In article ,
visionset writes:

New Vaillant combi.
Any cold mains tap or valve is closed boiler seems to sense increase in
pressure and turns itself on. I imagine without investigating further
that it is heating the small hot water store as it does do intermittently
anyway. This happens every time so it isn't coincidence.
Any ideas?

Possibly a deadleg of hot water pipework somewhere which is
full of air. The pressure increase in the cold supply as you
turn off a tap due to the momentum of the incoming water will
result in the air deadleg being compressed by water momentarily
flowing through the boiler. I don't know how the hot water store
works in that boiler, but maybe the air could be trapped in there?

--
Andrew Gabriel