The answer
Why can't an installer size a heat pump from floor area alone?
Two rooms of the same size may need different amounts of heat because their windows, walls and exposure differ. A survey should check each room before the installer chooses the pump and decides whether radiators need changing.
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See what the installer measured
Your report should show room sizes, walls, windows, ventilation and the temperatures used in the calculation. If the construction of a wall is uncertain, the installer should say what was assumed.
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The figures affect more than pump size
The total heat loss helps choose the heat pump. Each room's figure helps decide whether its radiator and pipes can deliver enough heat, and where insulation might make a useful difference.
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A calculation is still an estimate
Heat-loss figures are not a promise of future energy use. Ask about any assumptions and why extra capacity has been allowed, so the system is not made unnecessarily large.
What should your heat-loss report show?
The report should show each room's size, windows, wall and floor construction, ventilation and the temperatures used in the calculation. One total for the whole house helps size the heat pump, but it cannot show whether the back bedroom's radiator will keep that room warm. You need a separate figure for every room and its radiator or underfloor heating.
If an older wall or hidden floor cannot be checked, the report should say what was assumed. Ask whether measuring it later could change the pump size or only which radiators need replacing. Include any planned extension, because it may change the answer.
What to check next
Your next steps
Where we checked the facts
Check the original information.
These official pages back up the rules and technical details above. They can change, so check them again before spending money or starting work.
Apply this to your own property
Could a heat pump work at your property?
Every property is different. A free survey checks heat loss, hot water and radiators to see whether a heat pump may fit before a system is priced.