How to bleed a central heating system

Are your radiators not working as well as they should? If your central heating’s on but your radiators aren’t warming up properly, they may have air trapped inside them. So here’s how to bleed your radiators and get your central heating running smoothly again.

Peter Earl From the Energy team
minute read

What does bleeding a central heating system involve?

Bleeding your radiators releases any air that’s trapped inside your system, stopping the hot water from flowing and heating your home as it should.

Split 1

How do I know when to bleed my central heating system?

Is your room taking longer to heat than normal? If so, check the following:

  • Feel your radiators. Are there cold patches? If it’s cold at the top, there’s air in the system. If your radiator’s cold at the bottom, you may have a build-up of sludge. This is slightly more complicated and might involve an engineer call out.
  • Listen to your radiators. Is your radiator making gurgling or clanking noises? These are signs of air trapped in the system, which bleeding should fix.
  • Look for damp patches, condensation and mould. These can be caused by uneven heating –radiators that need bleeding might be the culprit.

How do I bleed a radiator?

You don’t have to be an engineer to bleed a central heating system. Anyone can do it. Just follow these simple steps:

  • Turn off your central heating and if your radiators are warm, leave them to cool down sufficiently to avoid scalds.
  • Start with the radiator lowest in the system and work towards the highest. If they are on the same level, start with the one closest to the boiler.
  • Place a tea towel/cloth or bowl under the valve to catch any escaping water.
  • Turn the radiator key anti-clockwise, about a quarter of a turn. You’ll hear a hiss as the air starts to escape.
  • Once the air’s out, it’ll start to drip water. When the water starts running smoothly, you’ll know that all the air’s out and you can tighten the valve again. Don’t over tighten it, as this can damage the valve.
  • Turn your central heating back on.
  • Check your radiator. Is it heated evenly throughout? If so, you’ve fixed the problem.

Repressure you central heating system

Now check your boiler pressure. Bleeding the radiators can cause the pressure to drop. It should read between 1 and 1.5 bar – check your boiler manual to be sure. Top your system up with water, if necessary.

Pressure is increased by adding water from your cold water main. The boiler will be connected to the main. You have to find what is called the filling loop in the system. It’s usually close to the boiler or even part of the boiler. If in doubt, check your boiler instructions or contact your installer.

Open the valve or valves on the filling loop – usually small handles – but if there isn't a handle you may need to use a screwdriver. Allow water to fill the system, while keeping a close eye on the pressure gauge, and turn off the valves as soon as the required pressure is achieved.  Be careful not to over-pressurise the system.

Check the pressure regularly and top up when needed.

What if I don’t have a radiator key?

Don’t worry, you can pick one up from any DIY store. You may be able to use a flat-edged screwdriver for some radiators.

Split 2

Can I bleed my radiator if the central heating is on?

No, because you’ll probably find that it’s too hot to touch. You’ll also be at risk from being showered with hot, dirty, water.

Can I get an engineer to bleed my radiator? Will it be covered under my warranty?

This is considered part of your home’s routine maintenance. So it’s your responsibility, as a homeowner or tenant, to keep on top of it.

Why will bleeding my radiators save me money?

If there’s air trapped inside your radiators, your boiler has to work harder to pump heat throughout your home. That means you’ll end up having the heating on for longer, or at a higher temperature – which will cost you more money.

Check out our energy-saving tips for more ways to cut your energy bills.

Compare energy suppliers

Get a quote in minutes and you could start saving

Get a quote