iain_b wrote:
On 20 Sep, 10:47, "Roger Mills" wrote:
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 theboileris 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 theboilerand pump.
[If you study the S-Plan wiring diagram inhttp://content.honeywell.com/uk/homes/systems.htmyou'll see what I mean]

If one of the valves sticks open (or if its secondary contacts weld closed)
theboilerand 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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Roger,

I checked the valves and the heating one was stuck, working it open
and shut with a spanner and a wee bit of WD40 loosened it off
completely, did the hot water one too for good measure. Everything is
working properly now. Thanks very much for all your help - this
central heating novice (could you tell?) was totally lost.


Be advised that once they stick once, they tend to do so again with
increasing frequency until they are irreperable.

Schedule a replacement for a quiet day next summer, and get the part in
now in case you need t do it on an icy Sunday morning in winter..


Cheers,

Iain.