On 8 Oct 2020 at 22:48:19 BST, "JohnP" wrote:



I'm trying to keep it simple.

I think a Programmable Termostat is the simplest - get rid of the Timer (or
leave it on 24 hrs.
Set a profile that turns it low in the night and it won't come on anyway.
Enjoy the benefit of holiday mode and a sensible daily demand profile.

If it is an old fashioned controller with a safety earth (usually) and three
further wires to the boiler, common, water demand and heating demand (or only
two if it doesn't control hot water) then putting the room thermostat in
series with the heating demand wire as Roland initially suggested is by far
the cheapest and simplest solution to do what Roland wants. (The fact some of
us wouldn't want to control the heating this way is not his problem.)

If however the controller has one of the fancy bus connections popular
nowadays the thermostat needs connection back to the "room thermostat"
terminals on the boiler. On the old system these were probably used for
Roland's controller, and in any case the boiler demand wire if it can be
identified was available at the controller.

--
Roger Hayter