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How to clear an airlock in a hot water system

man holding onto the side of a radiator

What is an airlock in a central heating and hot water system?

An airlock occurs when trapped air within a system collects into a single location, effectively cutting off a consistent water supply or flow of water to a specific area of your central heating loop or hot water supply pipes. Trapped air can manifest in a number of different ways:

  • Poor or loss of heating performance from the radiators in general (airlock in the pump)
  • Cold spots on an otherwise functioning radiator (trapped air in the radiator)
  • Tapping or banging sounds from your heating system (trapped air in the system)
  • Lower than normal boiler pressure (may occur after bleeding radiators)
  • No hot water from taps (air lock in the hot water supply pipe work)
  • Sputtering or inconsistent flow of water from taps (trapped air in the hot water supply pipe work)

Although there can be a number of reasons for loss of heating performance and we recommend reviewing all the possibilities, trapped air can be identified by the location of the performance loss. As air is lighter than water, it will typically rise to a high point within your system, such as at the top of radiators.

How to clear trapped air in a heating system

If you have diagnosed that the air is located within one or a number of your radiators, you don’t have to worry. Fortunately, this can be resolved by correctly bleeding your radiators. This process allows for trapped air to escape the system and be replaced by water.

We’ve written an in-depth guide on how to bleed a radiator, which will guide you through the process step-by-step to ensure you resolve the issue. As with all maintenance work on a central heating system, we always advise you to seek a qualified heating engineer to assess and fix any potential issues your system may have.

  1. 1

    Turn off the mains water supply

    Before doing anything else, you will need to locate the stopcock valve in your home, which is typically installed beneath a kitchen sink, and turn it clockwise to stop the flow of water to your system.

  2. 2

    Turn on all taps to drain the water supply

    In this step, you’re essentially removing the residual water from within your system now that the mains supply has been switched off. Start at the top floor of your house and be sure to turn every tap in your home on. Eventually, the water supply will diminish and the taps will stop dispensing water.

  3. 3

    Flush toilets until there is no water left

    Repeatedly flush the toilets in your home until the water supply has been used up. At this point, you have drained the internal water supply from your home and can begin to flush the airlock out of your system.

  4. 4

    Turn off taps completely, then turn them on slightly to allow a small amount of water flow

    You want to start from the top of your home again and turn off each of your taps. Once turned off, turn each tap on slightly - just enough to allow a small amount of water to dispense. This ensures that when you turn the water system on it can flow back through and push the trapped air out. Note that you won’t actually see any water running yet, as the supply hasn’t been turned back on, so don’t be alarmed when nothing happens.

  5. 5

    Turn the water supply back on

    Locate the stopcock once more and allow the mains water supply to resume. You should see water begin to flow through the partially-opened taps.

  6. 6

    Turn taps until they’re half open

    Starting from the top floor, turn all the taps on until they’re approximately half-open, delivering half the normal amount of water flow.

  7. 7

    Turn taps on full to push any remaining air out

    In this final step, you’ll turn all the taps in your home back on at full to flush any remaining trapped air from the system. Once all taps are successfully dispensing water, turn them off and the system should be free from an airlock.

    If you’re still experiencing issues with your water supply after performing the above, contact an engineer to further diagnose and resolve the issue.


Airlocks and trapped air can be a frustrating problem to deal with and can often go unnoticed for a lengthy amount of time. Fortunately, with simple procedures that can be done at home, many airlocks can be resolved relatively simply.

We recommend always keeping an eye on your home’s heating performance to ensure you catch potential airlocks and other issues early, as this makes it much simpler to resolve.