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Central Heating &  Gas Engineers

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How To Re-Pressurise a Central Heating System:

To keep your boiler working efficiently, it is essential that any air that has found its way into the system be removed or 'bled' from the radiators. Common symptoms of excess air are: 

  1. Cold/cool radiators

  2. Noise from the boiler, pump or system

You will first need to establish if you have pressurised central heating.  All combination boilers are pressurised, as are some conventional systems.  A pressurised system will have a pressure gauge which is usually located on the boiler facia however some models have the pressure gauge on the bottom, these will need to be viewed from below the boiler.  This gauge will be graduated in 'bars' and should read between 0.8 and 1.5 bar (usually 1 bar) when the system is cold.  Gradual loss of pressure is normal.

Combined Temperature/Pressure Gauge

Combined Temperature/Pressure Gauge


If your system is conventional and you have a feed and expansion tank (central heating header tank) your system will be topped up automatically and you will only need to bleed the radiators as detailed below.

If your system is pressurised, you should regularly check the pressure gauge and top it up if necessary.  

Pressure is increased by adding water from your cold water main. There will be a device called a filling loop somewhere on the system. The filling loop will usually be a flexible connection below the boiler (see Fig.1) but in some cases this is an integral part of the boiler (see Fig.2) or, it can be located behind the washing machine, in an airing cupboard or under the kitchen sink. If in doubt, consult your boiler installation instructions or your installer.

Open the valve or valves on the filling loop whilst watching the pressure gauge.  

Water can usually be heard moving into the system - do not over-pressurise. 

External Filling Loop

External Filling

Integral Filling Loop
Integral Filling Loop

When the gauge shows the correct pressure, turn off the valve or valves and bleed the radiators as detailed below.  Constantly check the pressure level at the gauge and top up when necessary, until all radiators have been bled of excess air.

Finally, ensure that the pressure reading is correct and that the filling loop valve/valves are closed.  

To Bleed a Radiator:

You will need a bleed key (available from any hardware store) and an absorbent cloth/tissue.

If the central heating is on, turn it off and wait until it has cooled down enough - a central heating system is filled with hot and sometimes extremely dirty water so precautions should be taken to protect floor coverings and decorations.

Start with the lowest radiator on the system and work towards the top. Some hot water cylinders also need to be bled.  Holding a cloth to catch any water spillage, slowly and gradually loosen the bleed screw on the radiator (do NOT remove the bleed screw) until any air (evidenced by hissing) or water starts to dribble out.  If there is no air in the system water will appear immediately. Close the bleed screw - do not over tighten.

Bleed Key

Bleed Key

Releasing Air from Radiator
Using Bleed Key

Releasing Air from Radiator Using Bleed Key

Finally, ensure that the pressure reading is correct and that the filling loop valve is closed.  

Regularly check the pressure level at the gauge and top up when necessary.

If your radiators are still cold/cool (especially at the bottom), or the water in your system appeared particularly dirty, your system may need powerflushing.Powerflushing Information