On Tue, 22 Mar 2005 23:18:05 +0000, Johan wrote:

John Stumbles wrote:

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.

The manual says there is a diverter valve, but this appears to direct
the water either through the central heating, or through the Domestic
heat exchanger (to produce hot water). How does this affect putting a
non-return valve in the hot water pipe after the boiler?
It does not say anywhere in the manual that a non-return valve should
not be used in the water supply, it also doesn't say that it is ok.....
Sorry to keep asking questions, but I'd really like to understand this
problem now.

When you had the cold water off air may have got into the Hot pipework.
If you have a 'dead' branch somewhere this could harbour some air. The
effect of the air would be for a small flow to occur through the boiler
when the cold taps are worked.

This boiler is able to dump residual heat after doing HW into the CH
pipework so this does not precluded the fitting of a non-return valve (NRV).

However since this boiler did work correctly before searching for the
trapped air is likely to be more fruitful. A NRV would solve problems. A
shock arrestor won't help IMHO.

Some boilers (e.g Ferolli) a NRV would be bad news, and the manual will
forbid it. Since residual heat would lead to expansion of the water in the
HW pipes with nowhere to go.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html