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Calculate the heating load

The heating load is the heat your building loses on the design day, in kW. It comes from heat flowing through walls, roof, floor and windows (fabric loss) plus heat carried away by air leaking or being ventilated out.

How

1. Get the area and U-value of each envelope element (walls, roof, floor, windows, doors) from your design module. A lower U-value means better insulation.
2. Multiply each U-value (W/m2K) by its area (m2) and add them up. This is the fabric heat loss coefficient in W/K.
3. Calculate ventilation loss as 0.33 x air changes per hour x house volume (m3), also in W/K.
4. Add fabric and ventilation coefficients, then multiply by your design deltaT to get the design heat load in watts.
5. Multiply the total coefficient by heating degree days x 24 / 1000 to estimate annual heating energy in kWh.
6. Use the heat loss calculator below, then rerun it each time the design changes (better windows, more insulation).

Do these first

• design-1

Sources

1. Energy in buildings: heat loss calculation. The Open University, OpenLearn, accessed 2026-11
2. HVAC heat loss calculations (course M319). PDH Online, accessed 2026-11

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