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TOPIC HUB 02 · SIZING
Heat loss & heat pump sizing: a balance, not a number
Correctly sizing a heat pump is not as simple as matching a kW figure to a house. A proper design needs to establish how much heat the property loses during cold weather, what flow temperature the heating system needs, how much output the selected heat pump can actually provide at those conditions and how well it can reduce its output during milder weather.
- Topic hub
- Updated August 2026
- By Phil · Heat Loss Hub
THE SIZING BALANCE
Heat-pump sizing is really a balance between maximum output, minimum output, building heat loss, flow temperature, emitter design and hydraulic performance. The aim is not simply to choose a heat pump that is “big enough” — it is to choose one that matches the property as closely as practical across the heating season.
The eight-step heat pump sizing process
Survey the building, calculate room-by-room heat loss, establish whole-house design heat loss, set the design outside temperature, determine the lowest practical design flow temperature, check the heat pump’s actual output, check minimum modulation, verify emitters and hydraulics. This is why a heat-pump design cannot be reduced to one number.
Why floor area, boiler size and EPC data are not enough
Rules such as 50 or 100 watts per square metre cannot account for differences in construction, insulation and exposure. A property with a 30 kW boiler does not necessarily have a 30 kW heat loss. An EPC is not the same as a detailed room-by-room heat loss calculation.
The design point: the number that actually matters
A whole-house heat loss of 7.4 kW at a design outside temperature of -3°C and design flow temperature of 45°C might be described as A-3/W45. Manufacturer performance data needs to be checked at conditions representative of the actual design — the number in a heat-pump model name should never be treated as the whole answer.
Why oversized heat pumps can cycle
If a property needs 2 kW but the heat pump cannot reduce output below 4 kW, it cannot continuously match demand — leading to cycling. This is known as cycling. Some cycling is normal, but excessive cycling can reduce seasonal efficiency and operating stability.
Imagine most rooms can meet their heat loss at a 40°C flow temperature, but one large room appears to require 50°C. A better design question is: can the emitter in that one room be improved so the whole system can operate at a lower temperature? The weakest room should be identified and addressed rather than forcing the whole system hotter.
Sizing does not stop at the heat pump
Suitable emitters, adequate water flow, suitable pipework, enough system volume and appropriate controls all matter. A perfectly selected heat pump connected to a poorly designed heating system can still perform badly.
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