Heating Load Calculator
Calculate heating BTU/hr from indoor design temperature, outdoor design temperature (by climate zone), envelope insulation, and infiltration.
Inputs
Sets outdoor design temperature.
Tight new build ≈ 0.3, average ≈ 0.5, leaky ≈ 1.0+
Results
Visualization
Where the heat escapes
On a cold design day, heat flows from inside (warm) to outside (cold) through every surface and every crack. The diagram below shows the two main paths — conduction and infiltration — and how they split for different home sizes and climate zones.
Heat-loss visualization
Where the heat escapes
Conduction (red arrows) is heat lost through walls, ceiling, windows, and floor. Infiltration (blue arrows) is heat lost to cold air leaking in through cracks. Both scale with the indoor-outdoor temperature difference at design conditions.
Formula
The heating load formula
Heating load is conduction (envelope U×A×ΔT) plus infiltration (1.08 × CFM × ΔT). Both scale with the indoor-outdoor temperature difference at design conditions.
U = U-factor (1/R), A = surface area in sq ft, ΔT in °F.
Infiltration CFM = (Volume × ACH) ÷ 60.
15% safety margin above design heating load.
Reference
Heating BTU/hr by climate and home size
| Home size | Zone 3 (warm) | Zone 5 (cool) | Zone 7 (very cold) |
|---|---|---|---|
| 1,000 sq ft | 20,000 | 35,000 | 50,000 |
| 1,500 sq ft | 30,000 | 55,000 | 75,000 |
| 2,000 sq ft | 40,000 | 75,000 | 100,000 |
| 2,500 sq ft | 50,000 | 90,000 | 125,000 |
| 3,000 sq ft | 60,000 | 110,000 | 150,000 |
Average insulation, 9-foot ceilings, 0.5 ACH. Older homes (1.0+ ACH) need ~25% more; well-insulated new builds (R-21 walls, 0.3 ACH) can drop 30–40%.
Pitfalls
Common heating load mistakes
- Using the wrong design temperature — pulling the historical low instead of the 99% design
- Ignoring infiltration in tight new builds — even at 0.3 ACH, infiltration is 10–15% of load
- Forgetting duct losses in unconditioned attic/crawl — adds 15–25% to the system size
- Sizing a heat pump on the AHRI 47°F rating instead of the design-temp rating
- Not accounting for window orientation — north-facing windows lose more heat than south-facing
- Skipping the night setback assumption — design load assumes 24/7 setpoint, not setback
Heating load FAQ
Quick answers to common HVAC sizing questions.