How to estimate radiator output for a room
A radiator needs to give out at least as much heat as the room loses on a cold day. Start from the room's heat loss in watts, correct for the water temperature your boiler or heat pump runs at, then pick a radiator whose catalogue output at ΔT50 is at least that big. Lower flow temperatures need bigger radiators.
Start from the room's heat loss
The heat a room loses depends on its size, its walls, windows, floor and ceiling, how well they're insulated, and how cold it is outside. The heat loss calculator works this out room by room, in watts. It's the figure everything else follows from.
Rules of thumb based only on floor area or room volume ignore insulation and windows, so they can be well out either way. If you're replacing a boiler or fitting a heat pump, a heating engineer will normally do a room-by-room heat loss calculation: ask for the figures.
What ΔT50 means
Radiator catalogues give each radiator's output at ΔT50, the rating set out in BS EN 442. ΔT (delta T) is the difference between the radiator's average water temperature and the room temperature. At ΔT50, the water is on average 50 degrees warmer than the room.
Most systems run cooler than that, so the radiator gives out less than its catalogue figure. The radiator ΔT converter shows how much less. For a radiator rated at 2,000 W at ΔT50, run at ΔT30:
- correction factor = (30 ÷ 50)^1.3 = 0.515
- output = 2,000 W × 0.515 = 1,030 W
The 1.3 is the radiator exponent: a typical figure that the calculator starts from. Use the maker's own figure or correction table where they give one.
Work out your ΔT
Your ΔT comes from the flow and return temperatures of your system and the temperature you want the room at:
- mean water temperature = (flow + return) ÷ 2
- ΔT = mean water temperature − room temperature
Ask your heating engineer what flow and return temperatures your boiler or heat pump will run at. Heat pumps and condensing boilers run cooler than older boilers to work efficiently, which is why radiators often need upsizing when the heat source changes.
Size the radiator
The radiator size calculator does the whole sum for each room: output to buy at ΔT50 = heat loss ÷ correction factor. For a living room that loses 1,000 W and is heated to 21 °C:
| Flow 70 °C, return 60 °C | Flow 45 °C, return 40 °C | |
|---|---|---|
| Mean water temperature | 65 °C | 42.5 °C |
| ΔT | 44 K | 21.5 K |
| Correction factor | 0.847 | 0.334 |
| Output to buy at ΔT50 | 1,181 W | 2,996 W |
The same room needs a radiator about two and a half times as big, by catalogue output, at the lower temperatures. You can enter the ΔT50 output of two or three radiators you're considering, and the calculator shows the smallest that's enough for each room.
Fitting more than one radiator
If one radiator big enough won't fit the wall space, split the output between two or more: their outputs at your ΔT add up. Taller, deeper or double-panel radiators give more output from the same wall width. Check the catalogue output for each size at ΔT50, then correct it to your ΔT.
When to get a heating engineer
These figures help you plan and compare radiators. A heating engineer should size the system, including the boiler or heat pump, pipework, pump and controls, and check the work. Any work on a gas boiler or gas pipework must be done by a Gas Safe registered engineer. The radiator size calculator links on to the boiler and heat pump size calculators for the next step.