Our room thermostat might be misbehaving so I've been having a look at it with a view to replacing it. And I've found something I don't understand.
It seems like a standard indirect S-plan system. Boiler, pump, dual channel timer and a valve each for CH and HW. The thermostat is a fairly old Honeywell mechanical dial with an anticipator heating resistor. But the thing that's puzzling me is that return core from the heater doesn't connect to the common neutral in the wiring box; it goes to the CH-OFF terminal on the timer. So when the timer puts the heating on, there's no return path and when the timer puts the heating off, there's 240V on the wrong side of the anticipator.
What explanation could there be for this? Is it an error by whoever wired it up in the first place or is it some variation of the standard setup?
Terminal 2 of a Honeywell stat would be connected to neutral. The best thing I can suggest is to go to www.honeywelluk.com and navigate to the reference guide for your plan. There is a wiring diagram you can follow.
I'm coming to the conclusion that whoever wired it in got a bit confused about what the terminals on the thermostat were.
On the HW side, the spare third wire from the cylinder 'stat is connected to the HW-OFF output of the timer. I'm wondering if the installer meant to connect the other (usually unused) switch contact of the room 'stat.
Could it be that once upon a time the valves needed to be powered in both directions and needed an OFF signal? I did replace the valves a couple of years ago and the new ones definitely have spring returns.
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