Why Correct Sizing Matters
Heat pump sizing is one of the most critical factors in system performance. An undersized pump will run continuously and struggle to heat your home on the coldest days. An oversized pump short-cycles, reducing efficiency and causing premature wear. The goal is to match the heat pump's output capacity to your home's heat loss at the design temperature.
What Is a Heat Loss Survey?
A heat loss survey (also called a room-by-room heat loss calculation) measures the heat demand of every room in your property at the design outdoor temperature (typically -3°C for most of the UK). It considers: room dimensions, insulation values of walls, roof, and floor, window and door areas and U-values, and ventilation rates. This gives a total heat demand in kilowatts.
Rough Sizing Guide
As a very rough guide (a proper heat loss survey should always be done):
- Well-insulated 2-bed flat: 3–5 kW
- Well-insulated 3-bed semi: 5–8 kW
- Average-insulated 3-bed semi: 7–10 kW
- 4-bed detached (moderate insulation): 10–14 kW
- Large detached house: 14–20 kW
The Impact of Insulation
Improving insulation before installation is always beneficial. Loft insulation, cavity wall insulation, and double glazing can reduce your heat demand by 20–40%, meaning you need a smaller, cheaper heat pump with lower running costs.
MCS Requirement for Heat Loss Surveys
MCS standards require that all heat pump installations are preceded by a detailed heat loss survey using approved software (such as Heat Geek or MCS-approved calculation tools). Any installer offering a quote without conducting this survey is not following MCS guidelines.
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