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How to Clear an Airlock in Central Heating System


The presence of an airlock in your central heating system may sound like a complex problem. However, you need not worry as there is a proven method of clearing it on your own. This quick-fix will save you the hassle and expense of having to call a professional plumber to resolve the issue. But first;

What is an airlock, and how did one get into my central heating system?

An airlock is just another word to describe the presence of trapped air in your central heating system. For example, draining and refilling your radiators in the wrong order is one-way air enters your central heating system and eventually leads to an airlock.

Refilling your downstairs radiators without properly closing the upstairs ones is another way that an airlock gets into your central heating system. Lastly, an airlock can be introduced into your system by the water vapour created by excessive heating of water.

As air is less dense than water, it always rises to the highest point of your central heating system that is normally your radiators. In other situations, the air or vapour can be trapped in hot water pipes, therefore, hindering hot water from flowing freely.

How can I tell if there is an airlock in my central heating system?

Diagnosing the presence of an airlock in your central heating system is easy. All you have to do is check for cold or partially cold radiators. Another dead giveaway of an airlock in your central heating system is hot water failing to come out of the taps despite the boiler being on. An airlock can also cause tapping and banging sounds in your boiler and sometimes even lead to low boiler pressure.

What tools do I need to clear an airlock from my central heating system?

Here are the tools that you’ll need to get an airlock out of your central heating system:

  • A hosepipe at least 1 metre long
  • A radiator bleed key
  • A container to catch water.
  • A large towel
  • A small flat head screwdriver

How can I remove an airlock from my central heating system?

So after diagnosing that your central heating system is suffering from an airlock problem, the next logical step would be to resolve the issue lest it keeps inconveniencing you. And the easiest way to remove an airlock from your central heating system is by draining your radiators.

How to remove an airlock by bleeding your radiators

If you have been an avid follower of this page, you have probably read one of our extensive guides on bleeding radiators. But here is a recap on how to efficiently drain your radiators.

Step 1:  Put your radiators to the highest settings and turn on your boiler. Allow the radiators to run for about fifteen minutes. With the central heating system running at full blast, any air trapped in the system gets pushed to the highest radiator in your home.

Step 2: Test and isolate the faulty radiators. As the boiler runs, test each radiator separately to identify the ones not warming up properly.

Step 3: Switch off the central heating. After marking the radiators that need bleeding, switch off the boiler and allow the system to cool down for about 20 minutes.  This time will let all radiators sufficiently cool down.

Step 4: Go to the faulty radiator that is closest to the boiler. Turn off this radiator at both the lock-shield and TRV end using the flatbed screwdriver.

Step 5: Lay the towel beneath the radiator to catch any water dripping out during the bleeding process.

Step 6: Use the radiator key to open the radiator valve slowly. Open the radiator by turning the valve in an anti-clockwise direction until you hear a hissing sound. The hissing sound is evidence of air escaping out of your central heating system. Wait until the hissing sound stops and water starts coming out instead of air. Once done, close the radiator valves.

Step 7: Alternatively, remove the radiator bleed point and attach the hose where the air bleed was. If the hose doesn’t fit properly, you might need to use a pair of tight grips to secure it in place.

Step 8: Run the hose outside or lead it into the container. Once done, open the TRV end of the radiator and flush it. As the air escapes from the central heating system, you will hear a gobbling sound coming out of the radiator. Close the TRV after around 20 seconds.

Step 9: Repeat the procedure on the lock-shield end of the radiator until all air is out of the radiator.

Step 10:  Close the two valves by turning them clockwise until the valve can’t turn any further. Next, remove your hose while ensuring no water spills on your floor or carpet.

Step 11:  Carefully re-install the bleed point and close it. After this, turn the radiator’s TRV and lockshield on and proceed to the next faulty radiator.

Step 12:  Once all the faulty radiators have been bled, top up your heating system until the boiler pressure reaches the recommended value of 1.5 to 2 bar.

Step 13: Turn on the central heating system and check the radiators for any cold spots. If there are none, then you have successfully cleared the airlock out of your system, and you deserve a pat on the back.

What if bleeding my radiators doesn’t work?

There is no guarantee that bleeding your radiators will remove the airlock in your central heating system. It is, therefore, normal to find that the airlock persists even after you attempt to clear it by bleeding your radiators.

If this is the case, you can try closing all the radiators in your house and forcing the hot water through the system. You can achieve this by turning up the pump pressure to the second or third setting. If this results in another failed attempt, you can try balancing your central heating system as it might by partially closing some valves.

After doing all this and the airlock remains in the central heating system, it may be time to call a certified emergency plumber as the problem is way above your capabilities.

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