Bleeding Radiators Combi Boiler

When the cold weather sets in and you need every radiator in the house to work at maximum, you don’t want to find a couple of radiators in your heating system that isn’t heating up properly because of trapped air or that have cold spots. In this article, you will find out how air gets trapped in the system and step by step how to bleed radiators.

Steps to bleed radiators with a combi boiler

Follow these steps to bleed a radiator with a combi boiler :

  1. Switch on the central heating and make note of the colder radiators.
  2. Turn off the heating and wait until it completely cools.
  3. Start with bleeding radiators on the ground floor.
  4. Locate and open the bleed valve on the radiator with a radiator key.
  5. Close the valve when you see water trickling out.
  6. Repeat for all radiators in the house, working your way up.
  7. Turn on your central heating and check if radiators are hot.
bleed a radiator attached to a combi boiler

Why does a radiator need bleeding?

The main reason radiators need bleeding is because of trapped air bubbles that occur as the water circulates around the heating system’s pipes and radiators.  The trapped air needs to be removed by bleeding if the system is to function optimally, otherwise, radiators will stay cold at the top while being hot at the bottom.

How To Bleed a Radiator: Step by Step Guide

To bleed a radiator you will need a radiator bleed key or something similar.

1. Switch heating on to locate cold radiators

For half an hour to find out which radiators are cold or making gurgling noises. That is caused by trapped air and will need bleeding.

2. Turn off your heating and wait for it to cool

And wait for all the radiators to cool.  Remember which radiators need to be bled.

3. Locate and open the bleed valve

Start with bleeding radiators on the ground floor. Place or hold a cloth where the bleed valve is at the top of the radiator and use your radiator key to turn the valve anticlockwise.  Don’t open the valve fully but in small turns.   When trapped air escapes you can hear a hissing sound.  The picture shows what a bleed valve looks like.

radiator bleed valve
radiator bleed valve

4. Close the valve when you see water flowing out

Clockwise when you see water starting to come out.  Use your cloth to catch any water.

5. Repeat for all the radiators

Repeat the steps for each radiator that needs bleeding working your way up each floor.

6. Turn your central heating system back on

check the pressure gauge at the front of your boiler. Bleeding your radiators can cause the pressure to drop and if it gets too low (below 1bar), you’ll need to top it up. Use the filling loop to top the water back up to between 1 and 1.5 bar.

7. Check that the radiators heat up

You have bled are now heating up properly and there are no cold spots.

How to bleed a towel radiator

Follow these steps to bleed a towel radiator:

  • Step 1: Turn off the central heating at the thermostat.
  • Step 2: Let the towel radiator cool completely.
  • Step 3: Turn on all of the valves. Turn anticlockwise to turn on if they aren’t open already.
  • Step 4: Find the bleed valve. Usually located at the top of the radiator.
  • Step 5: Rotate the valve anticlockwise. Open in small turns and you will hear a hiss of air.
  • Step 6 – Allow the water to flow. Leave valve open until water starts flowing out. It normally takes on 30 seconds to bleed a radiator fully.
  • Step 7 – Close the valve once water starts to come out.
  • Step 8: Check the boiler pressure. If it’s gone low, you will need to repressurise the system.

How to bleed radiators without a key

An alternative to the radiator key is to use a small flathead screwdriver.  Most modern radiator bleed valves have an indent for the screwdriver. You can also use some small-nosed pliers.  Some radiator bleed valves are hexagonal shaped so you can use an allen key.  

If you don’t want to go into your toolbox at all, I’ve managed to use the nail file of a nail cutter to get the job done too.

Why does my radiator need bleeding every week?

Air is entering the heating system and getting trapped, usually near the top of the radiator. Air can enter the system in many ways. Even if there are no leaks, the way a combi boiler works generates air which enters the sealed, pressurised system. Other ways are due to bad plumbing, or when repair and maintenance work is carried out.

how to bleed a radiator image

What happens if you bleed a radiator with the heating on?

You shouldn’t bleed the radiator when the heating is on, not only because it will be hot to touch, but hot water expands so will come out of the radiator quickly, and not all the trapped air will be released.  You have to turn off the heating and let the system cool down first before bleeding.

Bleeding radiator no water no air

If no water or air comes out when bleeding the radiator, this could be one of two issues: There is not enough pressure in the system; or the bleed valve has become blocked.

Both can be fixed easily. First, if there is not enough pressure in the system to force the air out of the radiator, you should top the system up with water until 1.5 bar, then bleed the radiator again. Carry on doing this until water starts coming out.

If the bleed valve is blocked, after turning the lockshield and radiator valve off at both ends, remove the centre bleed screw fully, and remove any blockage with a pin. Replace the screw and open both valves up again.

Too much water comes out when bleeding radiators

It is normal for water to come out when bleeding radiators as it shows all the trapped air has been released.  But be careful not to let too much water come out, otherwise the lost water in the system will have to be replaced for it to function properly. 

The problem here is that fresh water has a lot of air in it which can get trapped again.

What if a radiator needs bleeding frequently

If you find that a particular radiator needs bleeding frequently, it is often down to a leak in the system.  Even if it’s a sealed system, air can still be drawn into it. 

Usually, a towel rail on one of the upper floors of the house ends up having this problem, so check this out very carefully where the connections screw into the rail. 

If you don’t find a leak after a thorough search, it may be time to call in a gas professional to take a look inside the boiler for leaks around pumps or air vents, or pressure relief valves.

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*The information in this article should be used for general guidance only and not as financial or health advice.  Full details are on the link in the footer to our disclaimer page.  Always discuss your requirements with a competent and suitably qualified professional before undertaking any work.