simond said:If you send a switched live into pin 4 you will not be able to use the I/0 switch, because the switched live backfeeds the PCB.
A very common wiring fault, we often see it - not on our ones, of course!
The on/off breaks the permanent live but not the neutral. The sw/L from the clock then backfeeds the boiler.
All Vaillants are made this way so that the OH stat drops the controls, this functionality was used in the early Vantage cylinder which is why they could use a three port valve. The OH stat drops the live from the perm live input to pin 3.
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