Homeowners Professionals

How to clear an airlock in your central heating system

An airlock can occur when vapour becomes trapped in a heating system preventing water from passing into the radiator. If you’re experiencing cold radiators, your central heating or hot water system could have an airlock.

These are the two most common signs, however you may also be able to hear tapping or banging sounds and your boiler pressure may be on the low side. It’s worth noting that there can be numerous reasons why your radiators are cold, so you should rule these out first before attempting to remove the airlock. 

What is an airlock?

An airlock can occur when vapour becomes trapped in a heating system and prevents water from passing into the radiator, leaving the device cold.

Your heating system pushes hot water around your radiators to heat your home, however, as this water is heated, it can sometimes create excess water vapour. This gas is less dense than water, so it usually rises to the highest point in your system. This is what is known as an airlock and it can cause all sorts of problems with your central heating.

An airlock that occurs in your hot water system could prevent water from coming out of a tap, while an airlock in your central heating system could cause one or more radiators to stay cold, even when the boiler is on.

How to remove an airlock from your heating system

An airlock in your heating system can be removed easily by bleeding the radiators. Doing this will force the airlock out of the system, so you should attempt this task on all your radiators, including the ones that are cold all the way through and cold at the top.

You can read our full guide for more information around bleeding radiators, but we’ve also added a small recap here.

Step 1: Turn off your central heating and wait for around 20 minutes for the system to cool down.

Step 2: Lay a dry cloth or towel beneath the radiator you want to bleed, just in case any water escapes.

Step 3: Use your radiator key to slowly open the valve, turning it anti-clockwise to do so. You’ll hear a hissing noise which is the air escaping the system.

Step 4: When the hissing noise stops, all of the air is out of the system, so you can close the valve.

Step 5: Check your boiler pressure and turn the heating on. When it’s heated up, check your radiators for cold spots.

How to remove an airlock from your hot water system

Follow our steps below to remove an airlock from your hot water system.

Step 1: Turn off the mains water supply by locating the stopcock tap. This can usually be found under your kitchen sink. Turn it clockwise to shut off the water.

Step 2: Turn on all your taps, starting upstairs and working your way downstairs. Don’t forget about any downstairs toilet taps and bath taps. The water should eventually stop flowing.

Step 3: Flush all your toilets until there is no water left in the tanks.

Step 4: Starting upstairs, turn all your taps so that they’re almost off but not completely. A small amount of water should still be able to get through.

Step 5: Turn the water supply back on by adjusting the stopcock.

Step 6: Adjust each tap so they're about halfway. Then, after a few minutes, put each one on full. 

This should have allowed any trapped air within the pipes to escape and you shouldn’t have any more problems with your taps. If these problems still occur then contact a qualified heating engineer.

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