Where the radiator is cold tells you the fix. Cold at the top and warm at the bottom means trapped air: bleed it. Some radiators warm and others not, especially those furthest from the boiler, usually means the system needs balancing. Cold at the bottom and warm at the top usually means sludge, and the system needs cleaning, often with a powerflush.
Cold at the top: bleed it
Air rises, so trapped air leaves the top of the radiator cold. With the heating off and cool, put a cloth under the bleed valve at the top corner, turn the radiator key slowly anticlockwise and let the air hiss out. Close it as soon as water appears. Then check the boiler pressure: bleeding lets pressure out, so you may need to top it up (see our boiler pressure guide).
If you’re bleeding the same radiator every few weeks, something is letting air in or producing gas in the system, and it’s worth getting it checked.
Some rooms warm, others not: balance the system
Water takes the easiest route. Radiators closest to the boiler often get hot quickly while the furthest ones stay lukewarm. Balancing means adjusting the lockshield valve on each radiator (the one under a plastic cap at the other end from the thermostat) so the flow is shared out evenly. It takes patience and a thermometer; we do it routinely after fitting radiators or a new boiler.
One radiator stone cold: check the valve
If a single radiator stays cold all over while the pipes to it are hot, the thermostatic valve pin may be stuck. Take the TRV head off and check the small pin underneath moves in and out. A gentle tap can free it; if it’s seized, the valve can be replaced. Also check both valves are actually open.
Cold at the bottom: that’s sludge
Over the years, rust and debris form a black sludge that settles at the bottom of radiators. Bleeding won’t shift it, and it makes the boiler work harder. Other signs are black or brown water when you bleed, a noisy or kettling boiler, and radiators that take ages to warm up. The fix is to clean the system, usually with a powerflush, then add inhibitor and fit a magnetic filter to keep it clean.
Powerflushing is all our sister company Kent Powerflush does, with the same engineer and the same number. Its guide to radiators cold at the bottom goes into more detail.
When to call a professional
Call us if radiators stay cold after bleeding and balancing, if the water from the bleed valve is black, if the boiler is noisy, if you keep losing pressure, or if you’d like a radiator moved, upsized or replaced. We’ll tell you honestly which fix you need, and if it’s a quick one, we’ll say so.
Quick questions
Why is my radiator still cold after bleeding?
If it was cold at the bottom, the problem is sludge, not air, so bleeding won’t help. If it’s cold all over, check the TRV pin isn’t stuck and that both valves are open. If several radiators are lukewarm, the system may need balancing.
Do I need a powerflush?
Common signs are radiators cold at the bottom, black water when you bleed them, a noisy boiler and slow heat-up. Our sister company Kent Powerflush can check your system and tell you if a powerflush is worth it.
Should I bleed radiators with the heating on or off?
Off, and cooled down. Hot water can spray out of the bleed valve, and you’ll get a more accurate reading of the pressure afterwards.
Which radiator should I bleed first?
Start downstairs at the radiator furthest from the boiler, then work closer, then do the same upstairs. Check the boiler pressure at the end.
This guide is general advice for simple checks. Never attempt work on gas appliances yourself: by law, gas work must be done by a Gas Safe registered engineer. If you smell gas, call the National Gas Emergency number on 0800 111 999.