Vaillant EcoTec Plus 837 Problem

Have a problem with this newly fitted boiler - wonder if anyone has come across this before.

It appears that terminal 4 (CH switch) has a permanent 240V feed and is causing the boiler to fire even when the thermostat is not calling for heat.

We've disconnected the thermostat to eliminate that and the boiler still fires up, obviously the connection between terminal 3 and terminal 4 was removed on installation.

Anyone know why this is happening? We'll contact Vaillant on Monday but wondered it there was anything we could investigate before then.

Cheers

It must be an external control sending 240v to terminal 4.

Remove this wire, make safe, and see if the boiler stays off.

Fixed.

Despite reassurances he'd checked all the wiring I had a quick re-check myself today and found a badly terminated wire in terminal three.