Short answer: about 5 kW, not 8-10 kW - if the house is well insulated. HeatAlgo's reference house, 102.2 m², needs 4.59 kW to EN 12831-1 at a -20 °C Polish design temperature (Warsaw), and preliminary sizing looks for a heat pump of at least 5.14 kW (a 12% margin) at a 40 °C flow temperature. The 80-100 W/m² rule would give this house 8.2-10.2 kW.
"The company has come back recommending a 12kW," writes a homeowner on a UK self-build forum, against a calculation of about 7.6 kW. That is the usual picture: the calculation says one thing, the quote another, and floor area muddles both.
The calculation against the rule
Heat for a 102.2 m² house at -20 °C
80-100 W/m² rule: 8.2-10.2 kW
Building heat loss4.59 kW
fabricPreliminary sizing target (+12%)5.14 kW
- fabric (walls, roof, floor, windows) 3.44 kW
- ventilation 0.53 kW
- thermal bridges 0.62 kW
- sizing margin 12%
HeatAlgo's reference house is a fictional single-storey house in Warsaw: 102.2 m², eight rooms, heat recovery ventilation at 85% and airtightness n50 = 1.5 1/h. HeatAlgo's engine calculates it room by room to EN 12831-1:2017. Fabric losses are 3435 W, ventilation 534 W, thermal bridges 618 W. The total is 4587 W, or 44.9 W/m². Every input is published, so the result can be checked by hand.
The 80-100 W/m² rule knows nothing about insulation, airtightness or heat recovery. For this house it overshoots by one and a half to nearly two times. An oversized heat pump short-cycles: for most of the season the house needs less heat than the compressor can give at its lowest speed. It cuts the other way too: an uninsulated house of the same size needs several times more than 4.59 kW.
Floor area does not carry the load, even within one house
In the same house, the load per square metre varies nearly twofold between rooms:
| Room | m² | W | W/m² |
|---|---|---|---|
| Living room | 28.0 | 1396 | 49.9 |
| Kitchen | 10.0 | 400 | 40.0 |
| Bedroom | 16.1 | 786 | 48.8 |
| Room 1 | 11.9 | 473 | 39.7 |
| Room 2 | 11.9 | 473 | 39.7 |
| Bathroom (24 °C) | 8.1 | 329 | 40.6 |
| Hall | 10.2 | 573 | 56.2 |
| Plant room (16 °C) | 6.0 | 429 | 71.5 |
| Sum of rooms | 102.2 | 4859 | 47.5 |
The room that loses most per metre is not the living room but the small plant room: two outside walls, a window and an outside door over 6 m² of floor, although it is heated only to 16 °C. One figure for the whole house cannot see that. The rooms add up to 4859 W, more than the building's 4587 W: each room carries its own worst-case share of air infiltration, and for the whole house those shares cancel out (EN 12831-1).
Heat pump sizing: how to check an installer's quote
- Ask for the heat loss at your location's design temperature, not "the size for the floor area".
- Compare the heat pump's output at that temperature and at your flow temperature, read from the unit's data sheet. The model name gives the output in milder weather.
- A margin over the load is a tenth or so, not a half. HeatAlgo's preliminary sizing adds 12%. For this house its verdict is "tight headroom": the catalogue unit covers 5.14 kW, but with less than the 0.51 kW of headroom the module looks for.
Sizing is preliminary and does not replace an installation design. The result is for one well-insulated house; a table across insulation standards needs separate calculations. The -20 °C Warsaw design day is colder than anywhere in the UK or Ireland, where the same house needs less. What a heat loss calculation is, and what depends on it, is in the guide what a heat loss calculation is and what depends on it. A HeatAlgo report is not an MCS heat loss calculation.
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