Envelope · Thermal
Whole-house heat loss
A house needs exactly as much heat as it leaks, and it leaks through three things: surfaces, junctions and air. Add up the area of every surface times its U-value, add an allowance for the junctions between them, add the air that leaves and has to be reheated, and multiply the lot by how cold it gets. That number — in kilowatts — is what the boiler or the heat pump has to cover on the worst day of the year. It is almost always far smaller than the one on the appliance already installed.
The whole calculation
There is no cleverness in it. The care is all in the inputs.
The 0.33 is the heat capacity of air in convenient units: 1.2 kg/m³ × 1005 J/(kg·K) works out at 0.33 watt-hours per cubic metre per kelvin. So one air change an hour in a 250 m³ house costs 82 watts for every degree of temperature difference, whatever the walls are made of.
What the answer is for
- Sizing a heat pump. This is the number, and it is the only number. A heat pump is chosen so its output at the design outside temperature matches the design heat load — not its output at 7 °C, which is what the badge on the box says.
- Deciding whether radiators will work at 45 °C. Below roughly 40 W/m² a conventional radiator system can usually be run at heat-pump temperatures without replacing everything. Above 60 W/m², it usually can't.
- Knowing what to fix first. The breakdown above is worth more than the total. People spend on walls when the answer is air.
What the number should look like
| Design heat load | What that means |
|---|---|
| ≤ 10 W/m² | Passivhaus territory. The building can be heated through the ventilation air alone. |
| 10 – 30 | Very good. A small heat pump at low flow temperature, comfortably. |
| 30 – 50 | A decently insulated house. Heat pump works, emitters may need enlarging. |
| 50 – 80 | Insulated in places. Fabric work pays back before plant does. |
| > 80 | Largely uninsulated. Sizing plant for this is treating the symptom. |
Compare that with the rule of thumb still used to sell boilers — around 100 W/m², or "one kilowatt per room" — and you can see why so much plant is two or three times too big. An oversized boiler cycles, an oversized heat pump cycles harder and costs more to buy.
Worked example you can check by hand
The house this page opens with: 100 m² floor, 250 m³, 21 °C inside, −2 °C outside.
And the part that catches people out: the ventilation is 41.3 W/K, the single largest line on the sheet — bigger than all the walls put together, and two and a half times the roof and floor combined. Halve the leakage and the house needs 2.59 kW instead of 3.07, which is a whole heat-pump size. Insulating the walls from 0.30 to 0.18 saves less than that, and costs twenty times as much.
What this page does not do
- No solar and no internal gains. The design load is deliberately a worst-case: dark, cold, nobody home, no appliances running. That is correct for sizing plant and wrong for predicting a fuel bill. The annual figure is less strict — degree days to a 15.5 °C base already assume roughly 2.5 K of free heat from people, cooking and sunshine, which is why the base is not 18.
- One temperature everywhere. Real houses have a cold spare room and a hot bathroom, and the losses between rooms cancel out in the total but decide the radiator sizes. Room-by-room is a different calculation and you need it before you buy emitters.
- Ground floors are a fudge here. The 0.45 factor stands in for the whole of ISO 13370, where the real answer depends on the perimeter-to-area ratio and on the ground itself. For a compact plan it is close; for a long thin one it is optimistic.
- Steady state. No thermal mass, so no credit for a heavy building riding through a cold snap, and no penalty for intermittent heating.
- The y-value is an allowance, not a calculation. It is a reasonable stand-in until you have the junctions, and then you should work them out.