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...
On 10 Dec 2003 08:54:40 -0800, Robert Richards wrote:
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.
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
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.
On Wed, 10 Dec 2003 17:08:39 -0000, "Christian McArdle"
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)
Overlooking the Clyde, West Coast of Scotland
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.
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.
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.
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
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.
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
*The longest recorded flightof a chicken is thirteen seconds *
Dave Plowman firstname.lastname@example.org London SW 12
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