Bleeding a Radiator

By Rob Hull on July 23, 2010

Most domestic radiators will need bleeding at some point and it is important for the efficiency of your central heating boiler that it is done properly. Luckily it is a job that you can normally do yourself providing you follow a few basic rules.

When to bleed a radiator

It is quite easy to tell when a radiator needs bleeding as the top section will remain a lot cooler than the bottom section, or in severe cases the entire radiator will stay cold when the heating system is turned on. This happens because trapped air displaces the hot water that normally heats the radiator and it is this air that needs to be released during the bleeding process. Once the air is released, the hot water can flow freely again.

Where does the air come from?

Small amounts of air enter the central heating system whenever fresh water does, i.e. every time you use some water in the house. This means that in larger houses, where a considerable amount of water is used each day, the radiators may need to be bled more often than in smaller houses where water is used in less significant quantities.

Where to start when bleeding a radiator

The air in the central heating system will generally rise to the highest point it can – as air is lighter than water – and so you will probably find that the radiators on the upper floors will need bleeding before those on the ground floor. If you find that those on the ground floor are not heating properly then it might be worth bleeding all of the radiators in the house.

When bleeding a radiator it is important to have the central heating system turned off. This is because some water pumps – depending on where in the system they are fitted – can actually suck more air into the radiator and thus the heating system if they are turned on while you open the bleed valve.

How to bleed a radiator

Bleeding a radiator is a relatively simple task. You just need a bleed key, also known as a radiator key and an old piece of cloth to catch any drips. To start, fit the bleed key into the valve and turn it in an anti-clockwise direction for about half a turn. Once the valve is open you should hear the hiss of air being released. Continue to release the air until water begins to drip from the bleed valve and then close the valve by turning it half a turn in a clockwise direction. It is important not to over tighten the valve though. If you have a pressurised sealed system then releasing trapped air may cause the internal pressure to drop and so this should be topped up using the manufacturer’s guidelines.


The process of bleeding a radiator is a useful DIY skill to know, however, occasionally it doesn’t rectify the problem of cold radiators. In these cases a professional should be called as it may mean that the entire central heating system needs to be bled and this is often a slightly bigger job.

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