In an earlier contribution to this discussion,
doozer wrote:

Hi,

I really hope you can help me because this problem is driving me nuts!
There is something wrong with our heating system which is causing it
to just do random things. Sometimes it won't turn on and other times
it won't turn off (this morning is a prime example of that). At other
times it will come on maybe for a few minutes and that just turn
itself off.

I come to you in desperation. There is such a shortage of plumbers in
this area that they won't come out unless I can reproduce the fault
while they are here but the fault is random happening for maybe a day
a week. I did manage to talk to one plumber who thought it might be
the circuit board in the boiler (the boiler is a Glow Worm Space
Saver 50 MkII and looks quiet old) but I am really sceptical.

Anyway a few more details. The most dangerous thing that happens, IMHO
(please excuse the fact I don't know what all the various bits are
called), is when the boiler fires up but the hot water produced is not
pumped around the hot water tank or the radiators (the pump goes ten
to the dozen and sounds like it's trying to tear itself off the
pipe). I am fairly sure there must be a safety cut off because under
those circumstances the boiler does go off briefly. The Honeywell
controller / timer box, an ST6400C
(http://www.plumbworld.co.uk/acb-1357), has been bodged on to the
wall (previous owners fault) and tapping it, even very lightly, with
my finger can cause the heating or hot water to come on.

If I had to make a guess I would say that it is the controller / timer
box but I don't know for sure. Any ideas?

Bearing in mind our boiler is quite old I would also be interested to
know how if you think it might be worth ripping it our and sticking in
an instant heat system (or heat on demand)? At some point the house
has had a loft conversion so it has no header tank - it has some
complicated Megaflow thing-m-bob with a little red pressure tank
instead. Removing the hot water tank would give us a lot more storage
space so it is an option if repairing the existing system is going to
be very expensive.
Many thanks for getting this far down my rambling post )

It sounds to me as if the pump is cavitating through lack of water.
Presumably the little red pressure tank is the expansion tank for the
primary circuit (the water which flows from the boiler, through the
radiators and through the heating coil in the hot water cylinder)? If so,
there should be a filling loop for adding water, and a pressure gauge. Is
there? If so, what does the pressure gauge read (a) when the system is cold
and (b) when everything is hot?

Whatever you do, *don't* get rid of your stored hot water system!
--
Cheers,
Set Square
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