In an earlier contribution to this discussion,
iain_b wrote:


Hi Roger,

No work has been done on the system.
I'm now totally confused - both the hot water and heating are in the
'off'' position on the controller, but the boiler is still working -
I'm guessing that the controller's internal circuits are goosed and
hopefully a new controller will fix things. It's a British Gas UP1
controller, any ideas where I can get such a thing - I take it it will
be a Honeywell or similar re-badged.


No, there's a more likely explanation than a duff controller.

An S-Plan system works like this:
The controller and room stat control the CH zone valve.
The controller and cylinder stat control the HW zone valve.
Each zone valve has a secondary switch in its actuator which is electrically
isolated from the motor circuit, and which makes when the valve is fully
open.
These secondary switches have a permanent live input (independent of
anything the controller or stats are doing) and their outputs feed the
boiler and pump.
[If you study the S-Plan wiring diagram in
http://content.honeywell.com/uk/homes/systems.htm you'll see what I mean]

If one of the valves sticks open (or if its secondary contacts weld closed)
the boiler and pump will run even when everything is switched off. You need
to have a close look at these valves!
--
Cheers,
Roger
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