Curing An Air Lock In A Hot Water Pipe Leave a comment

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Curing an air lock in a hot water pipe. (No hot water from the hot tap!)

If no water, or a spluttering supply, comes out of the hot tap when its opened, the likely cause is an air lock.

If you have an automatic washing machine, and have both the hot and cold supplies connected, an effective remedy can be implemented quite easily.

The cold water to least one of the downstairs taps will be connected directly to the mains (this is generally the kitchen sink tap). The washing machine will be connected to this run of cold pipe as it needs mains pressure to work properly. The mains pressure of the cold water is much greater than that of the hot water and we can use this greater pressure to push any air locks back to the hot water storage tank.

Disconnect the cold water hose from the back of the washing machine (Turn the valve off first). Disconnect the hot water hose from the valve on the hot water pipe. (Turn this valve off first also). Now connect the free end of the cold hose, to the free, hot valve. Turn on the hot water valve and then the cold water valve. This will now allow the greater pressure of the cold water to push any air back up the hot water pipe.

Leave both valves open for approximately 30 seconds and then close, cold first. Now check the flow from the hot tap at the kitchen sink and the air should have been dispelled. If not repeat above. If, after 3 attempts, the problem is still there, please contact a plumber from our directory ( Locate a specialist ). There may be a problem with your system.

If your washing machine only has the cold connected, another way of utilising the higher cold pressure to clear an air lock is by connecting the cold tap to the hot tap. This can be done, in most cases, with a piece of hosepipe and a couple of jubilee clips. Make absolutely sure the connections are sound and do not leave the taps unattended. If you have a mixer tap in the kitchen, the same method can be used, but you will need a longer piece of hose to utilise the cold from the mains tap and the hot from another tap.

A Further method was sent in by a user, Dave Maynard:

When needing to pass mains cold water pressure across to air locked hot water supply using kitchen mixer taps: Easier than trying to find another tap and having to use a long hose which may, anyway, not fit the shape of some taps. Simpler than playing around with washing machine hoses.


  1. Squeeze the single mixer tap outlet so that the palm of your hand covers it firmly.
  2. Turn on the hot, then the cold, (this order is important to prevent confined mains pressure from forcing

your hand away from the tap outlet). The cold supply, now unable to exit from the hand blocked tap outlet, will instead flow across to the hot water pipe causing a backflow in the hot water system, clearing the airlock.

Note: If you have a similarly non-mains pressured COLD supply i.e. also fed from a header tank and which also has an airlock: Same as just described but this time you need to use the mains pressured water to purge the non-mains pressured COLD

supply, which, unlike the hot supply, is probably inaccessible from downstairs and therefore nowhere near any mains pressured water.

To do this you firstly need to create a path between the hot water supply and the non-mains pressured cold water supply.


Prior to starting the tap squeezing exercise, and if you have a non-electric shower mixer tap in the upstairs bathroom:

  1. Unscrew the showerhead from the pipe
  2. Place 2 or 3 layers of plastic bag over the end of the pipe to form a strong membrane
  3. Re-screw the shower head with the membrane still in place
  4. Go to the header tank and turn ONLY the hot outlet OFF. Leave the cold one ON.
  5. Go back to the shower taps and turn both hot and cold ON, with the diverter in the SHOWER position.
  6. Go back downstairs and carry out the procedure initially described i.e. using hand pressure over the mixer taps.

The mains pressured water will now back flow up into the hot water system but then flow across to the non-mains pressured cold water system via plastic bag blocked mixer in the bathroom thus clearing any air locks in the non-mains pressured cold supply.

If you don’t have a shower mixer as described, but do have separate hot and cold taps e.g. sink taps, then steps 1) to 3) can be replaced by just connecting a hose across the two taps. Step 5) would omit the requirement to set the diverter.

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