On 2020-10-09 22:08:30 +0000, John Rumm said:

Just because they are reliable as a group, it does not mean yours is
going to be perfect!

Indeed. Vaillant engineer fixed it on Wednesday in 15 minutes - it was
a faulty gas valve. But he let something slip - that the valves often
fail and he said he'd fitted an uprated component. This tells me that
there is stock being fitted by installers with a known issue, or at
least known internally to Vaillant.

Anyway it has a 7 year warranty so they can keep on coming to fix it
for free and I will ask for comensation for downtime.