Downstairs heating turning itself on, upstairs fine?

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Problem:
  • Downstairs radiators keep coming on by themselves, even when thermostat/programmer would indicate CH either shouldn't be on at current temp/time or programmer is deactivated but CH still comes on.
  • Upstairs rads function at the correct time/temp as per the programmer.
Tried so far...
  • I have switched the programmers around (up and downstairs, the display part of the body which contains the batteries is removable) but this does not solve the problem, downstairs still randomly comes on.
  • CH downstairs randomly comes on when the hot water programmer is off, which makes me think it's not the diverter valve stuck in position.
  • I can get the downstairs radiators to turn off by forcing the programmer to click on (audible click) then press deactivate, but if I leave it deactivated eventually it will turn on again by itself. Again, upstairs functions normally.

Plumbing Setup:

  • Boiler: Potterton Promax SL (frost stat in garage along with boiler)
  • HW Tank: Santon Premier Plus
  • Zone 1 (Downstairs) Diverter Valve: Honeywell 2 Port set to Auto
  • Zone 2 (Upstairs) Diverter Valve: Bosstherm 2 Port set to Auto
  • Thermostat/Programmers: Myson MPRT hard wired (one upstairs one downstairs, fresh batteries and have been reset).
I'm a bit stumped at this point - any help is very welcome! Some pics below if that helps...

Myson Programmer and Backplate
myson_prog.PNG

myson_backplate.PNG

CH Layout
ch_layout.PNG
 

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    diverters.PNG
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downstairs zone valve is sticking
I'm completely ignorant when it comes to these valves, should I move the arm to MAN and then back again a few times? Is there anything else that I could do to prove this is the issue as I would probably have to get a plumber in to replace it if that's the case.

Also, I can deactivate the heating for a few hours to the point where the rads are cold and leave the programmer off but it still comes on - doesn't that suggest that the diverter valve is fine but something is triggering it to open up specifically for heating?
 
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The lever that you refer to is simply for filling the system, with both zones switched off, when you try to move the lever across you should feel resistance, with both up and down calling for heat the lever should slide with no resistance as the valve is open, switch the d/stairs off and see if the valve lever is floppy , if it is it has seized
 
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The frost stat is adjustable, it would be set at around 5 degrees.
Are the rads only coming on when it’s cold
 
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The frost stat is adjustable, it would be set at around 5 degrees.
Are the rads only coming on when it’s cold

The frost stat is set to the minimum temperature (and has been since the house was built 5 years ago so nothing has changed there). It's obviously very cold right now so it could be triggering the frost stat, but should that connect to the radiators? I've no idea how to check.
 
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Problem:
  • Downstairs radiators keep coming on by themselves, even when thermostat/programmer would indicate CH either shouldn't be on at current temp/time or programmer is deactivated but CH still comes on.
  • Upstairs rads function at the correct time/temp as per the programmer.
Tried so far...
  • I have switched the programmers around (up and downstairs, the display part of the body which contains the batteries is removable) but this does not solve the problem, downstairs still randomly comes on.
  • CH downstairs randomly comes on when the hot water programmer is off, which makes me think it's not the diverter valve stuck in position.
  • I can get the downstairs radiators to turn off by forcing the programmer to click on (audible click) then press deactivate, but if I leave it deactivated eventually it will turn on again by itself. Again, upstairs functions normally.

Plumbing Setup:

  • Boiler: Potterton Promax SL (frost stat in garage along with boiler)
  • HW Tank: Santon Premier Plus
  • Zone 1 (Downstairs) Diverter Valve: Honeywell 2 Port set to Auto
  • Zone 2 (Upstairs) Diverter Valve: Bosstherm 2 Port set to Auto
  • Thermostat/Programmers: Myson MPRT hard wired (one upstairs one downstairs, fresh batteries and have been reset).
I'm a bit stumped at this point - any help is very welcome! Some pics below if that helps...

Myson Programmer and Backplate
View attachment 222274
View attachment 222275
CH Layout
View attachment 222272
Suspect valve no 8 in your schematic, sticking on and firing up the boiler.
 
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what was the result when you checked the valves
Not checked yet, system and pump is running so don't want to manually open the valve while the pump is running. Will report back when the current cycle turns off (it's going to be a cold one tonight so at least need to heat the house up before I play with valves!).

Additionally I waited till the rads downstairs went cold and just removed the programmer from the wall and so far they have stayed cold all afternoon. Bit of a crude stop gap at least.
 
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wont make the slightest bit of difference

I tried moving the leaver when all zones were off and felt steady resistance (and some small noise which I assume is the port opening). When I let go it moved back into the auto position. I moved the other leavers to compare and they all had the same feeling of resistance.

None of them really felt loose or flappy.
 
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There's an internal microswitch that fires up the boiler, that needs checking. Do you have a multimeter?
 

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