CH when calling for HW only

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I'm seeing a strange one and can't work out exactly the cause or reason.

When calling for just HW on the programmer I see 240v on the Orange wire and 63v on the white wire of the 3 Way Diverter Valve. The 63v is causing the diverter valve to hold in the mid position which turns on both CH and HW.

If I then turn the HW Stat down from 60degrees to something that triggers satisfied, the 240v flips to the Grey wire and the pump stops as expected, per diagram.

I then turn off HW at the programmer, then turn the HW Stat back up to 60 degrees, turn on the HW at the programmer again and then I no longer get the 63v issue.... for a while.

Any thoughts or suggestions?

The programmer is a Tado smart thermostat.

Wiring diagram...
upload_2022-4-20_22-51-10.png


3 way diverter valve positions diagram...
upload_2022-4-20_22-19-30.png
 
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What boiler is it? I believe a certain make/model can send a backfeed.
 
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mid-position-valve.jpg
White line (live) for no DHW and Grey line (live) for CH. Orange is output to boiler when central heating is required, but when DHW is required the supply comes direct from the tank thermostat.

In the main find one of the micro switches in the three port valve has got stuck. But you can't renew the V3 micro switch so have to replace whole head.
 
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What boiler is it? I believe a certain make/model can send a backfeed.

Baxi solo 50/4

In the main find one of the micro switches in the three port valve has got stuck. But you can't renew the V3 micro switch so have to replace whole head.

Originally thought it was the valve, so that has already been changed in the past 12 months. Unless I’m really unlucky and this new valve has a fault too
 
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The 3-port valve needs a HW off connection from the programmer / smart stat to the motorised valves grey wire. You have it from the cylinder thermostat (the black wire in your case) which is correct, but it should also be connected to the programmer.

Y-Plan-Wiring.gif
 
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When calling for just HW on the programmer I see 240v on the Orange wire and 63v on the white wire of the 3 Way Diverter Valve. The 63v is causing the diverter valve to hold in the mid position which turns on both CH and HW.
For DHW off should be 230 volt on the white wire. Sorry missed that bit.

It is powered for off not powered for on.
 
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Thanks, appreciate all the responses here. Apart from the Tado smart programmer which I installed, I’ve recently purchased the house and so inherited this!

Is the missing HW off wire the cause of my specific 63v issue, or is that just an observation? I can’t fully get my head around why it would be related, but also understand what you are all saying as I had seen similar wiring diagrams previously.

Currently the smart programmer is in a different part of the house which doesn’t have any spare wires… I can relocate it to the boiler cupboard and let you know how it goes!

Another thing I’ve noticed, and I don’t know if this is normal, if I need CH, I need to call HW and CH. It looks “expected” as I can see the pump is wired to only come on when HW is called. This is not an issue with my Tado as the company has reprogrammed it to call for both CH and HW whenever I need CH.
 
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Is the missing HW off wire the cause of my specific 63v issue, or is that just an observation?
Maybe... The problem is with heating systems is that various voltages get fed back from the boiler, and also via the diodes and resistors in the motorised valve, and it can change according to the status of the various other components in circuit. But in any case, to work properly the missing wire does need to be present.

I can see the pump is wired to only come on when HW is called.
No it's wired to come on with the heating too. The orange wire coming from the motorised valve will also start the boiler and pump. But having said that, actually it won't always now, because the valve won't trigger it under certain circumstances, when it should, but it will when the missing connection to the grey wire is added.
 
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Ahh makes perfect sense @stem. Yes I can see the orange wire should also drive the motor. Possibly it could have been C plan at some point @ericmark, that might explain why I don’t have enough wires behind the programmer which was in the kitchen!

I’ve now re-wired and added in the missing “HW off”; I had to relocate the programmer from the kitchen to the airing cupboard to sit with HW tank, valve and pump. Better in there anyway.

All working as expected for now, can also call HW and CH independently. Hopefully it has permanently fixed the 63v issue that I originally raised too!

thanks again all.
 

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