CH on continuously

[plumbamateur]

Hi all
I have a Vaillant Ecotec Plus 837, around 10 years old. The CH has been running constantly, and I thought originally this was a fault with the timer/thermostat (Drayton Wiser).
I have disconnected the external timer (Drayton heat hub), and fitted a used Vaillant mechanical Time switch 150 to the bay in the front panel. No change, CH still firing when 150 is in OFF position!
I have also removed a linkage between terminals 3 and 4, which my plumber installed when Wiser was removed. No change, CH still on.
Finally, I have removed the TS150 from the front panel, so there is no timer, thermostat or switch connected at all. CH still firing and rads hot.
I'm completely at a loss here, as there is no device calling for heat.
For info, the plumber has changed the PCB (with a refurbished one), which had no effect.
I'd be most grateful for your suggestions.
Thanks on advance.

 

[CBW]

Why was the pcb changed? I’d be thinking diverter valve.

 

[plumbamateur]

Thanks CBW. To be honest, I don't know.
Regarding the diverter valve, could a fault with that create demand for heat (and make the boiler in-effect switch on) even without any controls connected?
Many thanks

 

[CBW]

Thanks CBW. To be honest, I don't know.
Regarding the diverter valve, could a fault with that create demand for heat (and make the boiler in-effect switch on) even without any controls connected?
Many thanks

Unusual, but not unheard of. Is the comfort mode on? Maybe a link has been left in on the low voltage side?

 

[snb1]

Yes it will be trying to preheat the hot water but sending it round the rads and the Dhw plate.

 

[plumbamateur]

Thank you both. I think you're on to this. I've turned comfort mode off and it seems to have resolved it (at least, the CH is off).
Does it follow, therefore, that the next thing to test is whether the rads heat up when using DHW?
Regards

 

[CBW]

Thank you both. I think you're on to this. I've turned comfort mode off and it seems to have resolved it (at least, the CH is off).
Does it follow, therefore, that the next thing to test is whether the rads heat up when using DHW?
Regards

You could test it, but if it’s stopped by turning off comfort then it’s already proven really.

 

[plumbamateur]

Thank you CBW, understood. Can I attempt any troubleshooting on the diverter valve, or is it generally a replacement?

 

[jurassic sparkk]

Most likely the switch has failed inside it so a replacement be a better option.

 

[CBW]

Most likely the switch has failed inside it so a replacement be a better option.

They don’t have a switch, they work via a motor. Pcb sends voltage for it to move following signal from hot water or heating demand.