In an earlier contribution to this discussion,
ARWadsworth wrote:



The original Honeywell pictures are available

S plan

http://content.honeywell.com/uk/homes/Schemes/s1.jpg

Y plan

http://content.honeywell.com/uk/homes/Schemes/y1.jpg

C Plan

http://content.honeywell.com/uk/homes/Schemes/c1.jpg

W Plan

I cannot find it.


Yes, those are the ones - well, the wiring diagrams anyway. The original
document also had a plumbing schematic for each plan.

I'm not sure how common W-Plan systems are these days. They use a simple
diverter valve rather than a mid-position valve, and can thus do HW *or* CH
but not both at the same time - usually with HW priority, so you get no CH
until the HW demand is satisfied. AIUI, W-plan was invented *before* Y-Plan,
and the mid-position valve used by Y-Plan was subsequently invented to
overcome the problem of having to wait for the DHW to get hot before having
any space heating. However, with modern fast-recovery cylinders, that is
less of a problem - and I've heard it suggested that W-Plan systems are
coming back. They're certainly simpler from a wiring point of view, and have
less failure modes than Y-Plan.



I have driven over 800 miles today and so I am a bit knackered.

But my quick reply is , when the pump stopped it seems that the water
in the system was so hot it tried to go up the expansion pipe to the
tank in the loft. Lots of banging noises and the vent pipe got very
hot. If I kicked the pump back on then the vent pipe went cold and
the banging stopped.
I still suspect a plumbing problem somewhere. I can take pictures of
the plumbing if it helps.

Pictures would probably help - plus a diagram showing the overall pipe
layout.

On the face of it, it sounds as if there isn't a clear unrestricted path
from the boiler to the vent pipe but, if so, it's curious that this is fixed
by running the pump - unless that simply causes the residual heat to be
dissipated somewhere else, which I suppose is possible.

If my system over-runs for an insufficient period, the boiler's over-heat
stat trips - but there's no drama such as banging and gurgling etc.
--
Cheers,
Roger
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