Known as overrun & takes away residual heat from the core while the burners
and body cool down.

Steve


"Richard J." wrote in message
...
Giff wrote:
Thanks for you help. I've wired it all up and the heating is now
switched by the timer.

The only strange thing I've found is that when I switch the heating
on with the timer, I can hear the boiler fire up and you can see it
light. When I switch it off, the heating switches off but the boiler
is still making a noise. Sounds like a fan but the fan isn't on. If I
switch the heating off using the switch on the boiler then the noise
goes likewise if I turn the hot tap on and then switch it off, the
noise goes.

Our combi (not a Vaillant) does this too, and the noise is from the
central
heating pump. Even if we switch the CH off, the pump keeps on running.
Using a hot tap will always shut off the CH and the pump on our system,
and
this is why the noise stops if we turn a hot tap on for a second or so.
Maybe your system has the same logic.
--
Richard J.
(to e-mail me, swap uk and yon in address)