central heating

[ELZ4742]

its a common fault on these if you have any electrical indifferences in the system.

An easy way to test is to disconnect one of the thermostat wires going to the zone valve. If the valve stays open, the problem is with the zone valve.
Honeywell zone valves are particularly notorious for sticking either open or closed. This is due, 99% of the time. to a bad motor. The motor is far less expensive than a whole powerhead or zone valve. Replacing one takes me about 5 minutes. Shouldn't take you more than 15 minutes.

 

[ChrisR]

The wiring is like this:

I've not seen a "bad motor" make a valve stick open, as far as I can remember. Perhaps I just haven't noticed.
The mechanisms, and the microswitches, do stick though.

 

[ELZ4742]

inded chris, i meant to say microswitch.

 

[roygbmarshall]

Boys,
I've cracked it. It was the hint of a jammed motorised valve - many thanks - that made me check , and sure enough one was jammed open.
It would seem that that sends a signal back to the programmer to keep the system running. (The previous time it was jammed shut.)
A good squirt of WD40 and all is well.
Many thanks to all

 

[ELZ4742]

you cracked it?