On Sun, 21 Nov 2004 13:29:07 +0000, Pete C wrote:

On Sat, 20 Nov 2004 21:12:25 +0000, "Ed Sirett"
wrote:

Sorry I have now read the OP properly.
The combi is working correctly and the 'feature' does occur on many
models.

I'm familiar with several models although some models might not exhibit
the problem though most do I would guess. A model name would help here.
In fact the only ones which I reckon are ones with a sperate internal
bypass pipework.

On many models the secondary heat exchanger also serves as a part of the
bypass ciurcuit. This is easily implemented by arranging for the
diverter valve to bleed primary water through the secondary heat exchnager
when in CH mode. So in winter the 2ndry HE gets to the temperature of the
primary flow. This means that the contents of the 2ndry HE are hot, this
slug of very hot water will comes out first in DHW mode.

To mitigate the problem try turning the radiator tempertaure down.
Of course if the rads are not over sized then the house won't get
heated. 8-(.

All the boilers which have double primary heat exchnagers (eg Ferolli)
or integral tube-in-tube type like (Pott. Lynx, Saunier Duval)
will also have this problem and likely a worse version.

Hi,

Does it bleed water regardless of the flow in the primary and can it
be blocked somehow? If there is a differential pressure valve in there
or it can be blocked it may be possible to use a lower pressure one
elsewhere.

I'm fairly sure that modern Vaillants and Vokeras work as I describe.
The diverter-valve does not entirely block the path through the primary
side of the secondary heat exchanger. There is nothing smart about this
it is simply a deliberate hole in the valve. The effect is to slightly
reduce the output of the boiler in CH mode, since the output in CH mode is
usually far in exces of requirements this is not a problem.

The only way to prevent the primary water doing this wouod be to modify
the diverte valve. The effect of doing this would be disasterous.
1) The boiler would have no bypass.
2) On Vaillants there would be no sensing of primary water temperture
leading to a catastrophe.









--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at 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