How to bleed a radiator and fix cold radiators
Cold radiators usually mean trapped air or sludge. Learn how to bleed a radiator safely and how to tell the difference between air and a deeper problem.
A radiator that is cold at the top but warm at the bottom usually has trapped air inside. Bleeding the radiator releases that air and lets the hot water circulate properly again. It is a simple job most people can do safely in a few minutes.
However, a radiator that is cold at the bottom, or stays cold after bleeding, often points to a different problem. Here is how to do the task properly and when to call a heating engineer.
How to bleed a radiator safely
You will need a radiator key, a cloth, and a small container to catch drips.
- Turn the heating off and let the radiators cool down.
- Fit the radiator key to the bleed valve at the top corner of the radiator.
- Turn the key slowly anti-clockwise. You should hear a hiss as air escapes.
- Hold the cloth under the valve and close it the moment water starts to come out.
- Wipe the valve, turn the heating back on, and check the boiler pressure.
Bleeding a radiator releases air, which can lower the system pressure. If the pressure drops too far, you may need to top it up using the boiler filling loop.
Why radiators stay cold after bleeding
If a radiator is cold at the bottom but warm at the top, the likely cause is sludge — a build-up of rust and debris at the base of the radiator. Bleeding only removes air, not sludge.
Other causes of cold radiators include:
- A closed or stuck radiator valve.
- An unbalanced system where some radiators get more flow than others.
- A faulty pump or low water pressure.
If bleeding does not fix it, the system may need a professional clean called a powerflush. Our guide to what is a powerflush explains how it removes sludge and restores efficiency.
When to call a heating engineer
Call an engineer if:
- Radiators stay cold after bleeding and checking the valves.
- Several radiators are cold at once.
- The boiler loses pressure repeatedly (see our guide to boiler pressure).
- You notice unusual noises, leaks or a black, sludgy fluid from the bleed valve.
For help with bleeding, cold radiators or a full central heating check, our heating service covers homes, landlords and businesses across London. For wider winter preparation, our guide to boiler maintenance mistakes is worth a read.
The Energy Saving Trust has practical advice on running your heating efficiently and keeping radiators working well.
