In an earlier contribution to this discussion,
The Wallport wrote:


The backplate might be slightly damaged so I may have to replace it
anyway. Have figured the wiring out with a continuity tester.
Interestingly, the heating "Off" terminal is not wired but the water
"Off" is. What does this mean?

I assume that you have a Y-plan fully pumped system, with a 3-port valve to
direct the primary water to the HW or CH (or both) circuits?

This type of arrangement requires a HW not required connection from the
programmer, and also requires the cylinder stat to be of the change-over
type - so that a different wire becomes live once the hot water demand is
satisfied. Have a look at the Y-plan wiring diagram in
http://content.honeywell.com/uk/homes/systems.htm - when all should become
clear(ish).

Without these connections, there is nothing to drive the valve to the CH
position when the HW is off or satisfied - and until the valve gets to the
CH position, it doesn't switch the boiler and pump on - so cold house! [The
boiler and pump are fed from the HW side until the HW demand is satisfied].

Your replacement programmer must, of course also have a HW Off connection -
but they virtually all do, anyway.

--
Cheers,
Set Square
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