Heating zone remains on if another zone is on

[Brotheroak]

Question for the assembled mega-brain...
I am trying to get my head around the heating system in our recently purchased home. I'm a fairly competent DIYer, not a heating engineer, and I know not to mess with anything I don't understand, so at the moment just seeing how it all works.
Our home was once a simple bungalow, but has been added to over the years with extensions etc. It now has 5 zones plus HW, controlled by individual thermostats. S-plan system running off an oil fired system boiler (built in pump - apologies if I have the terminology wrong!)

It all looks well designed - the wiring centres are all labelled nicely, for example - but with so many zones, it is obviously complicated.
I have traced all the wiring and understand most of it, however have one outstanding question/issue:

The latest extension (built about 10 years ago) has 3 new zones - underfloor heating zones, though one also has a standard 2-way valve to feed two radiators. The other 2 zones are both underfloor and use the same pump, just different valves on the UFH.

However, all three of these zones, unlike the other zones in the house, show a strange behaviour - if they are the only one calling for heat, it all works fine - turns on when it should, and off when it should. However, if it calls for heat and turns on and then another zone also calls for heat (even the HW cylinder stat), then if it stops calling for heat at its own thermostat, their pump and valve still remains "on". It only turns off when no other zone is calling for heat. However, it doesn't *come on* when other zones call for heat, it just *remains on* where there are other zones on, even if it shouldn't.

I was assuming that the live from the boiler is feeding power back to the system, but I would have thought this would not matter as the thermostat would/should not be feeding power to the valve/pump. One difference between these zones and the others in the house is that these use Flomasta thermostats whilst the others use Danfoss. All 5/1/1 day programmable thermostats that in all other regards work ok.

I've actually mapped all the wiring, and it looks correct (albeit complicated!) but wondered if this was normal behaviour (though my other zones don't do this) and deliberate? I have lots more info, but this post is already long enough and don't want to put people off!

 

[Iddy]

From your description one or more of the zone valves have been wired incorrectly. The controls should operate the motor of the valve (brown wire live, blue neutral). When fully opened there is a switch inside the valve head that operates. This in effect which connects the grey wire to the orange. The grey should be connected to a permanent live, the orange goes to the boiler and pump 'on'. Sometimes an installer connects the grey and brown together - this works but can allow a live 'back feed' keeping zones on when they should be off. In short if you see a grey and brown connected to the same live feed it's wrong

 

[Brotheroak]

Nice, thanks - will check. It looked ok when I first looked, but with your explanation I can have a closer check. Will report back - thanks

 

[Brotheroak]

Thanks very much @Iddy !
Couldn't quite work out what you were getting at, but checked the wiring, and your explanation also led me to the following thread: https://www.diynot.com/diy/threads/need-help-understanding-my-s-plan-wiring.516302/
which talks about the same issue in more detail.

Turns out that was exactly the problem - the grey out of the valve was connected to the switched live from the thermostat, rather than the live. Switched it around and it is now working correctly.

Now to sort out the other zone that has the same problem (but the wiring is more complicated, as it uses relays too!)

Thanks again - much appreciated.