What size heat pump does my home actually need?

MIS 3005 requires a room-by-room heat-loss calculation using the fabric U-values, ventilation rates and design outdoor temperature for your postcode. The output is a peak load in kilowatts at the coldest expected day, plus a per-room emitter schedule showing whether each radiator is big enough at the proposed flow temperature. The quote should include both.

If a proposal shows a single headline kW figure and a total price with no room-by-room table, the sizing has not been done. Ask for it in writing before you sign.

A quick "one hundred watts per square metre" figure lands close for a mid-terrace with average insulation and completely wrong for a solid-walled Victorian end-of-terrace or a well-insulated new build. The same house at either end of the sizing error costs money for different reasons: undersized units run flat out and can't hold temperature in a cold snap; oversized units short-cycle, wear their compressors out early and hit a lower SCOP for the whole life of the system.

The lower the design flow temperature, the higher the SCOP; but the larger the radiators need to be. A well-designed retrofit typically lands between forty-five and fifty-five degrees flow, which usually means upsizing radiators in the two or three worst rooms rather than the whole house. Ask which rooms the survey wants to change and whether that fits how you use those rooms.