Short answer: what size heat pump for 1,500 sq ft? For a well-insulated house, about 21,000 Btu/h of heating, not the 38,040-47,550 Btu/h a per-square-foot rule gives. HeatAlgo calculated one single-story house room by room at five floor areas and three insulation levels: at 1,500 sq ft it needs a heating target of 21,292 Btu/h well insulated and 66,230 Btu/h uninsulated, at a -4 °F design day. Insulation moves the answer more than square footage does.
The usual quote starts from the floor area. That is the number every homeowner knows, and the one that says least about the heat a house loses. So below is the same house at 1,200 to 2,500 sq ft, calculated three times.
What size heat pump do I need: Btu/h for 1,200 to 2,500 sq ft
| Floor area | 80-100 W/m² rule | Well insulated | Partly insulated | Uninsulated |
|---|---|---|---|---|
| 1,200 ft² | 30,432-38,040 Btu/h | 17,880 Btu/h (5.24 kW) | 30,300 Btu/h (8.88 kW) | 56,300 Btu/h (16.50 kW) |
| 1,500 ft² | 38,040-47,550 Btu/h | 21,292 Btu/h (6.24 kW) | 35,862 Btu/h (10.51 kW) | 66,230 Btu/h (19.41 kW) |
| 1,800 ft² | 45,648-57,060 Btu/h | 24,465 Btu/h (7.17 kW) | 41,150 Btu/h (12.06 kW) | 75,715 Btu/h (22.19 kW) |
| 2,000 ft² | 50,720-63,400 Btu/h | 26,546 Btu/h (7.78 kW) | 44,597 Btu/h (13.07 kW) | 81,857 Btu/h (23.99 kW) |
| 2,500 ft² | 63,400-79,250 Btu/h | 31,665 Btu/h (9.28 kW) | 52,922 Btu/h (15.51 kW) | 96,700 Btu/h (28.34 kW) |
Each row is the same single-story house, grown or shrunk in both directions: the rooms, ceiling and slab grow with the floor area, the walls and windows with the perimeter, at the same 8.5 ft wall height. Well insulated is HeatAlgo's reference house as published: walls at U = 0.20, roof 0.15, slab 0.25, windows 0.90 W/(m²K), heat recovery ventilation at 85%, airtightness n50 = 1.5. Partly insulated has walls at U = 0.35, roof 0.30, slab 0.60, windows and doors 1.50 W/(m²K), no mechanical ventilation and n50 = 4.0. Uninsulated has walls at 1.00, roof 0.80, slab 1.00, windows and doors 2.60 W/(m²K), no mechanical ventilation, n50 = 7.0 and the thermal bridges of an unrenovated building. U = 0.20 W/(m²K) is 0.035 Btu/h·ft²·°F. These are example building elements, not any code's levels.
The second column is the European rule of 80-100 W/m², about 25-32 Btu/h per sq ft. It lands only near the middle column: for 1,500 sq ft it says 38,040-47,550 Btu/h, nearly twice what the well-insulated house needs and well under what the uninsulated one needs. Insulation moves the size more than threefold at the same floor area. And a bigger house needs more, but not in proportion: 2,500 sq ft is a little over twice the floor of 1,200 sq ft, and needs less than twice the heat.
How the reference house is calculated
The well-insulated house at its own size, 1,100 ft², loses 14,945 Btu/h (4.38 kW) on the -4 °F design day, calculated room by room to EN 12831-1:2017. HeatAlgo's preliminary heat pump sizing adds a 12% margin and looks for a unit of at least 16,754 Btu/h (4.91 kW) at a 104 °F water temperature. Every input is published, so the result can be checked by hand.
A rule per square foot knows nothing about insulation, air leakage or heat recovery. An oversized heat pump short cycles: for most of the season the house needs less heat than the compressor can give at its lowest speed. What a room-by-room heat loss calculation is, and what depends on it, is in the guide what a room-by-room heat loss calculation is.
How to check a heat pump quote
- Ask for the heat loss at your location's design temperature, not "the size for the square footage".
- Compare the heat pump's heating output at that temperature and at your water or air temperature, read from the unit's data sheet. The nominal tonnage is rated in milder weather.
- A margin over the load is a tenth or so, not a half. HeatAlgo's preliminary sizing adds 12%.
- Heating and cooling are two numbers. This page covers heating only; a heat pump that also cools is checked against the cooling load as well, which this calculation does not cover.
Sizing is preliminary and does not replace an installation design. The table gives an order of magnitude for one house shape on one design day; your house has its own walls, windows and design temperature. A quick estimate is in the heat pump calculator.
Work out your houseA HeatAlgo report follows EN 12831-1 and EN 1264, not ACCA Manual J, and is not accepted for permits or rebates.