If the MZV is not operating as you would expect, then determine whether or not the electrical supply to it behaves as you would expect according to your room thermostat and cylinder thermostats.
If u turn off the power & remove the tin cover off the valve u will see a small synchron motor that can be replaced quiet easily as long as u dont drop the tiny screw./s. u can remove the whole head to do this but only if the valve is the new type. not sure when they stopped makin the old one s softus may tel us that he, s a mine of infomation.
Thanks for the extra info guys - the motor's dead easy to get too, not at home at the moment so I don't for sure if it's a removable one but I'm 99% certain it is.
Another fault can also cause you to have your problem when demanding CH only even when motor functions.
Faulty micro switch of which there are two. One switches from the white wire supply to the grey wire supply. and one introduces a supply to the boiler via the orange wire. A fault on the first switch won't get you past the mid point, on the second switch you will pass mid point ok but won't get any supply to the boiler even though valve has gone to fully open position.
Can you not test the valve for the two separate movements A to A/B, then A/B to B.
I had this problem caused by the second micro switch. the good news was a replacement head cured it. the bad news was the old head was dismantled, micro switch contact points cleaned and now works ok, so £38 to have a spare head
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