In an earlier contribution to this discussion,
Roger wrote:


It's wired from permanent live on the switched fused spur that should
run the whole heating system..

How come? The microswitch only closes when the thermostat powers up
the actuator.

The microswitch is closed whenever the valve is open. The valve is opened by
a motor - when told to do so by the programmer and stat. It is closed by a
spring when the demand is removed.

HOWEVER, if the wet part of the valve jams open such that the return spring
is unable to close it, the microswitch remains closed, and the boiler
continues to fire - regardless of what the programmer and stat are doing.
--
Cheers,
Roger
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