Johan wrote:
Doctor Evil wrote:

snip


It could be that through the combi when a tap is opened the pressure
becomes
negative in relation to the rest of the cold water pipework. When you
close
the tap the pressure increases in the pipework and moves across the combi
and activates the flow switch, or the 3-way valve. What make of
combi, they
don't all work the same way?

You could

1. Insert a none return valve just before the combi. Then this will not
allow pressure to lower inside the combi and hot water pipework.


The boiler is a Vokera Linea.

I'm not familiar with this model but if it's a type that doesn't have a
diverter valve then you absolutely must NOT put a non-return valve in
the supply side (from the mains): it should say this in the manual.