On Thu, 31 Jul 2003 21:11:35 +0100, Ed Sirett
wrote:



At the merchant I use (Peter's Plumbing N11) we have had conversations
about which boilers have worked, gone wrong, were good value etc.

The opinion is that only the Keston boiler produces condensate. [I would
imaging the MAN would also].

Yes it does, by the bucketful. Not that it empties into a bucket I
should add, but through 22mm plastic and a trap to the waste.

I wonder whether modulating the pump as well as the burner has an
impact. The Keston, AIUI, does it by switching a Grundfos pump to
one of its three settings.

The MAN has a means of analogue or perhaps quasi-analogue control of
the pump (also Grundfos) from 35% minimum up to 100%. From the
display on the PC screen when the diagnostic software is run, the pump
rate adjusts to the firing rate, but they are not directly linked.
I've seen dT between flow and return on occasions of around 25
degrees, so even if the flow is approaching 80 degrees when it's
really cold or in HW mode, condensate is produced.
There is realtively little visible pluming, even on a cold day.
I wonder whether this has something to do with the cylindrical heat
exchanger and burner design.....

As I figure it, provided that the change from gaseous to liquid
state of the water happens either in the boiler or the part of the
flue within the building, the efficiency improvement should be
achieved since it is from the heat released by the latent heat of
condensation due to the change of phase. The visible "steam" is of
course water vapour and already in liquid phase from the latent heat
perspective.

Having it condense as water inside the boiler would appear to be more
of a convenience in that sense. Most installation guides that I have
seen talk about installing the flue such that it is at a slight angle,
sloping towards the boiler. I am told that this is mainly to avoid
acidic concentrate dripping down walls etc.

Perhaps this explains the "nuisance plume" aspect. If the heat
exchanger is rather too warm but nevertheless reasonably efficient and
the flue is cool enough, I would expect pluming to occur until the
water vapour re-evaporates. This would imply the best efficiencies
not being achieved, I think.


..andy

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