Recurring hot water cuts with Vitopend 100

[agarttha]

Hi,

I have a Vitopend 100 combi boiler. In the winter we have an issue where the hot water cuts off intermittently, for example during a shower, or at the kitchen sink. I have attached a link to a video which shows the front panel of the boiler as this occurs. The boiler is working fine, then it suddenly cuts out, tries to restart multiple times before it can get hot water going again, and the process repeats. I have had technicians replace almost everything in the boiler, including the electronics, the heat exchanger, flow sensors, and almost all temperature sensors. I am at a loss, and so are the technicians, from two different gas boiler maintenance companies, over several years. They keep proposing to replace another part, and I think the 3-way diverter is next on their list. But they haven't done electrical tests to determine which sensor/relay is tripping and preventing the boiler from maintaining heat on the hot water circuit.

This does not occur in the summer. Several days ago the heat exchanger was replaced, and that seemed to fix the problem, but it started up again yesterday. The technician now thinks that the new heat exchanger got plugged. But the old exchanger didn't seem particularly plugged to me when I ran water through it.

I have followed this forum over the years, hoping to find help. Perhaps the best is to ask you. If you need more info, some pictures, or more videos, please let me know. Anything to solve this problem.

Cheers

WhatsApp Video 2026-02-24 at 21.36.42.mp4

drive.google.com

 

[CountryFan]

Video won’t load for me. Could be crossover, eg cold leaching into the hot. If it’s only happening in winter, then could be what I’ve suggested. Diverter valve imo would be constantly, unless you are getting hot water when heating is on?

 

[agarttha]

I just updated the link to the right video. For the diverter valve possibility, we don't get hot water mixing in when heating is on. For crossover, is it possible to have intermittent crossover? Would that lead to a fault condition and the boiler stops heating?

 

[CountryFan]

For crossover, is it possible to have intermittent crossover? Would that lead to a fault condition and the boiler stops heating?

Not usually intermittent, however the water incoming is warmer in summer. If you isolate the cold inlet and run hot water outlets, if you get water coming out then it’s likely crossover.