The radiator is warm at the bottom but cold at the top. Or it gurgles when the heating comes on. That usually means air is trapped inside, and letting it out (bleeding the radiator) takes about ten minutes.
What you need
- A radiator key (from any DIY shop or hardware store). Some newer radiators use a flat-head screwdriver instead.
- An old cloth or towel
- A small bowl or cup
How to bleed a radiator
- Turn the heating on and wait until the radiators are warm, then turn it off and let them cool. Radiators can be very hot, and the water inside can scald.
- Find the bleed valve. It's a small square fitting at one of the top corners.
- Hold the cloth under the valve. Put the key on and turn it anticlockwise a quarter to half a turn. You'll hear a hiss as the air comes out.
- When water starts to drip out, turn the key clockwise to close it. Don't overtighten.
- Repeat on any other radiators that are cold at the top.
- Check your boiler pressure. Bleeding lets out a little water. If the gauge has dropped below 1 bar, top it up. See how to repressurise a boiler.
- Turn the heating back on and check the radiators heat evenly.
Still cold after bleeding?
- Cold at the bottom, warm at the top: usually a build-up of sludge. It may need a power flush.
- One radiator stays cold: check both valves are open. The thermostatic valve pin may be stuck.
- Needs bleeding every week: air is getting in somewhere, which can point to a leak or a system fault.
Water dripping from the valve after bleeding? See leaking radiator repair.
Common questions
Which way do you turn a radiator key to bleed it?
Anticlockwise, about a quarter to half a turn, to let the air out. Turn it clockwise to close it as soon as water appears.
Should the heating be on or off when bleeding radiators?
Off, and the radiators should be cool. Turn the heating on first to build up the trapped air, then switch it off and let it cool before bleeding.
Why is my radiator cold at the top?
Air is trapped inside. Bleeding the radiator lets it out. If it's cold at the bottom instead, it's usually sludge.