Guide to bleeding your radiators

Bleeding a radiator

If your radiators have not heated up properly and you can feel a patch of cold to the top of the unit, it’s more than likely that you have trapped air in the system which needs releasing so the water can flow around the pipes uninhibited.

Trapped air in a central heating system is not a good thing as it can cause the following issues:

  • the creation of an “airlock” literally stops water flowing around all or part of the heating/hot water circuits, resulting in cold radiators and cold rooms
  • reduction of your system performance, meaning more money spent on heating with little improvement to output
  • increase the chance of corrosion, reducing the lifespan of your heating system and boiler


Getting started

Before you get started you need to know what kind of heating system you have:

  • open vented heating system
  • sealed heating system

If you are not sure which heating system you have have a look in your loft space – if you have a header or expansion tank then your system is open vented, if not you have a sealed system.

If you want to find out more about header and expansion tanks to help you identify one, please read our Basic Guide to Header and Expansion Tanks.

Bleeding radiators is easy with an open vented system as it means the water will automatically be added to the system when you bleed out the air.

A sealed system has an expansion vessel (rather than a tank), a pressure gauge and a filling loop. The loop will refill the system with water once you have bled the radiators.

We recommend you only bleed the radiators on a sealed system when they are cold so you can obtain an accurate pressure reading on the pressure gauge.

You will also need the following tools:

  • a radiator key – which are available from most good DIY stores
  • a clean cloth – ready to dab up any water trickles

Step by step

Now you know what kind of system you have and you are ready with your tools, we can get started:

Step 1

Turn off your boiler so it won’t fire up or run any pumps.

Step 2
Go to the radiator which requires attention and locate the bleed vent. The vent is generally on the top corner of one side on your radiator.

Step 3
Insert your radiator key and turn it anti clockwise to gently open the bleed vent. (Be careful if the bleed vent is stiff. They are made of brass and you can easily round off the end or snap it if you are too heavy handed.) You will start to hear a hissing sound as the air escapes. Hold your cloth just under the vent as you do this.

Step 4
When water seeps out of the bleed vent gently close it until the water stops. The water may be dirty so be sure to have your cloth ready to catch any trickles that run down.

Water will seep out of bleed vent

Step 5
Turning the boiler back on:

For an open vented system
Turn the boiler back on, the expansion/ header tank should top up your system if needed. There is nothing else you need to do.

For a sealed system

You will need to check the pressure by looking at the pressure gauge generally located on the front of the boiler or on the expansion vessel if a separate one is fitted.

Typically your heating system will be set between 1 and 1.2bar. If it’s dropped below this you will need to turn the filling loop on. You will either have one black handle at one end, or two black handles at either end of the filling loop*. Turn the handle (or handles) just a quarter of a turn so they are inline with the loop, this will let the water in. Always turn the filling loop slowly as you are only topping up the system. Remember, this is mains water pressure so water will enter the system quickly if you fully open the valve(s). When the pressure reaches this level turn the filling loop off and check the needle on the gauge has stabilised. Now you are ready to turn your boiler back on.

* Some filling loops have a screw driver slot instead of a black handle but the principle is exactly the same.

Do I have to bleed my radiators in a particular order?
This is a question our heating heroes get asked quite a lot, and the answer is actually quite simple. Ideally you start with the lowest and closest radiator the boiler first, as the air rises through the system. For many people this is a radiator in the room where there boiler is situated – like a kitchen on the ground floor. As you make your way upstairs it doesn’t matter which radiator you start and finish with, even if your property is large enough to warrant a completely separate and managed heating zone.

Contact us

For further information about our services, please contact Clarity Heating, or call us on 01353 862315.