Mid Position Valve wiring - It's not functioning properly

[Citizen55]

Hi all,

Hoping there is an expert here able to help with weird CH and mid-position valve issues.

Setup:
Y-Plan with Honeywell ST9400 controller, mid-pos valve and cylinder stat. All these items are new. I replaced everything from an old Potterton controller which had the same problem. I thought it was either the controller or the valve sticking so I replaced the lot... only to find the exact same issue occurs.

The problem:
When both CH and HW (cylinder heating) is on and then the CH requirement is switched off, the valve stays in mid-position -AB- causing the radiators to stay hot. If I force the HW off, then switch it back on, only then the valve correctly goes to position B.

I did some testing with voltages below:

.........................Valve........Orange.........Grey...........White

Power OFF........... B...............3.................0 ...............0
HW......................B.............240...............80...............0
CH ......................A ............240..............240............240
HW + CH............AB.............240................80............240
Off (controller)......AB............95................240.............95
Turn CH off
when both were on...AB..........240..............0...............20

So, in my understanding, all these voltages are correct, so why, when CH and HW are on and then the CH is switched off, does the valve stay in AB causing the radiators to stay hot?

Happy to diagnose further if you need to know something.

Thanks for your help.
John

 

[45yearsagasman]

View media item 30560

 

[Magnatrumper]

Did you replace the valve body too?

 

[Citizen55]

Actually when I say I replaced the valve, I didn't actually replace the plumbing part of the valve, just the actuator part, - the motor and electronics.

 

[Citizen55]

View media item 30560

Thanks, your diagram shows exactly what I am getting, unless I am reading it wrong, so I don't understand still.

 

[D_Hailsham]

View media item 30560The explanation in the top right corner could be confusing.

1. Heating and HW: Only the White has 240V. The 240V on the Orange comes from the cylinder stat, NOT the valve.

2. HW and NO heating: The 240V on orange comes from the cylinder stat.

3. Heating and No HW: 240V on White, Grey and Orange. The 240V on the Orange comes from the valve, NOT the cylinder stat.

The only way to solve this problem is to go about it logically as explained in Mid-position valve operation and fault finding.

 

[Citizen55]

Ok thanks for the link, I will work through that and post my findings, thanks all for your help so far. Will let you know how I got on.

John