In an earlier contribution to this discussion,
fred wrote:


In answer to your question about why the boiler was staying on, it is
common to feed the demand from the a cylinder or room stat to the zone
valve and then use the output from the status microswitch in the zone
valve to create the demand for the boiler. That way the boiler isn't
fired until the zone valve is open.

.. . . but, as the OP has discovered, the downside of this system is that if
the valve sticks open, the boiler fires continuously - regardless of demand
from timers and stats.
--
Cheers,
Roger
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