Methodology
How the radiator check works
Estimates which rooms’ radiators would work with a heat pump, which could be swapped for a deeper radiator in the same space, and which may need a larger one. It uses the boiler’s setting and each radiator’s type, so nothing needs measuring.
How it works
If a radiator keeps its room warm now, it gives out enough heat at the boiler’s water temperature. A heat pump runs cooler water, and a radiator’s output falls with the difference between water and room temperature (to the power 1.3, under BS EN 442). The check works out how much bigger each radiator would need to be at the heat pump’s temperature. A radiator’s length cancels out of that, which is why nothing is measured.
The boiler’s water temperature comes from its heating dial. A 1–6 dial is read as 40–80°C, an unmarked dial as 40–80°C by position, and a dial nobody has changed as 70°C. Water is assumed to cool by 20°C on its way round a boiler system and by 5°C with a heat pump.
For each room, the household says whether the boiler can get it warm on a cold day. A room that overheats has radiator to spare, and one that never gets warm has too little (×0.8 to ×1.25). Rooms kept cool on purpose are not adjusted and are marked untested.
Swaps are compared by heat output per square metre of wall: a Type 22 gives about 1.8 times the output of a Type 11 in the same space. A room within 15% of enough is marked borderline, as that is within the method’s margin of error.
The dial setting is a maximum, and most radiators were fitted with some margin, so the check tends to overstate what’s needed. If the household has kept the house warm with the boiler turned down, that lower temperature is used. If not, the check suggests a week-long test at a lower setting (never below 50°C).
Figures used
Radiator output exponent
1.3
BS EN 442, the standard radiator outputs are rated to · Not re-checked
Temperature drop round the system
20°C boiler, 5°C heat pump
Typical design values · Not re-checked
Boiler setting if never changed
70°C
Where installers usually leave a combi’s heating dial · Not re-checked
Room temperatures
living room 21°C, kitchen 18°C, dining room 21°C, hall / landing 18°C, bedroom 18°C, bathroom 22°C, study / office 21°C
Design temperatures used in heat-loss calculations (CIBSE, as used by MCS) · Not re-checked
Panel radiator output per m² of wall
Type 10 1,070 W, Type 11 1,633 W, Type 21 2,242 W, Type 22 2,887 W, Type 33 4,167 W
Stelrad Compact 600 mm catalogue figures at a 50°C difference. Type 10 from typical catalogue values · Not re-checked
Column radiators and towel rails
Typical catalogue values
Not from a single product · Not re-checked
Comfort adjustment
×0.8 to ×1.25
Our judgement, kept coarse on purpose · Not re-checked
Borderline band
Up to 15% short
Our judgement of the method’s margin of error · Not re-checked
Running cost saving per degree lower
About 2%
A rule of thumb from a simple model of heat pump efficiency, not a measured figure · Not re-checked
Limits
- It assumes each radiator is about right for its room now. The comfort question and the turn-down test correct for this.
- Without a turn-down test it is cautious, and most homes will do better than it suggests.
- Pipe size doesn’t affect the result. Microbore pipes are flagged for the installer.
- An MCS-certified installer still needs to calculate the heat loss of every room before quoting.
Last reviewed 1 October 2026. Next review due by 1 October 2027. Try the radiator check.