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Heat pump sizing from your heat loss
Oversizing is the most common and most expensive mistake in a heat pump install: it costs more upfront and cycles badly at part load. This gives you a defensible starting number to check a quote against — it is not a substitute for a room-by-room heat-loss calculation.
Specific heat loss by fabric condition
| Fabric condition | W/m² |
|---|---|
| Solid wall, single glazing, no loft insulation | 100–130 |
| Cavity wall unfilled, double glazing, some loft insulation | 70–100 |
| Filled cavity, double glazing, 250 mm loft insulation | 45–70 |
| Post-2010 build, or a deep retrofit | 25–45 |
| Passivhaus or near it | 15–25 |
A quote 30 % above this figure is not automatically wrong — but the installer should be able to explain why, from a room-by-room calculation. “We always fit a 12 kW” is not an explanation.
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Get quotes for your property
Compare at least three installers. Ask each one for the heat-loss calculation and the design flow temperature in writing — that number decides your running cost more than the brand on the box.
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