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How to bleed a radiator

If your heating isn’t working, you should firstly bleed your radiators before assessing whether your system needs a repair.

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What does bleeding a radiator mean?

If your radiator isn't as warm as usual and you've noticed cold patches at the top, then you'll likely have air trapped within the radiator – this is a common issue for un-vented heating systems. To fix the issue, you'll have to bleed the radiator, this is a simple process to do with our friendly step by step guide - we'll even show you how to manage if you don't have a radiator key.

How to Bleed a Radiator

Before you get started, you’ll need a radiator bleed key and have a spare hand towel or mug handy to catch any drips. 

Turn your heating off – if you don’t, you’ll be at risk of scalding yourself and getting hot water on the floor.

Get the towel/mug ready – when you bleed a radiator, some discoloured water will come out and will need to be caught before falling on your carpet/floor.

Open the radiator bleed valve – using your radiator bleed key, insert it into your bleed valve, this is often found at the side of a radiator and looks like a round hole with a square inside. When you insert the key, you’ll feel them lock together, turn the valve anti-clockwise – you’ll hear a hissing noise which is the hot air escaping, so be careful.

Bleed your radiator – a quarter-turn of the valve will be enough; don’t turn it too far or else water will come rushing out. Hold the radiator valve open until the air stops coming out and water starts to drip out - this means you’ve successfully bled a radiator.

Seal the valve – turn the radiator bleed valve clockwise to tighten, making sure not to overtighten. Repeat the above processes on all the radiators that need bleeding.

Check the pressure on your boiler – once you’ve bled your radiators, check your boiler’s pressure gauge to see if your system needs to be re-pressurised. If it’s between 1.0 - 1.5 bars then you’re all done. 

Adjust boiler pressure - if your boiler pressure is too low then you need to manually increase the boiler pressure, this could be with a central filling loop that’s connected to your boiler. Find out what make and model your boiler is, then go to the manufacturers' website, they'll have a walkthrough explaining how to increase pressure for your exact boiler. 

How to bleed a radiator without a key

Some modern radiators are equipped with valves that are designed to be turned with a simple flathead screwdriver.

So, if you don’t have a radiator key handy then a flathead screwdriver could do the trick.

Why do you need to bleed a radiator?

Radiators need to be bled when they have trapped air within your central heating system. This trapped air can be introduced when new water enters the system from an expansion tank or could also be ‘created’ by the pump during movement as it turns.

This trapped air stops the heated water circulating your radiator, which often makes your radiator have cold spots at the top, but warm parts at the bottom.

You need to bleed a radiator to let the trapped air escape and let the hot water flow in your radiator again. 

How often should you bleed your radiators?

It easy to tell when a radiator needs bleeding as you'll notice cold spots at the top of the radiator while the bottom is often warm, or in some cases, the radiator doesn't heat up at all. This is due to a build-up of trapped air within the central heating system, so the air displaces the hot water that normally heats the radiator.

This air will be released when a radiator is bled and the hot water will be able to flow freely once all of the trapped air is released.

We would recommend you bleed your radiators once a year, even if you think they're working properly - better safe than sorry.

What should you do if bleeding them doesn’t work?

If your system isn't heating up after a bleed, there could be a build-up of sludge within - this will cause your boiler to work overtime to heat your home to the correct temperature, meaning your heating bill will be a lot more expensive.

To combat cold radiators after a bleed we recommend: 

  • Getting your central heating system serviced annually, so a gas safe engineer can clear your system as best they can to help prevent wear and tear and catch any problems early
  • Book a boiler repair, so an engineer can chemically flush away the build-up of sludge before your boiler completely throws in the towel. 

Here at Boiler Plan, we have professional engineers who can survey and check your heating system to make sure it’s running as efficiently as possible and, if necessary, can provide a free personalised quote for any replacement radiators or boilers.

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