S plan wiring woes

9 Nov 2016
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United Kingdom
Hi guys. I've got something strange going on with a s-plan system with two 2-port valves, dual programmer, room stat and cylinder stat.
Even when the programmer is not calling for heat there is power going to the ch valve causing the boiler to fire up. The only way to turn it off is to kill the mains power. Tried a new programmer and a new valve with no luck. Much head-scratching going on so any help greatly appreciated.
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Might be someone has not run a separate permanent live to the switches on the valves, if a switched live from the system is used (to save a bit of wiring) the feed from one valve can feed backwards into the other valve and hold it open. Try turning off the power then try one valve circuit if it works OK, turn off and cut the power again and then try the other circuit, if OK switch on both valves then try to turn one off the fault should have returned as soon as both are used.(y)
They grey on the zone valves are normally tagged into permenant live with the orange being switch signals outs.

Silly question but they arnt locked open manually by accident thus creating a demand when there actually isn't one
Thanks for the replies, it turns out that underfloor heating has been installed in a separate part of the house which the installer has just branched off the nearest radiator. He then put in a thermostat and linked that to the heating valve. So two thermostats control one valve, as soon as the weather turned cold the ufh stat turned the heating on in the whole house. It took some finding !
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