Does my boiler really need a 21vac input?

[Over Carl]

Was trying to diagnose a fault that mysteriously fixed itself when I managed to break a wire. (boiler is a recent valliant combo). I think all the wires to the boiler come from the controller, boiler had 3x21vac terminals to which one black wire was connected. On the 6x240vac terminals beside it were blue, brown and green all showing expected voltages, as well as one black wire which also seemed to be live, and one black wire that seems to be carrying around 3vac when boiled should be off and around 7.5vac when the boiler is meant to be on.

Strange thing is, even without the 21vac wire connected, it all seems to be working perfectly. Can I leave it this way as I think it came from the middle 21vac terminal but not 100% sure.

In case it has any relevance, the fault I wanted to fix (which I was told would most likely be a relay on the main board) was that it came on in the middle of the night one day for no reason at all. We temp fixed it by turning the thermostat and radiator temp down to min, but I couldn't get the fault to repeat at all today.

[coin-op]

Basically - yes. They wouldn't waste time and money adding a wire and terminal just to look nice would they?

[B B Leo]

defo need it m8.. it could control mortised values, thermostat, of temp stat for Ur cylinder etc, any of them disconnected could and i mean could over heat/fail