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:-

http://www.minda.co.uk/dave/heating.jpg

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 - )
--------------------------------------------------------------------------

In article ,
David Chapman wrote:
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.

Then there's no water circulating.

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.

The most likely 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.

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 cock using mains pressure water. This usually eliminates air pockets.

--
*I used up all my sick days so I called in dead

Dave Plowman London SW
To e-mail, change noise into sound.

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.

Then there's no water circulating.

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.

The most likely 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.

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 cock using mains pressure water. This usually eliminates air pockets.

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-cock. 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-cock - 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 - )
--------------------------------------------------------------------------

In article ,
David Chapman wrote:
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-cock. 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.

Are you certain there's not a drain cock 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.

--
*Avoid clichés like the plague. (They're old hat.) *

Dave Plowman London SW
To e-mail, change noise into sound.

Are you certain there's not a drain cock 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.

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.

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

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 - )
--------------------------------------------------------------------------

In article ,
David Chapman wrote:
Are you certain there's not a drain cock 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.

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.

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

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.

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.

--
*Never underestimate the power of stupid people in large groups

Dave Plowman London SW
To e-mail, change noise into sound.