How to remove an airlock in your central heating system

How to remove an airlock in your central heating system

Radiators that are not functioning properly and failing to get hot enough to heat the room could possibly mean that you've got an airlock in your heating system. An airlock is most often caused by excess water vapour building up in your system as a result of the water heating process. This vapour is not as dense as water and can become trapped in the system, usually at the highest point, stopping hot water flowing into your radiators. In this article, we'll look at what else might be causing your radiator to not get hot and how you can clear the airlock from your system.

Other reasons your radiator might be cold

Removing an airlock from your system is not the easiest task for those with little DIY or plumbing experience and there are several other common reasons why your radiators aren't getting hot, so It's always worth checking these first.

Thermostatic Radiator Valves (TRVs) off or stuck

A very common reason why radiators fail to get hot is because your thermostatic radiator valves are switched off or stuck shut. If you have a TRV on the cold radiator, check that it isn't set to 0 and turn it to 2 or 3. If water isn't coming in or out then of course the radiator won't heat up.

If your TRV is switched on (according to the dial) then the pin inside the valve, that controls the flow of water, may have got stuck and is not allowing hot water into the radiator. To fix this, you can remove the head/cap off the valve and wiggle the central pin with a pair of grips until it moves up and down freely.

Unbalanced heating system

If the radiator(s) that won't heat up is at the furthest point from your boiler, it might be that your heating system is unbalanced. This means that the water coming from your boiler is not being distributed evenly around your home which can result in some radiators getting hot quickly, while others aren't benefiting from the same amount of hot water and stay cool or cold.

This issue can be resolved by a process of switching your central heating system off and on several times and opening and closing your radiator valves to see which radiator is heating up the quickest, and effectively 'hogging' the hot water. You can see the step by step guide on balancing your radiators here.

Radiators need bleeding

If your radiators are actually getting hot at the bottom but cold at the top then it's likely you've got air trapped in the top of your rads. This is not the same as an airlock and can easily be resolved by using a radiator bleed key to allow the trapped air to escape.

Pump at the wrong speed

Central heating pumps can be set to variable speeds depending on the size of the property and how many radiators are in the system. If your central heating pump is set too slow, then the hot water might be getting cold before it actually reaches the radiators. Setting the speed to the next highest speed setting can help resolve this issue but make sure you don't set it too high or you could draw air into the system.

How to remove an airlock

If you've tried the solutions above and your central heating problem is still causing you problems, below are the steps for clearing the airlock yourself. If you are not confident about doing this then please call in a professional.

Step 1: Turn off your heating system and wait 30 - 40 minutes for it to cool down.

Step 2: Turn off both valves on the radiator that won't get hot.

Step 3: Use a radiator bleed key to allow the pressure in the radiator to dissipate. It's a good idea to have a cloth or container to hand to catch any water that comes out.

Step 4: Remove the whole bleed plug and immediate attach a hose with a 1/2" adapter and run the hose outside or into a bucket to enable the water in the radiator to drain.

Step 5: Open the TRV (flow) valve and you'll hear air and water rushing out through the hose. Allow about 10 - 15 seconds of this and then shut the valve.

Step 6: Open the lockshield (return) valve and repeat the process.

Step 7: Ensure both valves are closed, remove your hose and put the bleed plug back in.

Step 8: Top up or repressurise your system and switch your heating back on.

Step 9: To ensure you don't get another airlock you can top up the inhibitor levels in your system, check your pump is set to the right speed and check whether your expansion pipe is the wrong side of the pump.


Ben Clarke

I have over 10 years of experience researching and writing about a broad range of heating-related subjects. I understand the significance of home heating and strive to provide you with high-quality radiator content in an easy-to-understand manner.