Use this guide when a radiator is warm at the bottom but cold at the top, or you hear gurgling in the pipes. You will need a radiator key, an old cloth or towel, a small container and your boiler manual.
Step by step
Turn the heating off and let it cool
Switch the central heating off and wait until the radiators are cool to the touch. Hot water can spray out and scald, and a running pump can draw air into the system. This usually takes around half an hour.
Find the bleed valve
The bleed valve is a small square fitting at one of the top corners of the radiator. Fit the radiator key into it and hold a folded cloth and container underneath to catch drips. Start with the radiators on the lowest floor, then work upwards, and check your boiler manual for any specific advice.
Release the air
Turn the key anticlockwise a quarter to half a turn, no more. You should hear a hiss as air escapes. As soon as water starts to come out steadily, turn the key clockwise to close the valve firmly but without overtightening.
Repeat for other radiators
Bleed any other radiators that were cold at the top, working in the same order. Wipe away any drips straight away, as radiator water can stain carpets. Keep the key somewhere you will find it next winter.
Check the boiler pressure
Look at the pressure gauge on the boiler or its filling loop, as bleeding lowers the pressure. Compare the reading with the range given in your boiler manual. If it is low and your manual shows how to top up using the filling loop, follow those instructions exactly; if you are unsure, contact a Gas Safe registered engineer.
Turn the heating back on and test
Switch the heating back on and let it run for about 20 minutes. Feel each radiator from top to bottom to check it heats evenly. Recheck the boiler pressure once the system is hot and again the next day.
Ready-to-use checklist
- Heating switched off and radiators cool
- Radiator key, cloth and container ready
- Started with the lowest radiator nearest the boiler
- Valve opened no more than half a turn
- Valve closed as soon as water appeared
- Boiler pressure checked against the manual
- Radiators tested for even heat
Practical tips
- Bleed radiators at the start of the heating season so problems show up before the cold weather.
- Take a photo of the boiler pressure gauge when the system is working well so you know what normal looks like for your home.
- Some modern radiators have a slotted bleed screw that opens with a flat screwdriver instead of a key.
Common problems
The radiator is cold at the bottom rather than the top.
This usually points to sludge settling inside, not trapped air, so bleeding will not help. A heating engineer can flush the system or radiator and may recommend fitting a magnetic filter.
The pressure keeps dropping after I top it up.
A repeated pressure drop suggests a leak somewhere in the system or a fault with a boiler component. Check visible pipework and radiator valves for drips, and call a Gas Safe registered engineer to inspect the boiler.
Radiators need bleeding again every few weeks.
Frequent air build-up can mean the system is drawing in air or producing gas from corrosion. Ask a heating engineer to check the system and whether inhibitor needs adding, rather than repeatedly bleeding.