bleeding radiator

Learn How to Bleed a Radiator UK – Full Guide

Learn how to bleed a radiator from a professional UK heating engineer.

Your central heating system should be completely full of water at all times if you want it to work properly.

You need to remove any air in the system as it can cause rusting, your radiators to be cold, or even your boiler to not work at all.

To remove the air, you have to bleed it out.

This is a great DIY boiler repair that everyone should learn.

If you have to keep bleeding radiators regularly, then you probably have a leak on your heating system, or maybe another problem.


Here’s what you’ll need to bleed a radiator:

  • A radiator bleed key
  • An old towel or rag

how to bleed a radiator uk
Radiator Bleed Key

To know how to bleed a radiator, you will need to know where the bleed nipple is on the radiator.

This will be at the top of the radiator, usually on the right-hand side, as most people are right-handed.

But it could be on the left or on the back of the radiator panel.

How to Bleed a Radiator UK Steps

  1. Get your towel and hold it under and against the bleed nipple on the radiator.
  2. Place the radiator bleed key on the bleed nipple and turn anti-clockwise until you here the air coming out. You will need to keep going until water starts coming out.

When you bleed air out of a radiator on a pressurised heating system, it will lose pressure.

You will need to top this pressure back up every time, this is a crucial step for learning how to bleed a radiator UK.

The boiler will not fire up if there is not enough pressure in your heating system.

The pressure should be at least 1 bar on the pressure gauge.

If it’s below 1 bar, top the pressure up by adding some water to your heating system via the filling loop.

If it’s not a pressurised system, you won’t need to do anything as it should fill itself automatically from the header tank in the loft. 

You should know how to bleed a radiator by now, so let’s look at filling up a full heating system.

Filling up an Empty Heating System

If you have drained your full central heating system, you will definitely need to know how to bleed a radiator.

When filling the heating system up again via the filling loop, you should fill the system to 1.5 bar.

Once it reaches 1.5 bar, you can start bleeding the lowest radiator on your heating system.

When the radiator is fully bled of air, the water will start coming out, move on to the next radiator.

Then work your way around all the radiators on the same floor.

Then move up to next floor and bleed all of those radiators.

When the air stops coming out, you will need to top the pressure up again to 1.5 bar.

Keep doing this until all the radiators have been fully bled.

If you have 3 floors, you should bleed the ground floor first, then the 1st floor, then the 2nd floor.

How to Drain a Heating System

If you need to replace a radiator or a radiator valve, you will probably need to drain the central heating system.

The easiest way to drain a central heating system is to connect a long hose to a drain off valve and run it outside to a drain.

Open the drain off valve and look for water coming out and going down the drain outside.

When the water stops coming out of the hose and the pressure gauge is on 0, you should start venting all the radiators, starting with the highest first.

Venting means to open the bleed nipple on the radiator, like bleeding, except you are letting air in and water out.

Venting is the opposite to bleeding.

So, if you have 3 floors, you should vent all the top floor first and wait until the water stops coming out of the hose outside.

When it does, you should move down to the next floor and repeat until you’ve drained all radiators on all floors.


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