Heat Loss Calculator
Estimate a home's heat loss on a cold day and the furnace or boiler size it needs.
About the Heat Loss Calculator
Heat escapes through walls, windows, roof and floor in proportion to their area, their U-value (how easily heat passes through) and the indoor–outdoor temperature difference, plus heat carried away by air leaking in. This simplified whole-house estimate models the home as a box with typical U-values for its insulation level. Oversized furnaces cycle on and off and waste energy, so a proper load calculation is worth doing.
The formula
Q = Σ U × A × ΔT + 0.33 × ACH × volume × ΔT (W, with U in W/m²K and volume in m³)
U-values used (W/m²K) and air changes per hour
| Level | Walls | Roof | Floor | Windows | ACH |
|---|---|---|---|---|---|
| Poorly insulated (older, single glazing) | 1.5 | 1 | 0.8 | 4.8 | 1.5 |
| Average (some insulation, double glazing) | 0.6 | 0.3 | 0.5 | 2.8 | 0.8 |
| Well insulated (modern code) | 0.28 | 0.15 | 0.25 | 1.4 | 0.5 |
| Very high performance | 0.15 | 0.1 | 0.15 | 0.8 | 0.3 |
Worked example
Example: Design heat loss = 54,807 BTU/h (16.1 kW)
- Inputs: Heated floor area: 2,200; Area in: ft²; Storeys: 2; Ceiling height: 2.8 m; Insulation level: Average (some insulation, double glazing); Windows as % of wall area: 17%; Indoor temperature: 77; Outdoor design temperature: 11; Temperatures in: °F; Furnace / boiler efficiency (AFUE): 100%.
- Formula: Q = Σ U × A × ΔT + 0.33 × ACH × volume × ΔT (W, with U in W/m²K and volume in m³)
- Result: Design heat loss = 54,807 BTU/h (16.1 kW)
Things to keep in mind
- An estimate for budgeting; size equipment from a professional room-by-room load calculation.
Questions people ask
What's a design temperature?
A cold-day temperature exceeded 99% of the year in your area; local building codes list it.
Why include air leakage?
In older homes, draughts can be a third of the heat loss.