replying to Mike Humphrey, GeoffB wrote:
Many thanks for your reply ... I have been puzzling over this on my own during
the past week or so, and have come up with all sorts of theories, none of
which turn out to be valid as I delve deeper into the technicalities of the
system and wiring. So I had reached a dead end.

My S Plan system has worked fine for 30 years without a single hitch, but I
noticed recently that (1) the hot water wasn't getting very hot, and then (2)
that the pump & boiler kept cutting out (they would come on for 5 mins if the
water was completely cold, then go off for 30 or 60 mins, come back on for 1
min, or even just 10 seconds, then off again, and so on). I thought the most
likely cause was the timer, so I replaced it but that made no difference.
Then I began testing the cylinder stat, which appeared to be in order. In
desperation I replaced the boiler stat (the knob had become fixed in one
position, not having been touched for 30 years; I couldn't move it at all)
although I knew the boiler couldn't switch the pump off so it couldn't be the
cause.

So what else could be switching the HW off? I decided I had to really get to
grips with the thing, and so I analysed the entire system and the wiring
connections. I noticed that the wiring didn't correspond to the standard
S-Plan wiring which is shown in the timer (Honeywell ST699B1002) data sheet.
So that troubled me, but I carried on tracing a maze of wires through the
timer, junction box etc. At last I thought of the zone valve ... of course I
hadn't realised that the zone valve has a invisible micro-switch: I imagined
that it just opened and closed. So ... I tested the valve motor (resistance
cold about 2.35K ohms, which appeared to be ok, roughly) but while the valve
was closed the live feed from the cylinder stat was still at 240V ... so it
couldn't be anything else.

So I replaced the valve motor, and finally that fixed the problem; I
celebrated by having a hot shower. But by now I knew a great deal more about
the whole business, and wondered why the grey wire isn't connected to the
brown, for each zone valve separately. Hence my question.

Regarding your reply (which I do understand and is very useful), one point
still puzzles me. If the cylinder stat opens (i.e. HW is hot enough), that
cuts the live feed to the HW zone valve, so the valve closes under spring
pressure, opening the microswitch. Isn't that right? Why would the
microswitch stay closed when both brown and grey wires are no longer live?

The only reason I can see for the grey wire to be permanently live, is to
separate the timer/valve circuit from the pump/boiler circuit, so that the
pump/boiler current doesn't flow through (and possibly overload) the timer
switch. But that doesn't make sense, because the timer switch is rated at 3A
and the valve microswitch is rated at 2.2A. And the whole system is subject
to a 3A fuse at the mains switch.

So I would appreciate any additional wisdom you may be able to offer.
Thank you.



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