I have scanned through the numerous queries regarding problems with
Central Heating when the Hot Water part of the system is working fine
(the problem I also have) and noticed that the majority of times
replacing the motor sorts the problem.

I had a heating engineer out for 2 1/2 hours on Friday trying to find
the cause of the problem and needless to say he thought the motor
needed replacing but it is fine.

The question I had was what triggers the boiler to start up if the
microswitch wire on the central heating motorised valve unit is
deliberately capped off with a junction box?

The engineer and myself both looked for a stray wire to attach to the
other side of the junction box to no avail.

Seems every issue I ever have with houses I own is a 'non-standard'
setup.

Thanks in advance,

John