On most valves the motor assembly can be removed without opening the water system. There is a lever on the side of the motor that operates the gearing which will indicate if the mechanism is damaged. Test with the power off. Then listen to the motor as the heating is turned on. Do you hear the motor running?
Turn the valve head using a pair of pliers. Is it seized? To replace the valve body some water will have to be drained from the circuit.
Thanks for your reply. Problem is rads are hot when only HW is asked for.
So I presume it is the motorized valve.
With motor body removed but still connected the valve head turns, the black lever moves, what does the position of the lever represent which pos is HW or CH or both as there is nothing on the plastic body. The lever does move when HW or CH is selected.
The valve naturally lies in the hot water only position with the CH port closed. The black lever (You haven't said what make of valve it is) when pushed across moves the valve to the centre, (both CH and HW open) position. The valve cannot be moved fully over to the CH only position manually.
Turn all power to the CH system off and force the lever over by hand. You should feel (and hear?) the clockwork gears turn as the valve opens. It ought to take one or two seconds for the lever to be moved fully across, the same time it takes to return to rest when you let go.
Switch the heating on an select CH only. The lever should move without resistance along the slot.
Most likely causes of your fault.
Faulty time clock / programmer
Incorrectly wired circuit
Worn gears in 3PV valve head
Seized 3PV valve assembly
Worn / broken 3PV ball valve seal.
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