47 °F / 8 °C. At −8 °C a standard model may deliver 55–70% of that; a cold-climate inverter model holds far more. Get this figure from the manufacturer's extended performance table, not from the model number.
Frequently Asked Questions
What size heat pump do I need?
Take the cooling load in BTU/h and divide by 12,000 for tons, rounding to the nearest half ton. A house with a 24,000 BTU/h cooling load needs a 2-ton unit. Then check what that unit still delivers at your winter design temperature and size the backup heat to cover the shortfall.
Why does a heat pump need backup heat?
Because its capacity falls as the outside temperature drops, while the heat the house needs rises. The two curves cross at the balance point; below that the heat pump alone cannot keep up, and backup heat covers the difference.
Can I size a heat pump on heating load instead?
For a heating-only installation, yes — and this calculator does that when the cooling load is zero. Where the same unit also cools, sizing on heating produces a unit that short-cycles all summer and leaves the house cold and damp.