HEATLOSSPRORoom-by-room heat loss from a whole-house figure
A gas bill, an energy certificate or a quick estimate gives one number for the whole house. To size radiators or underfloor heating for a heat pump you need a number for every room. Here is why sharing the total by floor area goes wrong, and how far apart the rooms can be.
In short: dividing the total by floor area gives every room the same W/m², but a corner room under the roof can lose twice as much per m² as a hall in the middle of the house. Use the share below only to see the spread; size the radiators from a real room-by-room calculation.
Share the total over the rooms
Why floor area is not enough
Heat leaves a room through its outside surfaces and with its air. Two rooms of the same size can be very different:
- Outside walls and windows. A corner room has two outside walls and often two windows; a room in a terrace or in the middle of the house may have one or none.
- Roof and floor. Rooms under the roof lose heat upwards, rooms on a solid ground floor lose heat downwards, and rooms on a middle floor of a flat lose very little either way.
- Room temperature. A bathroom at 22 °C next to a bedroom at 18 °C has about a fifth more temperature difference to the outside at −3 °C.
- Air. Kitchens and bathrooms with extractor fans, or rooms with an open chimney, lose more with their air.
The whole-house figure is still useful: it sizes the heat pump, and it is a good check on the sum of the room figures.
What the room figure is for
Each radiator or underfloor loop has to cover its own room's heat loss at the heat pump's flow temperature, which is much lower than a boiler's. A radiator that was fine at 70 °C may give less than half as much at 45 °C. See radiator output at heat pump temperatures and underfloor heating output per m², and how to calculate one room's heat loss by hand.
In the HeatLoss Pro designer you trace the floor plan once and every room gets its own heat loss from its walls, windows, roof and floor, which add up to the whole-house figure. Each radiator and underfloor zone is checked against its room at the flow temperature you choose. Drawing is free; a full calculation with a PDF report costs €1.
Frequently asked questions
Can I divide the whole-house heat loss by floor area to size radiators?
Only as a very rough first guess. Rooms with two outside walls, big windows, a roof above or a solid floor below lose far more per m² than a small room in the middle of the house, and a bathroom kept at 22 °C loses more than a bedroom at 18 °C. A share by floor area is often 20–50% out for single rooms, which is the difference between a radiator that is big enough and one that is not.
How do I get room-by-room heat loss?
Calculate each room on its own: the area and U-value of each outside wall, window, roof and floor, times the temperature difference, plus the ventilation loss of its air. The room figures add up to the whole-house figure. Drawing the floor plan in a heat loss program does this for every room at once.
Why does room-by-room heat loss matter for a heat pump?
A heat pump runs at a low flow temperature, so radiators give much less than they did with a boiler. Every radiator or underfloor loop must cover its own room's heat loss at that temperature, so you need each room's figure, not just the total.
What W/m² should a room have?
It depends on the house and the room. In a well-insulated house 30–50 W/m² is common; in an older house 80–120 W/m², with corner rooms under the roof higher still. The calculator above shows how far rooms in the same house can differ.
Do the whole house in 5 minutes
Trace your floor plan and see every room's heat loss, the heat pump size and the radiator check as you draw. Free to try, nothing to install, no sign-up.
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