[?] Possible airlock on boiler feed to indirectly heated HW cylinder.

[David Chapman]

The central heating boiler in the house is heating all of the radiators perfectly well, but the gravity-fed indirectly heated HW cylinder in the airing cupboard above the boiler only heats up very slightly.

Two feet above the point where the 1" (?mm) diameter pipes exit the boiler itself, they are too hot to touch for long, but those same pipes are only warm to the touch as they enter and exit the cylinder.

There is a small header tank in the loft that feeds this closed-loop system, and some time ago I found it to be dry because of a sticking ball-valve. I think the hot-water problem started shortly after I replaced the ball-valve and refilled the header tank, and am wondering if an air-lock may be the cause.

I still don't know why the header tank ran dry - there were no obvious leaks anywhere in the system beneath it. Also, I can't quite understand how air can be trapped in pipes where there is a header tank above - surely air would just rise up that pipe and bubble out once and be gone.

If any clarification of my description is required, I've put a diagram of the complete system at:-

I'd appreciate the thoughts of any of the DIY experts in this NG. If an airlock could be the problem, I'd be most grateful to know how I can clear it.

TIA - Dave

David C.Chapman - ( [e-mail])

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[Dave Plowman (News)]

Then there's no water circulating.

The most likely cause.

Unfortunately not. Funny stuff, water. Never seems to run where you want it to.

If there's no bleed device in the hot water circuit, I'd consider fitting an automatic one - commonly called an 'air bottle'.

As a short term fix, you might partially undo the highest fitting in the loop in an attempt to bleed the air out. But this could be messy.

Another way might be to partially drain the system then re-fill via the drain c*ck using mains pressure water. This usually eliminates air pockets.

[David Chapman]

Thanks for the interesting suggestions.

The water connections at the back of the boiler comprise two manifolds, one 'flow' and the other 'return'. The pumped heating connections are separate from the gravity hot water feeds (of course) and the 'return' manifold has two connections for the hot-water feed 'flow' and returns, one at each side of the boiler.

The unused feed holes in the boiler manifold are sealed by blanking plugs which have a square-section head. I'm not sure what size thread they have. There is no drain-c*ck. Removing the plug from the 'return' manifold would appear to be the only way that the hot-water circuit can be drained, albeit VERY messily. With that arrangement at present, the simple back-feeding idea suggested seems to be a bit of a a non-starter.

Ideally I suppose, I should tie up the ball-valve and empty the header tank, remove the plug, drain the hot-water feed pipes completely, clean up the flooded kitchen, then replace the plug by a drain-c*ck - assuming that I can easily connect it to the threads on the return manifold. I would then be able to do a back-feed to remove the NEW air in the system, and also be better suited to deal with the problem if it arises again in the future.

But is there a simpler, less messy, way?

I certainly could try and loosen the compression fitting where the hot feed from the boiler goes into the cylinder, even though that will also be pretty messy.

I did a search on Google and read that someone had fixed an air-lock problem by running the boiler for a few minutes with the radiator pump switched off and the boiler thermostat removed so that the water in the hot water feed pipes actually boiled and released the air in that way. That solution sounds rather hairy to me - but does it work?

Any other suggestions for clearing this annoying airlock will be gratefully received.

ATB - Dave

David C.Chapman - ( [e-mail])

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[Dave Plowman (News)]

Are you certain there's not a drain c*ck elsewhere? Might even be under the floorboards somewhere? Most would fit it close to either the front or back door so a hose can be run out easily.

It's usually at the lowest part of the pipework - which isn't often at the boiler.

[David Chapman]

Yes, I'm sure. The cylinder is upstairs, in the room directly above the boiler, and the hot-water feed pipes leave the boiler and go straight up and through the ceiling above.

There is no part of the hot-water feed circuit in this house that is lower than the boiler itself. However, the feeds for the central-heating radiators do go under the downstairs floorboards as well as up through the ceiling to feed those radiators upstairs. I am assuming, I *DO* hope correctly, that the hot-water and central heating sections do not join up within the boiler, and that I can drain the hot-water circuit without affecting the one for the radiators. If they are joined within the boiler and draining involves both circuits, then the air-lock problem is going to be MUCH more difficult to resolve.

Many thanks for your comments so far.

- Dave

David C.Chapman - ( [e-mail])

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[Dave Plowman (News)]

Ahh. I'm afraid this is the root of your problem. Although they may well be separate pipes from the boiler, they share the same water within the heat exchanger.

In other words, draining down the heating side will also drain down the hot water heating side - although like all these things it's not an absolute as some water can remain anywhere. And air when you re-fill.

So I'd still try draining down the heating and re-filling as described previously. If you think about it, the water all comes from the same header tank so must mix somewhere.

Might be a good time to add inhibitor anyway. Sounds like it's not been done recently.