On Thu, 27 Nov 2003 11:32:18 +0000, John Laird
wrote:

On Thu, 27 Nov 2003 09:17:22 -0000, "Christian McArdle"
wrote:

How do I know if I have a condensing boiler? It was in the house
when I moved in, and there are no instructions for it.

It probably isn't. Does it have a make or model? What does the flue exit
look like and what route does it take? If it is short (i.e. straight through
the wall), it will plume noticeably if condensing.

I would have thought the whole point of a condensing boiler is that the
exhaust contains little or no water vapour. A condenser will have a
permanent drain, although this could of course be coupled to the pressure
relief plumbing and not be immediately obvious.

It very much depends on the design and operational conditions of the
boiler.

The important thing to realise is that the extra energy recovered when
condensing is due to the latent heat of condensation. This is the
change of state of water from the gaseous phase (steam) to the liquid
phase (water). Heat is given up as a result of this change, just as
extra heat is required at boiling point to turn water to steam.

It's important to note that water vapour (that you see) is *not* steam
- it is fine droplets of liquid water. By the time that you see the
plume, the energy release has already occurred.
Thus from the perspective of efficiency, it doesn't matter whether the
water vapour is collected inside the boiler or goes out through the
flue.

Some designs of boiler and heat exchanger appear to be better than
others in terms of how much pluming happens, but it does not appear to
significantly affect the efficiency. If you look at the SEDBUK
tables, most recent boilers are in the 90-91.3% range and the
variation is fractions of a percent between them and not statistically
significant. There are still a few older models that are in the mid
to high 80s range, but are older designs.

It is possible that in some of the older designs that the heat
exchanger(s) has/have not condensed so much of the steam to water
vapour and that some is escaping in gaseous phase through the flue,
and condensing to water vapour as soon as it hits the outside air.
Since there are so many models bunched together in efficiency terms at
the top 90-91% range, it suggests that they are reaching the limits on
design and physics for the technology.

There are certainly different burner designs. Most of the good
quality products have a downward facing burner and stainless steel
heat exchanger, to improve performance and deal with the acidic
condensate. The older ones tended to have more conventional burners
and heat exchangers and a second heat exchanger after the main one.

There are also implications in the burner design with the amount of
pollutant emission, especially of NOx gases.

I have a German made boiler from a company called MAN Heiztechnik
(division of the same company that makes trucks and diesel engines).
This is towards the top end on efficiency and one of the lowest NOx
emitters in the industry. The burner and heat exchanger design is
cylindrical with the burner being a wire mesh arrangement sitting at
the centre of a stainless steel chamber with pipes around the
periphery. The condensate falls to the bottom at the back and passes
to a trap system for disposal. Very little water vapour leaves the
boiler flue, even under quite high firing conditions.

There are photos at
http://www.man-heiztechnik.de/index.php

Click on the second thumbnail picture with the flashing red "test"
logo and then at the resulting Micromat EC product page, the link
under the photo marked "Mehr fotos und details". There is a gallery
of pictures and the heat exchanger is shown as the third and the
burner as the fourth.
The manufacturer won a best on test award in August for this product
even though it was originally introduced in 1995.


..andy

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