Since my boiler was replaced (to Viessmannn Vitodens 100-W) have been having an intermittent fault, boiler would shut down and a pair of alarms .... over temp, and low voltage. Sometimes 2 or 3 weeks with no issue then, 5 or 6 times in a day ?
Fun to run in a new 5 core cable to boiler all overhead, but got it done.
Viessmann connected it at the boiler .... initiated call for heat from system .,.. run up OK, cancelled call for heat ..... RCD tripped.
Repeated - and the same result each time.
After some investigation found the L,N,E which were the feed for the previous BAXI, was supposed to have been used for this boiler, the installers though decided only to connect the L (to perform switched live) They needed permanent live so it took that from nearby mains outlet, and also used N from that.
The issue was traced to System controls (switched Live) being on one RCD and the main socket (ring main) was on another RCD ..... the crossed Neutrals were causing the RCD's to trip.
Must have been just on edge of N imbalance threshold as was intermittent, untill pump overrun added.
If we had not added in the pump overrun and made this a repeatable fault could have been chasing this for months.
Luckily had run in a new 5 core cable so could provide permanent L&N as well as pump overrun form my system, and remove the local socket permanent live.
Fair play to Viessmann - they knew it was not a boiler fault, but were prepared to work with me to find the cause, installer had just passed the issue direct to Viessmann.