S Plan Zone Valve Grey Wire To Brown Wire?

[John Rumm]

The full S plan circuits are available here:

or download a full set of wiring diagrams:

[GeoffB]

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.

[GeoffB]

replying to Mike Humphrey, GeoffB wrote: PS I have studied your reply and thought about it some more. I would like to add the following note: let's assume that the grey wire from the HW zone valve is connected to the brown wire, but before the thermostat. So in the case you describe, the brown wire from the timer to the thermostat would be live, I agree. But not the wire from the thermostat to the HW zone valve, because the thermostat switch is open. The more I think about this, the more befuddled I become.

[GeoffB]

replying to ARW, GeoffB wrote: Thanks for your reply. Do you mean that there is some sort of connection between the HW and CH switches inside the programmer? If so, then your explanation begins to make some sense. For the case you describe, if the CH switch is still live, owing to 'back feeding' from the live HW switch, then the CH valve would stay open. But how does 'back feeding' work?

[GeoffB]

replying to GeoffB, GeoffB wrote: I've got it, finally. Once you understand it, it's simple and obvious, but before that it seems incomprehensible. I now see that there is a 'crossover feedback effect' which keeps the valve (although switched off by the timer) open while the other valve is also open. I was fixated by the idea that the timer would cut power to the valve, which would then close, thus opening the microswitch. What I didn't grasp is that the feedback effect is instantaneous, so the valve wouldn't have time to close.

Regarding my PS about connecting the grey to the brown BEFORE the cylinder thermostat, I have thought about how things would work out if you did that. After all, the thermostat provides a secondary level of HW control, after the timer. It appears to me that these are the possibilities:

1. If either HW or CH are on, but not at the same time (no overlapping either), there is no feedback effect.
2. It HW and CH both switch off at the same time, there is no feedback effect.
3. The effect arises when both HW and CH are on at the same time, then one of them is switched off by the timer.
4. In case 3, if the HW is switched off, the HW valve stays open (feedback effect) until switched off by the thermostat.
5. In case 3, if the CH is switched off, the CH valve stays open (feedback effect) until the thermostat switches off the HW valve, so ending the feedback effect and so switching off the CH valve.

[GeoffB]

replying to GeoffB, GeoffB wrote: In case 4, all that would happen is that the HW would continue to run until it reaches the temp set by the thermostat -- which is what you would want, anyway. In case 5, the CH would continue to run until the HW reaches the temp set by the thermostat, then both CH and HW go off.

My conclusion from all this is that the practical effect of wiring grey to brown (for the HW, before the thermostat) seems to be minimal. Ok, the CH would run on for a bit until the HW was hot enough (which it probably would be, well before the CH was switched off), but that's a fairly minor consideration. Overall, you really wouldn't notice any difference in the operation of the whole system.

Anyway, I've wired up the whole system in accordance with the standard S Plan diagrams, grey permanently live. Plus notes attached to the wires so I know where they come from and go to. I've also discovered why the installer (30 years ago) didn't follow the standard wiring -- he cut the wires too short, so couldn't reach the correct terminals (this had baffled me for weeks until I decided to change it).

So that is the end of the story, hopefully for another 30 years. Many thanks for all the helpful replies I've received. GeoffB.