Heat Pump Sizing Calculator
One System, Two Loads to Satisfy
A heat pump has to be sized for both jobs it does — cooling in summer and heating in winter — and in most climates those two loads aren't equal. Sizing to the cooling load alone can leave a home under-heated on the coldest days; sizing to heating alone can mean short-cycling and poor humidity control in summer. This calculator computes both loads and sizes the equipment to whichever one is larger.
The Formula
Heating Load = Area × Heating Factor × Insulation Multiplier × (Ceiling Height / 8)
Design Load = max(Cooling Load, Heating Load)
Cooling factor runs from 20–30 BTU/sq ft depending on climate; heating factor runs 25–50 BTU/sq ft, generally higher because heating loads in cold climates tend to exceed cooling loads in the same climate. Whichever load is larger determines both the equipment tonnage and whether the system is described as heating- or cooling-driven for that climate.
A Worked Example
| Load Type | Value |
|---|---|
| Cooling Load (1800 × 20 × 1.0 × 1.0) | 36,000 BTU/hr |
| Heating Load (1800 × 50 × 1.0 × 1.0) | 90,000 BTU/hr |
| Design Load (heating-driven) | 90,000 BTU/hr (7.5 tons) |
In this cold-climate example, heating load is more than double the cooling load — the equipment has to be sized to the heating side, not cooling.
Where This Calculation Matters
- Heat pump replacement or new installation — avoiding a common mistake of sizing purely to cooling tonnage, which can leave a cold-climate home under-heated.
- Cold-climate heat pump evaluation — understanding how much larger the heating load is than cooling helps when deciding whether supplemental (backup) heat is needed.
- Dual-fuel system design — some installations pair a smaller heat pump sized to cooling load with a furnace for the coldest days rather than sizing to the full heating load.
- Comparing climates — the same home's design driver can flip from heating- to cooling-dominated depending on climate zone.
How to Use This Calculator
- Enter the Room/Space Area in square feet.
- Enter the Ceiling Height in feet (defaults to 8).
- Select the Climate Zone and Insulation quality.
- Select Calculate to see cooling load, heating load, and the design tonnage.
Related Calculations
Compare against a straight AC/furnace split system using the HVAC Load Calculator and Furnace Size Calculator.