Heat Pump Sizing Calculator
A heat pump that is too small will not keep up on the coldest days, while one that is too large will short-cycle and waste energy. This calculator estimates the right BTU capacity and tonnage for your home by factoring in square footage, IECC climate zone, insulation quality, ceiling height, and number of stories.
Heat pump sizing starts with a base cooling load in BTU per square foot, which varies by climate zone. Hot-humid zones (1-2) need about 25 BTU/sq ft because of high outdoor temperatures and humidity. Cold zones (6-7) need less cooling capacity at 15 BTU/sq ft but significantly more heating capacity.
Unlike traditional furnaces that produce the same BTU output regardless of outdoor temperature, heat pump capacity drops as it gets colder outside. In zone 5-7, the heating load is 30-60% higher than the cooling load because the system must overcome a larger temperature difference. This calculator accounts for that by increasing the heating BTU estimate in colder climates.
Insulation quality has a major impact. A poorly insulated home can need 30% more capacity than the same home with standard insulation. A well-sealed, modern build can get away with 15% less. If you have had a professional energy audit (blower door test), use those results over the estimates here.