Strange heating operation

[wilbye]

You hijacked another thread so it was split to a new one.

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This has just started happening and seems intermittent.
Potterton Boiler and controller

Siemens 3 port diverter valve MA-V322C

Problem is that the heating will sometimes turn on even when the thermostat is set below the room temperature. The only way to cure this is to switch off the AC supply to the controller. If the controller is changed the fault persists.

Switch on Central heating on controller,
Turn thermostat above room temperature, valve moves to CH and boiler starts
Turn thermostat off, valve remains in CH position
Turn CH off at controller, valve remains in CH position
Turn off at ac supply to controller and valve returns to HW position

 

[oilhead]

Faulty head on diverter valve. Unless youhave a special programmer, it is usual for the valve to stay in the CH position when turning off on the programmer or thermostat.
I am assuming that the problem has only just started. If it is long standing, you may have a wiring problem such as the permanent live to the grey wire commoned up with the heating demand supply.