"John Stumbles" wrote in message
...
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.

I'm pretty sure it has a 3-way valve and a flow switch, hence the
momentarily turning on of the fan. Then you can. I would check with makers
first though. When the DHW is heated via the main gas heat exchanger (no
3-way valve) it uses the cold supply as an expansion route. Using a plate
heat exchanger the temperatures will get nowhere near as hot. I would put a
shock arrestor on the hot draw-off side as well. As near to any lever tap as
possible, as these can cause clunks when turned off, and the arrestor will
absorb any expansion from the combi too.



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