Category - Boilers
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Will Scholfield, Engineer

31 Jan : Updated 19 Apr ● 7 min read

How to bleed a radiator

Are your radiators not getting as hot as usual? Are they only warming up towards the bottom or in certain areas? Or are they making some odd noises as they heat up? It may be time to give your radiators a bleed.

Despite this task sounding slightly menacing, all it involves is allowing any pockets of trapped air to escape. If trapped air is left inside your radiators with nowhere to go, it will cause the radiators not to heat up fully and become less efficient. If left long term it could also promote corrosion within your central heating system..

Having to bleed your radiators doesn’t automatically mean that there is a larger or more serious issue with your central heating, so don’t get on the phone with your engineer straight away. It’s simply something that needs to be done regularly to allow your heating system to be as efficient as possible and do its job of keeping you cosy.

Bleeding your radiators semi-regularly (around twice a year) means your central heating can run efficiently, and you can rest easy knowing you’re keeping a lid on your energy bills.

So, let’s look at how to bleed a radiator. It’s quick and easy, so there really is no excuse for neglecting your radiators!

How to know if your radiator needs bleeding

Luckily, it’s easy enough to spot whether your radiators will need bleeding:

  • Is your radiator cold at the top? If so, this probably means it’s time to bleed it. The trapped air rises and gathers mostly towards the top end of your radiator, which causes cold spots at the top of your radiator
  • Are there any damp patches or condensation surrounding your radiator or above it?

What do you need to bleed a radiator?

  • A radiator bleed key or screwdriver, depending on the bleed valve type
  • A cloth or container to catch any leaking water

You probably have everything you need around your home already. The only item you may be missing is a radiator key, which you can find in most DIY shops, large supermarkets, or online for a small price.

A step-by-step guide on how to bleed a radiator

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1. Identify your boiler type

If you have a sealed system or a combination boiler make sure you are able to re-pressurise the system before you start if not the system pressure might drop too low, and the boiler may not fire up at all once you have completed bleeding your radiators.

2. Turn on your boiler

Start off by turning on your boiler as you normally would, allowing your radiators plenty of time to heat up fully. This way, you can quickly identify which radiators need some TLC and which can be left alone, remember we are looking for cold spots at the top of the radiator.

3. Identify which radiators need bleeding

Have a feel of your radiators to see if they require bleeding. As mentioned above, this is usually the case when there are cold patches at the top whilst warm at the bottom.

If your radiators aren't heating up as quickly as they should be, or you hear gurgling noises, this is also an indication that they might need bleeding.

Why not just bleed all your radiators to be on the safe side, you might ask? This can actually do more harm than good. If you bleed a radiator with no trapped air inside, you risk your boiler pressure dropping too low and your whole central heating system failing - which is not what you want.

But if multiple radiators do require bleeding, start with the one furthest away from the boiler. And if your house is more than one storey, begin by bleeding the downstairs radiators first.

4. Turn off the heating and wait for the radiators to cool

Make sure to let your central heating system cool entirely before taking the following steps, or you may end up spilling hot water.

5. Locate the bleed valve on the radiator

The bleed valve is usually located at the top of the radiator, on either side. However, this can vary depending on the radiator type. It is round and will have a square section for a radiator key or a slit for a screwdriver.

6. Place an old cloth or container below the valve

When you bleed your radiators, water will escape from the bleed valve. So, you'll need a container or cloth to catch the water.

7. Use a radiator bleed key or screwdriver to release the air

Depending on the type of radiator, you'll need to use a bleed key or screwdriver to release the trapped air.

8. Turn the device anti-clockwise so the air can escape

Next, you can get stuck into your radiator bleeding and remove the air blockages from your boiler. Keep in mind that you shouldn’t get carried away and open your radiator valve entirely or keep it open for too long - too much water will pour out, and your boiler pressure will drop dramatically.

Simply turn the bleed valve enough anti clockwise to hear the hissing sound of air escaping. Allow all the air trapped inside to escape until the sound stops and a steady stream of water leaks from the valve.

This means that you have mastered how to bleed radiators and can re-tighten your valve by turning your bleed keys clockwise - well done!

To avoid rust forming around your radiator valve, wipe away any excess moisture or condensation once you’ve finished.

9. Check your boiler pressure is between 1.0 and 1.5 bars

It's normal for your boiler pressure to drop if you've bled your radiators. If your radiator pressure is below 1.0 bar, make sure to repressurise your boiler using a filling loop or call out a heating engineer to give you a helping hand.

10. Turn the central heating on

If your boiler appears fine, it's time for a 'heat test'. Boot up your central heating and have another feel of your radiators.

All being well, there should be no more cold patches, meaning you and your family can feel the full benefits of a warm home and save money on your energy bills - it’s a win-win situation.

Do your radiators still have cold spots?

Radiator bleeding is a job that should only need doing twice a year. If you have consistent and frequent issues with cold spots on your radiators, despite bleeding them regularly, there is probably a more serious issue that needs attention.

In this case, check around your home, boiler, and underneath radiators for any leaks that could be causing trouble. Leaks can present themselves as damp patches, puddles or could also be identifiable by rust.

If you can’t find any leaks and find a radiator cold at the bottom, you may need your system flushing to reduce the build-up of sludge. Over time, dirt naturally collects inside your boiler, within your pipes and at the bottom of your radiators. Draining this dirty water away can allow the hot water to flow around your home without restrictions.

If you are a boiler novice, which most of us tend to be, it is worth contacting an engineer to help drain and or even power flush your system if they are really blocked. They will carry out the process safely, without damaging your home, and help identify any potential leaks and faults around or within your boiler.

Most of the time, an engineer can easily identify any issues and carry out a boiler repair. However, sometimes it’s best to accept defeat and choose a boiler replacement instead - it could be more cost effective in the future, especially if your heating system is breaking down regularly.

FAQs about bleeding radiators

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