Honeywell motorised valve not working

[xepedro]

<5year old heating system, comprising of (relative to this discussion) Worcester 30CDI gas boiler, honeywell ST9400c programmer (HW & CH) & 2 port Honeywell motorised valves.

Heating is working fine BUT HW is being stopped automatically entering an unvented cylinder by the 2port Honeywell motorised valve.
If I test it by turning controls on/ off & listen for the valves to open/close, they sound like it works as should but when I put the system back on timer then the HW fails to get past the motorised valve again.
If I manually open the motorised valve, the hot water floods in.
Ive changed the motorised valve head BUT still the same problem - any ideas?

 

[petertheplumber12]

If you have changed the head then I suggest it may be a wiring prob. You need a multi meter or someone who has one to check it out. The motor in the head is not getting a signal to turn.

 

[xepedro]

Appreciate the advice.
The Valve definitely has power running to it (I have checked), could the problem be:
1) part of the programmer failed? OR
2) maybe a part of the boiler board failed?
Thanks in advance
(Plumber who fitted the system says it is electrical, so not down to him, electrician has vanished)

 

[oldbuffer]

Check that the overheat thermostat on the unvented cylinder has not operated. If it has, then it should prevent the 2 port valve from operating, to prevent any additional heat being supplied to a hot water cylinder the thermostat already considers "too hot".

The overheat thermostat is not self re-setting, but requires intervention, usually pressing a small button.