Y Plan + 2 port valve wiring

[flameport]

So if the thermostat in the cylinder fails its still no help.

Unvented cylinders have two thermostats, one for normal operation and another one which opens when overheated, typically non-resettable, which will cut power to the valve and prevent water flowing.

The valve is what switches the boiler, so normally the valve closing also stops the boiler.
If the boiler continued to heat when not requested due to some other fault, the valve would still be closed, and the result is the boiler overheating and switching off, as the boiler will have some means of detecting overheating.

 

[AndyPRK]

I've not seen this safety stat in wiring diagrams. Maybe built into the factory stat. I know immersions have them.

I was thinking of the condition of the valve stuck open and still closing the micro switch running the boiler. I know it's rare. But still as likely Some if the still dreamt up failure modes.

The spring could break or the spring mount (which I have seen recently)