Hot water pipe airlocked

[Robert Richards]

Hi,

The hot water pipe from my hot water cylinder appears to be airlocked. When I turn on a hot tap, I get a reasonable (but not massive) flow for about half a second, then just a trickle. There is a header tank in the loft, with feed going to the hot tank which is on the ground floor of the house. The pipework is a bit of a mess, and it seems that somewhere it's airlocked between the tank and the rest of the system. I found a bill the previous owners had paid for the freeing of an airlock. Written on the bill was a warning that the problem would recurr unless the problem was sorted. I can't get a plumber before Christmas. Can someone please tell me how to free an airlock so that I can have some hot water until I get the pipework sorted? Do I have to drain the whole system? If so how, given that it's airlocked...

Cheers,

Robert

[John Armstrong]

Connect a hose pipe between that hot tap and a mains fed cold tap. Turn both on and that will force water back through the system. This will also push the water out of the cylinder back through to the header tank, so have somebody watching to make sure the overflow is coping.

[Christian McArdle]

A trick that can sometimes work on a gravity hot/mains cold system with a kitchen mixer tap is:

1. Ensure the header tank overflow looks secure (it may be needed!)
2. Place a slightly yielding pad over the tap nozzle (i.e. a large pencil eraser)
3. Place a cloth or teatowel over this and press tightly against the nozzle.
4. Turn on the hot tap.
5. Turn on the cold tap for about 10 seconds. (You may get some water in the tank overflow at this point).
6. Remove the stuff from the tap and see if the hot now flows.

Christian.

[Peter Andrews]

Connect a hose between the hot (airlocked) tap and a cold tap that is on the rising main - turn both on and the cold water should force it's way up the hot pipe, hopefully dislodging the air! However this may not be popular with the water company due to the small risk of water flowing the other way, but it would be pedantic to expect you to install a check valve in the hose.

Peter

[Olav Marjasoo]

Excellent technique, I've used it successfully. BUT it will only work if water mixes within body of tap. If hot and cold leave tap via seperate "holes" it will not work. You could also try disconnecting washer and looping back there( if cold is MAINS fed)

-- Olav Marjasoo Overlooking the Clyde, West Coast of Scotland

[Christian McArdle]

Ah, but this technique works because it causes mixing at the nozzle. Some spout designs would actually block both outlets, rather than mix at the spout, but I haven't yet had to overcome such a problem, as all the taps I've tried it on have have about 5mm between the separate channel endings and the rim of the spout, enough for the water to leave the mains cold channel and enter the gravity hot one.

Obviously, any check valves in the hot supply will cause problems here, but you shouldn't have check valves in a gravity system.

Christian.

[Dave Plowman]

Tape a strong polythene bag round the mixer spout.

[Set Square]

You've had lots of good advice about how to back-flush in order to get rid of an airlock.

BUT, there is a remote possibility that it may not be an airlock - but rather a blockage in the feed pipe from the cold header tank to the hot tank. This could possibly explain your symptoms because the initial higher flow could be funded by the water in the vent pipe.

Is there a tap or gate valve in the cold feed to the hot cylinder? If so, make sure that it is fully open. If there *is* a blockage in this pipe, back-flushing may not clear it becuase the water will take the easier route up and over via the vent pipe. You really need to temporarily block up the vent pipe to ensure that water *does* go up the feed pipe.

[Robert Richards]

Thanks everyone for your advice. I will try backflushing it and seeing if that works, if not I will try again with the vent pipe blocked. I guess there could be stuff in the feed pipe because the loft tank isn't covered and appears to be full of a mixture of thatch and rubble. One thing I do not understand is why backflushing will move the air bubble whereas simply opening the tap will not? Presumably the air bubble is trapped at some "hump" in the pipe, so it has to down before going up either way.

Cheers,

Robert

[Andrew Mawson]

It's a well known fact that water defies gravity where troublesome airlocks are present !

Andrew Mawson

[Dave Plowman]

The airlock will probably be near the top of the system where there is little 'head'. Mains has a greater pressure so should overcome it. Funny things, airlocks. They can happen in the best designed systems after a drain down.

[The Natural Philosopher]

Usually that is the case. Its also usually fairly near the tank and at very low pressure as a result. The mains has some sereious pressure and will fart that airlock right out of the header tank.