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How to Bleed Radiators with a Combi Boiler

Have you noticed your home isn’t feeling as warm and toasty as it should? Then now is a good time to check your radiators. If your radiators make strange sounds or are getting hot at the bottom but remain cold at the top, you may need to bleed them. 

The process of bleeding your radiators is straightforward and, thankfully, doesn’t require a qualified heating engineer. That means you can safely bleed them yourself if you’re comfortable doing so. 

Why do I need to bleed my radiators?

Over time, air bubbles can find their way into your system and reduce heating performance. The air bubbles eventually make their way to the top of your radiators and stay there, preventing warm water from circulating and resulting in cold patches on your radiators. 

Trapped air causes your radiators to take longer to heat rooms and can sometimes cause them to make odd noises. Air bubbles that get stuck inside your radiators also make your heating system use more energy to generate heat, which leads to higher energy bills.

Bleeding your radiators means that your rooms warm up more quickly, and you can keep your heating costs down. 

When should I bleed my radiators?

The common signs that your radiators need bleeding include:

Cold spots

When your heating is on, your radiators should be hot all over. If they’re not, that means warm water isn’t circulating freely. A quick and easy way to determine whether your radiators need bleeding is to check for cold patches near the top of your radiators.

Strange noises

Radiators often make clicking and creaking noises when heating up or cooling down. But if you hear strange gurgling or rattling noises when your radiators heat up, then chances are they need bleeding. 

High pressure

Your combi boiler’s pressure gauge should read between 1 and 1.5 bar when your central heating system is off. You can check the ideal pressure for your combi boiler by looking in your user manual. If the pressure is too high, you can fix this by bleeding your radiators. 

How long does it take to bleed a radiator?

It usually takes about 30 seconds to bleed a radiator. But it may take up to a minute or longer, depending on the radiator size and the time it takes to remove the trapped air. You have to wait for the hissing sound of escaping air to stop and will see a steady trickle of water to know that you have successfully bled a radiator. 

What equipment do I need when bleeding my radiators?

Bleeding your radiators is a simple fix that pretty much anyone can do, but be sure to contact a qualified heating engineer if you prefer not to do it yourself. 

You only need a few small tools to bleed your radiators, including:

  • A bleed key
  • A bucket or large pan to collect the water that may drip from your radiators
  • A cloth or towel 
  • A pair of gloves to protect your hands when checking for cold spots

Don’t panic if you can’t find a bleed key at home. They come in a standard size, and you can purchase one at any DIY or hardware store. 

How do I bleed my radiators?

Got all the tools listed above? Good. Now you’re ready to bleed your radiators. But there are a few things you need to do first before bleeding them. 

Below is a step-by-step guide on how to check and bleed your radiators. 

1. Turn up your combi boiler to the highest setting

Turn on your heating and make sure you turn your combi boiler up to the highest setting, then wait for your radiators to heat up. 

2. Check your radiators for cold spots

Put on your gloves and feel the top of each radiator in your home. If you find any of your radiators are hot at the bottom but cold at the top, you will need to bleed them. 

3. Turn off your heating and wait for your radiators to cool

Now you know which radiators need bleeding, you must turn off your heating and wait for all the radiators to cool down. If you don’t wait for your radiators to cool, you can end up injuring yourself and making a mess when hot water escapes from your radiators.

4. Place a cloth below the radiator and a bucket beneath the bleed valve

Start with the radiator furthest away from your boiler. Place a cloth or towel under the radiator to catch any water and put the bucket or pan beneath the radiator valve. The radiator valve should be at the top corner of your radiator. 

5. Open the radiator valve using the bleed key

Place the bleed key in the radiator valve and slowly turn it anti-clockwise. A quarter or half turn should be enough to open the valve and let the air escape. You will hear a hissing sound when the air begins to drop out of your radiator (this tells you that you’re bleeding your radiator correctly), and you need to stop turning the valve as soon as you hear the hissing noise. 

6. Close the radiator valve

Once the hissing stops and you see a steady trickle of water, turn the bleed key clockwise to close the valve. Always allow some water to run out into your bucket or pan before closing the valve to ensure you get rid of the trapped air. Be careful not to close the valve too tight, or you might damage it. You can then give your radiator a wipe using your cloth to clean up any trickles of water.

7. Repeat the above steps for each radiator that needs bleeding

But don’t repeat steps 1, 2 or 3. Starting with step 4, you can repeat the above steps for the other radiators in your home that need bleeding, ending with the radiator nearest your combi boiler. 

You can even check and bleed the radiators that don’t have cold spots if you wish. If they don’t hiss when you try and bleed them, that means they’re fine as they are. 

8. Check your boiler’s pressure gauge

Once you finish bleeding your radiators, you should check your boiler’s pressure gauge to make sure it reads between 1 and 1.5 bar (or whatever it says in your boiler manual). Combi boilers have a habit of losing pressure when you bleed your radiators, so you may find you need to re-pressure your boiler.

9. Re-pressurise your combi boiler

If your combi boiler pressure is too low, you should turn off your heating and look for the filling loop. The filling loop is usually a flexible silver pipe underneath your boiler with one tap or a tap at both ends.  

To add pressure using the filling loop, open the taps, one after the other, to let water into your system. Once the gauge displays between 1 and 1.5 bar, you can close the taps.

Before attempting to re-pressurise your combi boiler, check the instructions in your user manual. If you can’t put your hands on the manual, you should be able to find the information you need by visiting the manufacturer’s website. 

10. Turn on your heating

You can turn on your heating after you bleed your radiators and re-pressurise your boiler to check them for cold spots again. If any of your radiators contain a lot of trapped air, they may need bleeding more than once. 

How often should I bleed my radiators?

Bleeding your radiators is a simple process and a task you should carry out at least once a year, even if they are working fine. You should also check your radiators for cold spots every few months to ensure they don’t have trapped air inside them.