How to bleed a radiator
Quick answer: Turn the heating off and let the radiators cool. Fit a radiator key to the small valve at the top corner, hold a cloth underneath and turn it a quarter turn anticlockwise until air hisses out. Close the valve as soon as water comes out, then check the boiler pressure.
If a radiator is cold at the top and hot at the bottom, there is air trapped inside it. Bleeding lets the air out so the hot water can fill the whole radiator again. It is one of the most searched heating jobs in the UK, and it takes minutes.
Autumn is the usual time to do it, when the heating goes on for the first time after the summer.
What you will need
- A radiator key (a few pounds from any DIY shop, and many radiators also take a flat screwdriver)
- An old cloth or towel
- A small container or tray to catch drips
- Your boiler manual, if you need to find the pressure gauge or filling loop
Step by step
Turn the heating on and find the cold ones
Run the heating for 10 to 15 minutes and feel each radiator. Any that are cold at the top but warm at the bottom need bleeding. If the whole radiator is cold, that is a different problem (see below).
Turn the heating off
Switch the heating off at the thermostat or programmer and wait until the radiators are cool. This stops the pump pushing more air in and avoids hot water coming out of the valve.
Find the bleed valve
It is a small round or square fitting in the top corner at one end of the radiator, usually with a square or slotted centre.
Get the cloth ready
Hold the cloth or a small container under the valve. A little water will come out with the air.
Open the valve slowly
Fit the key and turn it a quarter turn anticlockwise. Do not take the valve right out. You should hear a hiss as the air escapes.
Close it when water appears
Keep the key on the valve. As soon as a steady trickle of water comes out, turn the valve clockwise to close it, hand-tight. Do not overtighten.
Check the boiler pressure
Bleeding lets water out of the system, so the pressure drops. Look at the gauge on your boiler: it should be about 1 to 1.5 bar when the system is cold. If it is low, top it up using the filling loop (see our boiler pressure guide).
Heat up and check
Turn the heating back on and check the radiator warms evenly. Check the valve is not weeping. Repeat on other cold-topped radiators, working from the one nearest the boiler outwards, ideally downstairs first.
Tips
- Bleed downstairs first, then upstairs, and re-check the pressure after each one.
- If a radiator needs bleeding every week, there may be a leak or a fault such as a failing pump or corrosion. Get it looked at.
- Some radiators have an automatic air vent. They do not need bleeding by hand.
Stop and get help if
- The radiator is cold at the bottom but warm at the top, or has cold patches in the middle. That points to sludge in the system, which needs a system flush by a heating engineer.
- Water keeps coming out and does not stop when you close the valve.
- The boiler pressure keeps dropping or the boiler locks out. Leave the boiler alone and call a Gas Safe registered engineer.
- Never open anything on the boiler itself or any gas pipe.
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Common questions
How do I know if a radiator needs bleeding?
It feels cold at the top and warm at the bottom, it makes gurgling or clicking noises, or the room does not warm up as it should while other radiators are fine.
Do I need to turn the boiler off to bleed a radiator?
Yes. Turn the heating off and let the radiators cool first. If the pump is running, it can pull more air into the system as you bleed.
Why is my boiler pressure low after bleeding?
Letting air and a little water out reduces the water in the sealed system. Top it up through the filling loop until the gauge reads about 1 to 1.5 bar cold.
More guides
Last reviewed 17 October 2026. This guide is general information for UK homes and is not a substitute for the manufacturer's instructions. If the job involves gas, mains wiring, asbestos, structural work or working at height, or you are unsure, ask a suitably qualified person.