Bleed a RadiatorBleeding a radiator is an easy job which you may need to do from time to time to produce an efficient heating system. On this page we explain how.

Radiator Problems

A regular problem with wet central heating systems is radiators which are not as warm on the top as they at the bottom. The problem is usually caused by a build up of air pockets in the radiator. The most likely reasons for air being trapped in the system are:

  • Corrosion in the system releasing gas.
  • Air entering your central heating system when the water is being topped up.

Although this is nothing serious it is clearly not energy efficient and you need to fix it. It can be fixed easily by bleeding the radiator.

How to Bleed a Radiator

How to Bleed a Radiator

You need to open the ‘air bleed valve’ or ‘bleed screw’ as it is known usually found at the top of the radiator. You need to use a special ‘bleeding key’ or ‘radiator key’ for this. This will allow the air to escape and lift the water up to the top. The water system will then be filled up.


How to Bleed a RadiatorBleed Screw

The bleed screw is the device which siphons off the air or allows it to bleed. It basically removes the air or gas from the system and allows pressure and density differences to force the water up. The screw is normally a hexagonal or square shape found inside a small round knob sticking out at the top side of the radiator.

How to Bleed a RadiatorBleed Key

The bleeding key is similar to one used for winding a clock. You simply attach and turn and you will hear the air hissing as it escapes. The main thing to remember when operating the bleeding key is be ready to close the key as soon as water starts to escape.

How to Bleed a RadiatorIt’s worth switching off the central heating system while you do this. Another useful tip is to wrap a towel around the key and valve to catch any water that bleeds out. Once the radiator is topped up the radiator should heat evenly and your system will now be working more efficiently.

Cold Radiators

How to Bleed a RadiatorRemember, if you do find the top is significantly cooler than the bottom then the radiator definitely needs bleeding. But if the radiator is almost completely full of air you may find the whole radiator becomes cool. Again, you should use the bleed key. You will find that this should rectify your cool radiator problem.


It is often necessary to bleed the entire boiler/radiator system. If you bleed all the radiators you will flush the boiler/radiator system out and help clean it of rust. You do this by continuing to drain the radiators and refill until you notice clear water replacing what is likely to be slightly dirty water on account of the build up of rust. It is also a good idea to put anti corrosion liquid into the system once you have done this to try and prevent further corrosion.

Condenser Boilers

If you have serious problems with your boiler it may be worth considering upgrading to a condenser boiler or High Efficiency or HE boiler as they are also known. These boilers are more expensive to buy but will save you money in the longer run.

Basically, they extract more heat from the flue gases and are more energy efficient. You can easily save about a third of your standard gas bills compared to a conventional boiler. They are also made from corrosion resistant materials. So as well as saving you money, they run better, are more environmentally friendly and can add value to your home.