Heat Loss and Design · 4 lessons
Heat loss: how big a heat pump does a house need?
A heat pump is sized from how much heat the house loses on a cold day, room by room, not from the size of the old boiler.
Have a go
0Which is a fabric-loss path?
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Two ways heat escapes
Heat leaks out through the walls, roof, floor and windows: that's fabric loss. It's also carried away by air leaving the house and being replaced by cold air: that's ventilation loss. A heat loss calculation adds both up for every room.
One wall, worked through
Fabric loss is the U-value times the area times the temperature difference. A wall with a U-value of 0.4 W/m²K, 15 m² in area, with 20 °C between inside and out, loses 0.4 × 15 × 20 = 120 W. Do that for every surface in every room.
The design day
The calculation uses a cold outdoor design temperature for the location and a target temperature for each room. The MCS design standard, MIS 3005-D, points to the BS EN 12831-1 method; a simple air-change sum is a way to learn the idea, not the full method.
Why bigger isn't better
Rooms are matched first, then the building's total sets the heat pump. A unit much bigger than the loss switches on and off too often, which is called short cycling, and that wastes energy and wears the compressor.
Key numbers
- 120 W
- A 15 m² wall at U = 0.4 W/m²K with a 20 °C difference: 0.4 × 15 × 20.
Common mistakes
- Sizing the heat pump to match the old boiler's output.
- Adding up the whole house without checking each room can be heated.
- Forgetting ventilation loss and counting the walls and windows only.
The lessons this guide comes from
- Where a house loses heat
- Fabric heat loss
- Ventilation loss and design temperatures
- Room-by-room assessment and sizing