Is it air?
Air collects at the top of a radiator, so the classic sign is a radiator that is hot at the bottom and cold at the top. You may also hear gurgling or trickling in the pipes. If that sounds familiar, bleeding it should help.
What you need
- A radiator key, from any hardware shop. Some newer bleed valves take a flat screwdriver instead
- An old cloth or towel
- A small container to catch drips
Step by step
- Turn the heating off and wait until the radiators are cool. Hot water can scald, and a running pump can draw air in rather than let it out
- Find the bleed valve: a small square fitting at one of the top corners of the radiator
- Hold the cloth under it, fit the key and turn it anticlockwise about a quarter of a turn. You should hear air hissing out
- When water starts to spit or run steadily, close the valve by turning it clockwise. Firmly, but do not force it
- Wipe up, and repeat on any other radiators that need it. If several do, start downstairs and work up
- Turn the heating back on, and check the radiator is warm all over once it has had time to heat up
Check the boiler pressure
Bleeding lets out water as well as air. On a combi or system boiler with a pressure gauge, check it once the system is cold again. If it has dropped below the figure in your manual, top it up with the filling loop; our guide to low boiler pressure explains how. Regular boilers with tanks in the loft refill themselves.
When bleeding does not fix it
- Cold at the bottom, warm at the top: that usually means sludge, a build-up of rust and debris that settles in the radiator. Bleeding will not shift it. A powerflush or chemical clean usually will, ideally with a magnetic filter fitted afterwards
- Needs bleeding every week or two: air or gas is getting into the system, often through a small leak or corrosion. Worth getting looked at before it causes damage
- One radiator cold all over: check the valves at both ends are open. The pin under a thermostatic radiator valve head can also stick
- The whole house is cold: it is not the radiators. Check the boiler is on, the pressure is right and the thermostat and timer are calling for heat. If they are and nothing happens, book a repair