How to Bleed a Radiator Step by Step

If a radiator is cold at the top but warm at the bottom, or gurgles, there is probably trapped air inside. Air gets in over time and prevents hot water filling the radiator, so the room heats less well. Bleeding the radiator lets the air out. It takes a few minutes.

What you will need

A radiator key, available cheaply from hardware shops, or a flat-bladed screwdriver for some valves.
A cloth or small towel and a small container to catch drips.
Something to protect the floor.

Step by step

Turn the heating on and let the radiators warm up fully, so you can see which ones have cold patches.
Turn the heating off and wait for the system to cool. Bleeding a hot system risks scalding and can draw more air in.
Find the bleed valve. It is a small valve at the top corner of the radiator, on one side.
Hold the cloth under the valve and fit the key. Turn it slowly anticlockwise about a quarter turn. You should hear a hiss as air escapes.
Wait for water. When the hissing stops and a little water appears, close the valve by turning the key clockwise. Do not overtighten.
Wipe up any water, and repeat for other radiators, starting with the one lowest in the house and working upwards.

Check the boiler pressure

Letting air out can reduce the pressure in a sealed heating system. Check the pressure gauge on your boiler. Most combination boilers should read within the range shown in the manual, commonly about 1 to 1.5 bar when the system is cold. If it has fallen too low, top it up using the filling loop as shown in your manual. If you are not sure how, ask a Gas Safe registered engineer rather than guess.

When bleeding does not help

The radiator is cold at the bottom and warm at the top. This is often a sign of sludge or debris building up inside, which may need a power flush or a magnetic filter and a cleaning treatment from an engineer.
The whole radiator is cold. The valve may be stuck or the system may need balancing. Check the thermostatic valve first.
You need to bleed often. Air repeatedly entering the system can indicate a leak or a fault and is worth having inspected.

How often should you bleed?

Once a year before winter is a sensible routine, or whenever you notice cold spots. A well-maintained system works more efficiently, so it also supports your efforts to cut bills. For more ideas see boiler settings that save energy and our low-cost energy saving guide.

If you smell gas or suspect a carbon monoxide leak, turn off appliances, open windows and call the national gas emergency number straight away. Bleeding radiators does not involve gas, but it is a good moment to check that your carbon monoxide alarm is working.