Heat pump tons = heated square feet ÷ square feet per ton, so a 2,000 sq ft older home in a mixed climate at 500 sq ft per ton needs a 4 ton unit, which is 48,000 BTU per hour of cooling. Enter your home for tons, then read how much of the winter heating load that unit covers and how much backup heat is left.
How the heat pump size calculator works
A heat pump is sized in two directions. The calculator starts with cooling, the way contractors usually size it: FSEC's study of Florida contractors found working rules of 350 to 700 sq ft per ton (weighted average 502), Energy Vanguard puts the rule at 400 to 600, and FSEC's modified guideline for newer, tighter homes is 700 to 1,000 sq ft per ton. We place your home in that range by climate: hot zones take the low end, mixed the middle, cold the high end (our placement).
Tons = floor area (sq ft) ÷ sq ft per ton, then the nearest size of 1.5, 2, 2.5, 3, 3.5, 4 or 5 tons
Rated tons for the whole heating load = heating BTU/h ÷ capacity kept at design temperature ÷ 12,000
Heating starts from the BTU per square foot for your climate zone, the same numbers as the furnace size calculator. A heat pump loses capacity as it gets cold, so you enter the share of rated capacity it keeps at your design temperature. The chip uses the Mitsubishi MSZ-GS12 in the ENERGY STAR data set: 10,100 BTU/h at 5 °F against 12,000 rated, or 84%. One ton is 12,000 BTU/h.
Worked examples
- 2,000 sq ft, zone 4, older home: 500 sq ft per ton gives 4.0 tons, a 4 ton unit, 48,000 BTU/h. The heating rule of thumb is 2,000 × 47.5 = 95,000 BTU/h. At 84% the 4 ton unit delivers 40,400 BTU/h at design temperature, 43% of the heating load, leaving 54,600 BTU/h or 16 kW for backup heat. Heating the whole house with the heat pump alone would take 9.4 rated tons.
- Same home, newer and tight with good insulation: 850 sq ft per ton gives 2.35 tons, so a 2.5 ton unit; heating rule 90,000 BTU/h.
- 1,200 sq ft in zone 2, older home: 3.0 tons; heating rule 45,000 BTU/h, so a 3 ton unit at full capacity covers 80% of it.
The heating bands are furnace guidance and run high for a tight house, so the backup figure is an upper estimate. Newer or better insulated homes come out lower, and a Manual J load calculation will be lower still.
Heat pump size by square foot and climate
| Home | Sq ft per ton | 1,500 sq ft | 2,000 sq ft | 2,500 sq ft |
|---|---|---|---|---|
| Older, zone 2 (hot) | 400 | 3.75 tons | 5 tons | 6.25 tons |
| Older, zone 4 (mixed) | 500 | 3.0 tons | 4 tons | 5 tons |
| Older, zone 5 (cool) | 600 | 2.5 tons | 3.3 tons | 4.2 tons |
| Newer, zone 4 (mixed) | 850 | 1.8 tons | 2.4 tons | 2.9 tons |
Above 5 tons a house usually gets two systems, one per floor or wing.
What size heat pump for a cold climate
NEEP's guide for cold climates gives two strategies: match the unit's capacity at the design temperature to 100 to 115% of the estimated heating load without backup heat, or size for 75 to 85% of the load and add just enough supplemental heat. It also insists on a full ACCA Manual J load calculation and on manufacturer performance data at your design conditions. The second field above is where that data goes: the lower the capacity at your coldest temperature, the more rated tons you need, or the more backup heat.
Why not just buy bigger?
NEEP warns that grossly oversizing equipment "can lead to excessive cycling, low efficiency and ineffective summer dehumidification". A variable-speed unit softens that, but the cooling size still matters in summer, so check it in the AC size calculator. For one room, an addition or a house without ducts, use the mini split size calculator.
Next steps
Airflow for a new system is about 400 CFM per ton (Trane), which the ductwork calculator uses to check your ducts. To weigh the running cost against another fuel, compare with the firewood calculator, the propane usage calculator and the BTU calculator for one room. All of these are in the heating and plumbing calculators hub.