Heat Pump Sizing Calculator

The model

Each climate zone has a baseline heating and cooling intensity in BTU per hour per square foot at design conditions. That baseline is then adjusted by four multipliers:

load = area × base(zone) × envelope × glazing × ducts × (height / 8)

Ceiling height enters as a ratio against a standard 8 ft because the baselines are expressed per square foot of floor, and a taller room holds proportionally more air to condition.

Baseline intensities

These will look low if you have seen the rules of thumb that circulate in the trade, which put zone 4 heating nearer 35–40 BTU/h·ft². Measured Manual J design loads in real existing homes usually come in well under that — commonly 15 to 28 BTU/h·ft² for heating — and the inflated rules of thumb are a large part of why so much residential equipment is oversized. We would rather start from the measured range and let the envelope multiplier move you up if your house deserves it.

They remain averages across a very wide spread of buildings, which is exactly why the result is a screening figure and not a specification.

Why oversizing is the common failure

Contractors routinely size heat pumps from the furnace being replaced, and that furnace was usually oversized too. An oversized heat pump short-cycles: it reaches setpoint fast, shuts down, and never runs long enough to dehumidify or to settle into its efficient modulating range. The result is worse comfort, worse humidity control and a shorter compressor life, at a higher purchase price. If in doubt, size to the load, not to the old equipment.

Cold climate note

In zones 5 to 7 the published capacity of a heat pump falls as the outdoor temperature drops. A unit rated at 36,000 BTU/h at 47°F may deliver 22,000 at 5°F. Size against the manufacturer's capacity table at your design temperature, not against the nameplate, and decide deliberately how much backup heat you are willing to accept.

Sources

The constants and default values above come from these public sources. Every one is free to read and none of them are ours — check the figures against them.

• ACCA Manual J — Residential Load Calculation The actual standard. This calculator is a screening estimate; Manual J is what your installer should be running.
• DOE — Building America Climate-Specific Guidance IECC climate zone definitions and the design conditions behind our baseline intensities.
• NEEP — Cold Climate Air Source Heat Pump List Capacity at low outdoor temperatures for specific models — essential in zones 5 to 7, where nameplate capacity misleads.
• ENERGY STAR — Air Source Heat Pumps Efficiency criteria and product listings.