On Fri, 15 Sep 2006 19:51:02 +0000, Andrew Gabriel wrote:

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?

I have a deadleg as a temporary measure when swapping over from the stored
water conventional system to this one. As I have good ceiling and floor I
have capped off the hot supply from the old cylinder, so maybe it is
there. I'll try flexing the capped end down and whilst SWMBO runs the
taps bottom to top.
The boiler itself has a bleed program that does heating and hot water.
I've run this twice, so I'd expect internal components to be properly
bled. Shag, I've just thought - haven't bled the washing machine...

Thanks AG!

--
Mike W