How to use this calculator
- Enter or choose units.
- Enter or choose length.
- Enter or choose width.
- Enter or choose side-wall (eave) height.
- Enter or choose ridge height.
- The heating load updates as you type. Use Copy result or Share to keep it.
Method
Q = U × A × ΔT + 1206 J/(m³·K) × V × ACH × ΔT ÷ 3600; A = walls + gable ends + roof
- U
- heat transfer coefficient of the covering
- A
- covering area
- ΔT
- inside minus outside temperature
- V
- house volume
- ACH
- air changes per hour; air holds 1.2 kg/m³ × 1005 J/(kg·K)
Worked example
- Area = walls 2 × 39 × 3 = 234 m² + gables 9 × 2 = 18 m² + roof 2 × 30 × √(4.5² + 2²) = 295.47 m² → 547.47 m².
- Covering loss = 4 × 547.47 × 20 = 43.80 kW.
- Leakage = 1206 × 1080 × 1 × 20 ÷ 3600 = 7.24 kW; total 51.03 kW (174,133 BTU/h).
Holding 15 °C at −5 °C outside needs about 51 kW (174,000 BTU/h) of heat for this 30 × 9 m house.
What the result means
Choose a heater with output at or above the load (allowing for its efficiency), and consider a second skin or thermal screen to cut the covering loss.
Assumptions and limits
- Steady-state loss at the design temperature; no solar gain.
- The floor and soil heat exchange is ignored.
- Wind, radiation to a clear night sky and thermal screens change the real load.
Questions
Why does the covering dominate?
Glazing and films conduct heat far more readily than insulated walls, so area × U-value is usually most of the load.
How do I convert kW to BTU/h?
Multiply by 3,412.14.