How to match a radiator to a room, read the output figures properly and spot when the problem isn't the radiator at all.
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What size radiator do I need for my room?
M Spencer Plumbing & Heating
If a room never quite gets warm in winter, or you're replacing a radiator, the size you need depends on how much heat the room loses, not just how big it is. A small room with two outside walls and a big window can need more heat than a larger room in the middle of the house. The right radiator is one whose heat output, measured in watts or BTUs, matches that heat loss.
This post explains what decides how much heat a room needs, how to read radiator outputs without being caught out, the checks you can make yourself, and when a bigger radiator won't fix a cold room.
What decides how much heat a room needs
Heat escapes from a room through its walls, windows, floor and ceiling, and through draughts. The more it loses, the bigger the radiator needs to be. The main things that affect it are:
Size: the room's floor area and ceiling height.
Outside walls: each wall that faces outdoors loses far more heat than an inside wall.
Windows and doors: glass loses more heat than a wall, and single glazing much more than double.
Insulation: solid walls, common in older houses, lose more heat than insulated cavity walls. A room under an uninsulated loft or over a cold garage needs more too.
What the room is for: living rooms and bathrooms are usually heated to around 21 to 22°C, while bedrooms are often comfortable at around 18°C.
Online radiator calculators ask for these details and give a rough figure. They're a reasonable starting point, but a proper heat loss calculation, room by room, is more accurate.
Watts, BTUs and what the figures mean
Radiator output is given in watts (W) or British thermal units (BTU). One watt is about 3.4 BTU, so a 1,000W radiator is about 3,400 BTU.
The catch is the "delta T" figure next to the output. Delta T (ΔT) is the difference between the average temperature of the water in the radiator and the temperature of the room. UK manufacturers now quote outputs at ΔT50, which assumes the water is around 70°C. Some older catalogues and cheaper sellers quote ΔT60, which makes the same radiator look about 25% more powerful.
So always compare radiators at the same delta T. A radiator listed as 1,000W at ΔT60 gives roughly 790W at ΔT50.
Why a modern boiler changes the sums
Condensing boilers work most efficiently when the water coming back from the radiators is cooler, ideally below about 55°C. Running the heating at a lower temperature saves gas, but the radiators then give out less heat. At ΔT30, a radiator gives roughly half its ΔT50 output.
If you're planning to run your heating cooler, or might switch to a heat pump one day, it's worth sizing radiators a little larger now. A bigger radiator at a lower temperature can heat a room just as well, and more cheaply.
Single, double or triple panel
Radiators of the same height and width can give very different amounts of heat:
Single panel (type 11): one panel with fins on the back. Slim, but the lowest output.
Double panel (type 21 or 22): two panels, with one or two sets of fins. A type 22 gives roughly twice the heat of a type 11 the same size.
Triple panel (type 33): the most heat for the wall space, but deep, so it sticks out further.
If you can't fit a wider radiator, a double or triple panel in the same space is often the answer.
Checks you can make before calling us
These are safe to do yourself and help whoever looks at the room:
Measure the room: length, width and ceiling height.
Note the outside walls and windows, and whether the glazing is single or double.
Look at the radiator: measure its height and width, and count the panels and sets of fins from above.
Feel the radiator with the heating on. If it's cold at the top, it needs bleeding. Our guide on how to bleed a radiator shows how.
Compare it with the others. If radiators near the boiler are hot and those further away are lukewarm, the system may need balancing. See how to balance a radiator.
Don't remove a radiator or drain the system yourself: getting the pressure and inhibitor right afterwards is a job for an engineer.
When a bigger radiator won't help
A cold room isn't always down to the radiator's size. If a radiator is warm at the top but cold at the bottom, sludge has probably built up inside it, and a new or bigger radiator will soon clog in the same way. Cleaning the system with powerflushing usually brings it back.
A stuck thermostatic radiator valve (TRV), the dial on the side that controls each radiator, can also stop one heating up. And if every room is struggling, the boiler or the heating controls may be the problem rather than any single radiator.
Getting radiators sized and fitted
M Spencer Plumbing & Heating fits new radiators and whole central heating systems. Our Gas Safe registered engineers can look at the rooms that never warm up, check whether the radiator, the system or the controls are at fault, and recommend radiators that suit each room and your boiler.
If a room in your home never quite gets warm, you can book a visit through our central heating and radiator installation service.
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