What size radiator does a room need?

Radiator sizing goes wrong in one specific way: the number on the box was measured under conditions your heating system does not run at.

Step by step

1. Measure the room properly.Length, width and height in metres. Height matters — a Victorian room with three-metre ceilings needs far more heat than a modern one of the same floor area.
2. Use an online calculator.A BTU or watt calculator asks for dimensions, glazing, insulation, external walls and room type, and gives a requirement. It is more reliable than any rule of thumb.
3. Answer the questions honestly.Two external walls, single glazing and an uninsulated floor all raise the figure considerably. Optimistic answers are the commonest reason a correctly-sized radiator underperforms.
4. Adjust for the room's use.Bathrooms want warmer than living rooms; bedrooms are usually kept cooler. Most calculators ask, and the difference is significant.
5. Check the delta T of the quoted output.This is the crucial step. Output is quoted at a temperature difference between water and room — commonly ΔT50, meaning roughly 75°C flow. If your system runs cooler, actual output is much lower.
6. Allow for a condensing boiler running cool.Modern systems are most efficient at lower flow temperatures. At ΔT30 a radiator gives roughly half its ΔT50 figure, so sizing on the headline number leaves the room cold.
7. Size generously for a heat pump.Heat pumps run at much lower flow temperatures still. Radiators are typically two to three times larger than the equivalent for a gas boiler, and this is the single biggest cause of disappointing heat pump installations.
8. Understand the panel types.Type 11 is one panel; type 21 and 22 have two panels with fins between; type 33 has three. A double-panel radiator gives far more output in the same wall space.
9. Choose dimensions to fit the space.Once you know the watts required, height and length are interchangeable. A tall narrow radiator and a long low one can deliver the same output.
10. Prefer slightly larger over slightly smaller.An oversized radiator with a TRV simply reaches temperature sooner and shuts off, and it lets you run the system cooler and more efficiently. An undersized one cannot be fixed.
11. Check the pipe centres.The distance between the valve tails. Choosing a replacement with different centres means altering pipework, which turns a swap into a plumbing job.
12. Consider what will be in front of it.A sofa, long curtains or a radiator cover can absorb a large share of the output. Size for reality rather than for an empty room.
13. Total up the whole house before replacing several.A boiler has a maximum output. Substantially upsizing every radiator in a house can exceed what the boiler and pipework can deliver.

Tips

• Check what delta T the output is quoted at. It is the number that makes correctly-sized radiators underperform.
• Match the pipe centres on a replacement and the swap stays a swap.
• Slightly oversized is fine and lets you run the system cooler. Undersized cannot be corrected.

Common mistakes

• Taking the headline output figure at face value — It is usually quoted at ΔT50, meaning around 75°C flow. A condensing boiler running at 55°C delivers roughly half that, so the room never reaches temperature.
• Replacing like for like without recalculating — The existing radiator may have been undersized all along, or the room may have been insulated or extended since. The old size proves nothing.

If it doesn't work

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