How To Bleed Radiators

In this article learn how to bleed radiators and make your heating system warmer and more efficient.

Stelrad Radiator

Bleed a radiator - what do you mean?

When you bleed a radiator you release air that has become trapped inside your heating system

The air could have been introduced when you changed or removed one of your radiators for decorating. 

Trapped air prevents water from travelling around the radiator stopping it from heating fully.

This can mean your house feels cold during the Winter period.

You’ll also hear some strange noises from your boiler as it struggles to pump water efficiently around

Play Video

This video from our friends at Viessmann Boilers explains what to do.

Things you'll need

Radiator bleed key
Cloth for drips
Bowl for drips of water

A radiator bleed key (like the one above). You can get these from the local DIY store if you haven’t got one.

A cloth or a towel because you’ll need to catch the drips – it’s sensible to have a small bowl handy too.

OK let's begin

First, you’ll need to switch the boiler on as the system water needs to be hot.

This will force the trapped air around the system as pressure in the radiators will now build.

Now you’ll need to check which radiators have cold spots. This is the tell tale sign of trapped air.

You’ll now need to prepare to bleed the first radiator.

Caution emoji

Move cautiously now as the water in the system can get very hot and it is also under pressure.

This means it can shoot out when you open the radiator bleed valve and scald you.


It’s good practice to turn off the boiler now as the air in the system will be ready to escape.

Next step - be prepared

The water in your radiator is likely to be dirty and will stain your carpets badly if allowed to spill.

Make sure that you have your cloth ready to catch any discoloured water that escapes.

Now you can insert the bleed key over or into the valve.

When the bleed key is engaged turn the valve anti clockwise very slowly and it will start to open.

You will now hear a hissing, gurgling sound as the air rushes to the opening you have made.

Timing is everything now because you will need to close the valve as soon as the air stops escaping.

Turn the valve clockwise now and wipe any excess water away.

Relieved emoji

⭐️  Handy Hint ⭐️

It’s a good idea to start with the lowest radiator in the property and work upwards as the trapped air will rise.


That’s the first radiator done, now bleed all your radiators that have cold spots in just the same way.


Final step - check your system pressure

Combi filling loop

It’s only necessary to learn how to bleed radiators if you have a system boiler or a combi boiler.

You’ll now need to check the pressure gauge on your boiler and it’s possible you’ll need to add new water by opening the tap on your filling loop.

Here’s what to do…

The standing pressure on your boiler should be between 1.0 bar and and 1.5 bar.

If your water pressure is below 1.0 bar slowly open the tap and allow more water to enter the system.

When your pressure gauge looks like the image above close the filling loop tap.

Thumbs up emoji

Your system is now free of air and it’s standing pressure is correct.


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