In an earlier contribution to this discussion,
iain_b wrote:

Thanks Roger,

Further investigation shows that I've got the S plan system - I think.
Also turning the heating to 'on' won't start the boiler and if the
boiler is on with the hot water control, turning the heating to 'on'
will switch off the boiler altogether, I'm confused.

Thanks,

Iain.

You ain't the only one!

Correctly functioning S-Plan systems will run the boiler whenever either or
both HW and CH are calling for heat, and turning on one won't stop the
other!

Has any work been done on the system recently - particularly replacing one
of the motorised valves (or just its actuator)? I can't explain the symptoms
unless there's a wiring fault, and even so, you'd have to be quite
'clever'[1] to produce this particular effect.

[1] The secondary switches on the valve actuators are supposed to be wired
in parallel. To get this effect, you'd have to wire them in series *and* use
the NO contact on one and the NC contact on the other!

--
Cheers,
Roger
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