In an earlier contribution to this discussion,
wrote:

Ta to Set and Andy for their replies.

Conventional arrangement in the house: hot tank upstairs with
thermostat;
programmable thermostat in hall downstairs; three-port diverter valve
midships in
the house.
The conservatory will be tacked on to a gable wall - the route from
the existing
three-port valve locus to the gable end would be tortuous (not
impossible, but just
outside my "contemplated" range just now! :-)

Mungo

OK then, Plan B!

I assume that CH is set to Constant on the programmer and *actually* timed
by the programmable stat in the hall?

If so, how does this grab you? Have a programmable stat in the conservatory
to control your new zone valve. Wire the secondary contacts of this zone
valve in parallel with the prog.stat in the hall. Then, whenever the
conservatory is calling for heat, it will bring on the house heating even if
it isn't otherwise scheduled to be on. If the house rads have got TRVs,
having to have the house heating on to heat the conservatory shouldn't be
too much of a problem.
--
Cheers,
Set Square
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