Arrg! Impossible airlock (combi)


29 Aug 2008
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United Kingdom
Saturday afternoon I turned off my mothers main stopcock, opened downstairs taps to drain hot & cold water. Replaced two taps in a downstairs loo (took half hour or so), stopcock back on, cold water returns with good pressure but hot water only ever a weak dribble - this is every hot tap in the house NOT just the one I played with.

If I leave the hot tap off for a few minutes and turn it on it's built up some pressure that quickly drops off back to a dribble. Left it overnight, didn't clear. Spent a good few hours today trying every combination of draining down and refilling with various taps open/closed etc. No joy.

Boiler is definitely a mains fed combi (Baxi Duo) - seen plenty of answers saying an airlock should be impossible but it sure seems like one.

Boiler is in an upstairs airing cupboard and in it is a stopcock that isolates the boiler AND the cold supply to the bathroom - stopcock works and good cold pressure at it. There is a second valve on the boiler inlet which isolates the boiler and it "sounds" like there is good pressure there too.

It's definitely not me forgetting to turn something back on - I only used the main stopcock and that's definitely back on.

Any suggestions?
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It does sound like you have a blockage of sorts on the hot side.
If you have a mixer tap you could try and force mains cold water down the hot side by using a cloth, hold it over the spout as tight as possible, open the hot then the cold. You might get a bit wet in the process but it may help.
It will not be an air lock. Blockage is almost sure the fault. You're gonna have to start undoing a few joints.
Ensure cold feed to combi is OK, by disconnecting and proving flow.
If you can, disc the outlet and connect straight across and if you have good flow at the hot taps, the fault is your combi.
as above it wont be an airlock, check the inlet filter to your combi sounds like you may have disturbed some debris when you turned the water off and when you turned it back on it has blocked the filter/strainer
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I sort of thought an airlock was unlikely - have drained it down and refilled with no problems before.

The cold water feed is joined to a manifold (one of 4 or 5) on the bottom of the Baxi. The Manifold sticks directly out from the rear wall (points toward me looking at it from the front) and has a black 90 deg turn appliance type valve on the bottom of it.

Is any kind of pressure reduction/filter going to be on the outside of the Manifold or somewhere deeper in?
Please come back and update once you find the problem for the benefit of other users

To complete this for future reference...

Ended up getting someone to look at it, all inlet filters etc clear, nothing on outlet side that could cause a restriction leaving only the heat exchanger. When we pulled it out it was obviously badly full of limescale - you could put a finger in and dislodge some or shake it and hear the loose deposits. Exchanger replaced, problem solved.

Key thing here was that an upstairs tap had been leaking in an unused part of the house for a long while (disabled relative), discoverrd and fixed over Christmas but had clearly been leaking for a long while - not enough to fire up the hot water but enough to drain off lots of slow running water heated by the radiators being on.

Wordy answer for posterity and the sesrch engine.
If you had told us about the leaking tap then we could have suggested that to you!

I had a customer with a completely blocked plate and he refused to admit a leaking tap EVEN though there was the obvious lime scale marks on his kitchen sink!

Sorry :) if I'd had a better idea about the innards of a combi boiler I'd probably have put 2+2 together more quickly myself. Anyway, problem solved. Thanks for the responses.

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