How Do I Know If My Radiator Is Big Enough for The Room?

If a room never gets properly warm, even when its radiator is hot all over and the rest of the house is comfortable, the radiator may simply be too small for the room. Every radiator gives out a set amount of heat, and if the room loses heat faster than that, it will always feel chilly. A bigger radiator, or a different type, is often the fix.

This post explains how to tell an undersized radiator from other common problems, what decides how much heat a room needs, the difference between single and double radiators, and when it's worth getting an engineer to work it out. You can make the checks below yourself, but swapping a radiator means draining the heating system, so it's a job for a heating engineer.

First, rule out the usual problems

Before blaming the radiator's size, make sure it's working properly. With the heating on for half an hour, carefully feel the radiator from top to bottom:

• Cold at the top, warm at the bottom: usually trapped air. Bleeding the radiator should fix it.

• Hot at the top, cold at the bottom: usually sludge (a build-up of rust and dirt) sitting in the bottom of the radiator.

• Warm in some rooms, cold in others: the system may need balancing, which means adjusting each radiator's valves so hot water reaches them all evenly.

• Hot all over, but the room is still cold: this is the sign the radiator may be too small.

Also check the thermostatic radiator valve (the numbered dial on the side) is turned up, and that furniture or long curtains aren't blocking the heat. Our posts on radiators not getting hot and how to balance radiators go through these in more detail.

What decides how much heat a room needs

Radiator output is measured in watts, or in BTUs (British Thermal Units, another unit of heat; one watt is about 3.4 BTUs an hour). How much a room needs depends on more than its floor area:

• Size: length, width and ceiling height. High ceilings need more heat.

• Walls: outside walls lose more heat than inside ones, and older solid walls lose more than insulated cavity walls.

• Windows and doors: large windows, single glazing and outside doors all add to heat loss.

• What's above and below: a room over a garage or under a cold loft loses more heat.

• What the room is for: living rooms are usually heated to around 21°C, bedrooms to around 18°C.

So two rooms the same size can need very different radiators. A front room in an older house, with a bay window and two outside walls, may need far more heat than a bedroom of the same size in the middle of the house.

Single and double radiators

Radiators of the same height and width can give out very different amounts of heat, depending on their type. You can usually tell yours by looking down through the top grille:

• Single panel: one flat panel, sometimes with metal fins on the back.

• Double panel: two panels side by side, usually with fins between them.

Fins (called convectors) heat the air passing over them, so they add a lot of output. A double panel radiator with fins can give out roughly one and a half to two times the heat of a single panel the same size. That's why swapping a single radiator for a double, in the same space on the wall, is a common way to warm up a cold room without needing a wider radiator.

One of our customers had 8 single radiators replaced with 7 doubles. As they put it: "The house is so much warmer".

Working out whether yours is big enough

Online radiator calculators can give a rough idea. You enter the room's size, its windows and outside walls, and they suggest the output you need. You can then compare it with your radiator's output, which is usually on its label, in the paperwork or on the manufacturer's website if you know the model.

Treat a calculator as a guide only. A heating engineer can work out the heat loss of each room more accurately, and also look at whether the pipes feeding it are big enough. In homes with microbore pipework (very thin pipes, often 8mm or 10mm), a larger radiator won't help if the pipe can't carry enough hot water to it.

Why bigger radiators can also save energy

A modern condensing boiler works most efficiently when the water coming back to it is cooler, generally below about 55°C. With radiators that are big enough for each room, the boiler can heat the house at a lower temperature, so it spends more of its time in its most efficient range.

If you're thinking about a heat pump in the future, larger radiators matter even more, as heat pumps run at lower temperatures than boilers. Sizing radiators properly now can save changing them twice.

What's involved in changing a radiator

Changing or upsizing a radiator means draining down part or all of the heating system, fitting the new radiator and valves, and refilling and bleeding the system. If the new radiator is a different width, the pipes may need moving too. Once the system is refilled, it should be dosed with inhibitor, a liquid that slows down the corrosion that causes sludge.

It's also a good time to think about a magnetic filter, which catches sludge before it reaches the boiler, or a power flush if the system is already dirty. New radiators work best in clean water.

Getting it right first time

Green Flame Plumbing and Heating is based in Porth and is Gas Safe registered, OFTEC registered and a Which? Trusted Trader. We fit, move and replace radiators, whether that's one cold bedroom or a whole house, and we'll explain the options before any work starts. If a room in your home never feels warm enough, find out more about our central heating installation service and book a visit.