In an earlier contribution to this discussion,
wrote:

Dear All,
My domestic Central Heating Pump is running continuously despite the
fact that the CH mode on the Landis & Gyr mini programmer is switched
off. The only method of switching the pump off is to disconnect the
electrical supply at the local junction box. Not a particularly good
method.
Can you point me in the right direction please and advise the heirachy
of the control system. By this I mean, does the programmer have the
sole control of the central heating pump by virtue of the CH mode?
The system also has 2 off 2 port motorised valves.

Advice gratefully received,
Regards
Mark A Shaw

Has it ever worked properly? Does the boiler fire as well, or is it just the
pump. If it hasn't ever worked, and is just the pump, it could be wired
wrongly.

It sounds as if you have an S-Plan system. [See
http://content.honeywell.com/uk/homes/systems.htm and scroll down to S-Plan]

In an S-Plan system, the programmer and thermostats control the motorised
valves (one for HW and one for CH). Each motorised valve has an independent
set of contacts which close when the valve is open. These are used to
control the boiler and pump. When everything is wired correctly and working
properly, the boiler and pump *only* come on when either or both circuits
are demanding heat.

One variation to the standard wiring can occur if the boiler required pump
over-run. That is, if the boiler needs the pump to continue running for a
little while after it stops firing, in order to carry away the residual
heat. In this case, the pump will be controlled by the boiler, and will be
connected to the boiler's 'pump' terminals rather than just being wired in
parallel with the boiler, as per the Honeywell diagram. If this is true in
your case, there is a possibility that something could have gone wrong with
the boiler's pump control logic - causing the pump to run all the time.

Let us know what you find!
--
Cheers,
Set Square
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