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 - )
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