The heating's on, the boiler's running, but one radiator is stone cold. Or it's warm at the top and cold at the bottom. Where it's cold tells you a lot about what's wrong, and most causes are easy to fix.
First, feel where it's cold
- Cold at the top, warm at the bottom: there's air trapped inside. You need to bleed the radiator.
- Cold at the bottom, warm at the top: usually sludge (rust and dirt) sitting in the bottom of the radiator.
- Cold all over, other radiators hot: a valve is shut or stuck.
- Several radiators cold, or all of them: the problem is with the system or the boiler, not one radiator.
If one radiator is cold all over
- Check the thermostatic valve (the numbered knob). Turn it fully up to 5 or the highest setting.
- If it still won't heat, the pin inside the valve may be stuck. Take off the numbered head (it unscrews or unclips) and you'll see a small metal pin. Press it gently a few times with your fingers or tap it lightly. It should spring up and down.
- Check the valve on the other side (the lockshield, usually under a plastic cap). If someone has turned it fully closed, open it a turn or two.
If it's cold at the bottom
Sludge builds up over years, especially in older London systems. It blocks the bottom of the radiator so hot water can't flow through. Bleeding won't fix it. The radiator needs flushing out, or the whole system needs a power flush if several radiators are affected. A plumber can do this, and fitting a magnetic filter afterwards stops it coming back.
If several radiators are cold
- Check the boiler pressure. If it's below 1 bar, the heating may not circulate properly. See how to repressurise a boiler.
- Upstairs cold, downstairs hot: often low pressure or air in the system. Bleed the upstairs radiators, then top up the pressure.
- Downstairs cold, upstairs hot: often sludge collecting in the lowest radiators, or the system needs balancing.
- No heat anywhere: check the thermostat, the timer and the boiler display for a fault code. Boiler faults need a Gas Safe registered engineer.
When to call a plumber
Call us if a valve is leaking, the radiator still won't heat after bleeding and checking the valves, you need a flush, or a radiator is dripping. Read about leaking radiator repair.
Common questions
Why is my radiator cold at the bottom?
Usually sludge, a build-up of rust and dirt, sitting in the bottom of the radiator. Bleeding won't fix it. It needs flushing out.
Why is my radiator cold at the top?
There's air trapped inside. Bleed the radiator with a radiator key until water comes out, then check the boiler pressure.
Why is only one radiator not working?
The thermostatic valve may be turned down or its pin stuck, or the lockshield valve may be closed. Check both before calling a plumber.