Motorised Valve not operating as expected. CH Stays on.

[leso]

First may I appologise if this has been covered before, I have searched back quite a few pages and cant find this exacr fault.

Specs.
Potterton Ultra 2 DV Sealed Syetm Boiler
Potterton (in built) 3 way valve MSV322/07
Landis & Staefer RWB9 Programmer
Honeywell room stat

Symptoms
The CH thermostat does not seem to be able to switch the MV to the HW only position when the room is up to temperature, it does however turn the MV when CH is required.

What I know so far.
Boiler fires and runs normally.
Responds to programmer switching on and off.
Reponds to HW thermostat on and off.
Responds to CH thermostat on only.
MV will reset to HW only when I switch off HW & CH on the programmer and then switch on HW.
HW works fine on its own.

I have so far replaced the room stat as a cheap tryout.

Also for a different reason I have changed the control circuit board.

Symptoms persist


Thanks to anybody who is out there.

[Perry525]

You do not name the motorised valve.
There are a number of different types of valve and two basic methods of operation. The common valve motor open, spring closed and motor open motor closed.
With the latter, the open signal comes from the thermostat and the close signal from the constant main.
If it is a Sunvic again there are different types spring close or motor open with four wires or five.
You need to pull the head off, and switch the valve to watch the operation, it may be a simple bad connection, or an intermittent fault.

[leso]

The valve is a Potterton 3 way valve MSV322/07, not sure if it is motorised or sprung.

I have a multimeter so if you could explain the principles of how it operates I can check the continuity and if the 'signal' is getting through.

Thanks