How do I bleed a radiator?
Updated: May 19
It’s normal to have to bleed your radiators from time to time to remove air from the heating system. Here, we’ll explain step-by-step how to bleed a radiator.
Over time, air trapped within a heating system can accumulate at certain points within the heating circuit. Most commonly, air will accumulate at the highest points in the system. This is often at the top of radiators or towel rails on the top floor of your home or building. This accumulation of air will stop your heating system from working as effectively as it should be.
When should I bleed a radiator?
A good indicator that an air pocket has accumulated in a radiator or towel rail is that it will be hot at the bottom and cold at the top. If you find your house isn’t warming up as expected, check all the radiators in the home are hot from top to bottom. If you find one or two that seem cooler at the top, it’s probably time to bleed the radiator.
Step-by-step: how do I bleed a radiator?
To remove air from a conventional radiator you’ll usually need a radiator bleed key and an old towel or handful of tissue paper or kitchen roll.
First, identify where air pockets may have built up by feeling how hot each radiator is when the heating is on. Take note of where this is, then turn the boiler off and leave them all to cool.
Once the system is cool, use the bleed key to turn the square screw in the centre of the bleed valve (photo below) located at the highest point of one end of the towel rail or radiator.
The screw should not be fully removed but can just be turned clockwise in quarter turn increments until air can be heard releasing from the radiator.
Once air is heard, have your towel or tissue at the ready to catch any drips of water that are released. Once a steady stream of air-free water is being released from the vent point, the bleed key can then be used again to close the screw by turning it anti-clockwise.
This process will need to be repeated on all radiators where a build-up of air is suspected. On sealed systems, it will also be necessary to top up pressure in your system from time to time to allow all air to be removed (see our guide to topping up pressure in your boiler). Once all air has been removed, ensure the system is topped up (if necessary) and switch on the boiler.
How often should I bleed my radiator?
It is worth noting that although bleeding radiators is normal, this should only need to be done a few times a year. If you find that air is accumulating on a regular basis or at lower points in the system, you should contact your service engineer to investigate the cause.
The need to remove air on a regular basis can be an indicator of serious underlying faults within the system, and if these aren’t addressed, it could lead to expensive failures within your heating system.
Got more plumbing questions? Read through our FAQs here, or give our expert engineers a call to find out how we can help you.