How to bleed a radiator

2 min read · 8 steps · Updated 3 October 2026

A radiator that is hot at the bottom and cold at the top has air trapped inside. The air takes the place of the hot water, so the room stays cool while the boiler keeps burning fuel. Letting the air out is called bleeding, takes a few minutes for each radiator, and needs only a key that costs very little in any DIY shop.

If you rent, tell your landlord about problems with the heating or with appliances that belong to the flat before you try anything beyond these simple steps.

Step by step

1. Find the radiators that need it

   Turn the heating on and, once it is warm, feel each radiator carefully. Cold at the top and hot at the bottom, cold patches, or gurgling and hissing sounds are the signs of trapped air.

   Done

2. Turn the heating off and wait

   Switch the heating off and let the radiators cool down. This matters: with the system hot and the pump running, hot water can spray out of the valve and scald you.

   Done

3. Get the tools ready

   You need a radiator bleed key, or a flat-head screwdriver for many modern radiators, a cloth to grip with, an old towel for the floor and a small container to catch the water.

   Done

4. Open the bleed valve a little

   The bleed valve is a small square or slotted screw at one top corner of the radiator. Hold the cloth and the container under it. Turn the key anti-clockwise, slowly, a quarter to half a turn. Do not unscrew it completely.

   Done

5. Let the air out

   You hear a hiss as the air escapes. Keep the valve open until the hissing stops and water comes out in a steady dribble.

   Done

6. Close the valve

   Turn the key clockwise until the valve is closed. Firm is enough: do not force it. Wipe the valve dry and go on to the next radiator.

   Done

7. Check the boiler pressure

   Bleeding lets pressure out of the system. Look at the pressure gauge on the boiler. With the heating off it should show about 1.0 to 1.5 bar. If it is lower, top the system up as the boiler manual describes, or ask your landlord or the building manager.

   Done

8. Turn the heating back on

   After a while, feel the radiators again. They should now be warm from top to bottom.

   Done

Good to know

In houses with several floors, plumbers usually start with the radiator furthest from the boiler on the lowest floor and work towards the top.

The water that comes out is often dark. That is normal, but it stains carpets, so keep the towel in place.

In many blocks of flats with central heating for the whole building, tenants cannot top up the pressure themselves. Bleed only if the landlord or the house rules allow it, and report it afterwards.

Call a heating engineer if a radiator stays cold after bleeding, if you cannot find the valve, if the pressure keeps dropping, or if you see a leak.

More ways to spend less on heating: how to lower your energy bill.

In short: Heating off and cool, cloth and container under the valve, key anti-clockwise until the hissing stops and water comes, close, check boiler pressure (about 1.0 to 1.5 bar cold).