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. This air gets released when you bleed your radiator. The hot air will be able to flow freely when the air is released.

Where does the air come from?

When fresh water enters the central heating system a small amount of air will do so too. Basically whenever you use some water. 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

Air being lighter than water, it means that any air in your central heating system will rise to the highest point possibly. It generally means that the radiators on the upper floors of your home will require bleeding before radiators on the ground floor. If  you do have ground floor radiators which aren’t heating properly it’s a good idea to bleed every radiator in the house.

Don’t forget to make sure you have the central heating system switched off before you start the process of bleeding a radiator. This is very important because some water pumps  - depending on where in the system they are fitted – will actually suck more air into the radiator and consequently the heating system if they are turned on while you open the bleed valve.

How to bleed a radiator

It’s a pretty easy and simple task – although you do need a radiator key, also known as a bleed key, and an old piece of cloth to catch any drips. Start by fitting the radiator key into the valve and then turn it in an anti-clockwise direction for about half a turn. When 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.

Conclusion

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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