Short answer: not in a new, well-insulated house. The 60 W per m³ rule gives HeatAlgo's reference house 15.9 kW of radiators. A heat loss calculation to EN 12831-1 gives 4.66 kW for the rooms added up, and 4.38 kW for the whole building. In every room the rule gives 2.2 to 4.1 times too much.
"A radiator sized by the 60 W per m³ rule - is that right?" The question keeps coming back on forums in Poland, Germany and the Netherlands. The rule is simple: the room's volume times 60 W. We checked it on one calculated house, room by room.
The 60 W/m³ rule against the calculation, room by room
HeatAlgo's reference house is a single-storey house in Warsaw, well insulated, with heat recovery ventilation. Every room is 2.6 m high, so the volume is the floor area times 2.6 m. The heat loss of each room comes from the heat loss calculation module, to EN 12831-1.
| Room | Volume | 60 W/m³ rule | Heat loss (calculated) | Difference |
|---|---|---|---|---|
| Living room | 72.8 m³ | 4,368 W | 1,340 W | +3,028 W (+226%) |
| Kitchen | 26 m³ | 1,560 W | 381 W | +1,179 W (+310%) |
| Bedroom | 41.86 m³ | 2,512 W | 753 W | +1,759 W (+233%) |
| Room 1 and Room 2, each | 30.94 m³ | 1,856 W | 450 W | +1,406 W (+313%) |
| Bathroom, 24 °C | 21.06 m³ | 1,264 W | 311 W | +953 W (+306%) |
| Hall | 26.52 m³ | 1,591 W | 553 W | +1,038 W (+188%) |
| Plant room, 16 °C | 15.6 m³ | 936 W | 418 W | +518 W (+124%) |
| Total | 265.72 m³ | 15,943 W | 4,655 W | +11,288 W (+242%) |
The rule oversizes every room of this house. The plant room least, the two small rooms most. The rooms add up to 4,655 W because each room is calculated for its own worst case. A heat pump is sized, preliminarily, for the whole building's 4.38 kW.
The 60 W/m³ rule against the room's heat loss (W)
Living room4,368 / 1,340 W
Kitchen1,560 / 381 W
Bedroom2,512 / 753 W
Room 1 and Room 2, each1,856 / 450 W
Bathroom24 °C1,264 / 311 W
Hall1,591 / 553 W
Plant room16 °C936 / 418 W
- The room's heat loss (calculated)
- Extra from the 60 W/m³ rule
Why the rule misses
The rule knows only the volume. Heat leaves through outside walls, windows, doors, the roof and the floor, and with the ventilation air. The rule sees none of it.
- Outside walls and windows. The plant room has two outside walls, a window and an outside door. It needs 26.8 W/m³ - the most in the house, although it is kept at only 16 °C.
- The room's temperature. The bathroom is at 24 °C, the warmest room, and needs only 14.8 W/m³. It has one outside wall and a small window. The rule treats 16 °C and 24 °C the same.
- Insulation and ventilation. Well-insulated walls, roof and floor and heat recovery cut every room's loss. The two small rooms need only 14.5 W/m³.
Even the right average for this house does not work. The rooms need 17.5 W/m³ on average. That average gives the plant room -35%, the hall -16% and the kitchen +20% against the calculation. No single number per m³ fits every room.
What it means for the radiator
Take the living room and a type 22 panel radiator, 600 mm high. With water at 75/65 °C and a 20 °C room, one metre of it gives about 1,694 W. The rule buys about 2.6 m of radiator, the calculation 0.8 m. At a lower water temperature the same metre gives less. Where the Radiators module takes a radiator's output from is in Radiator output: where the numbers come from.
A radiator that is too big will not overheat the room, because the thermostatic valve closes it down. With a heat pump a bigger radiator can even help, because it lets the water run cooler. But then you size it to the room's heat loss at that water temperature, not to the room's volume.
How to check your own room
- List the room's building elements: outside walls, windows, doors, the roof or ceiling and the floor.
- Calculate the room's heat loss to EN 12831-1. What that is and what depends on it is in the guide what a heat loss calculation is and what depends on it.
- Size the radiator for that loss at the water temperature you will really have.
What this number does not show
The figures are one house: new, well insulated, with heat recovery, calculated for Warsaw, whose design day is colder than anywhere in the UK or Ireland. In an old, poorly insulated house the loss per m³ is higher. There the rule may come closer, or undersize a room with big windows. That is why one number per m³ cannot replace a calculation. Heat pump sizing from a calculation is always preliminary. A HeatAlgo report is not an MCS heat loss calculation.
Check the radiators in your house