On Thu, 27 Nov 2003 11:45:48 -0000, "Christian McArdle"
wrote:

I would have thought the whole point of a condensing boiler is that the
exhaust contains little or no water vapour.

This would only happen if the flue is long enough for all the water to be
removed, or the return temperature is very low. If the return temperature is
around 60C, then the boiler fundamentally can't cool the flue gases any
lower that this. Those gases would then still contain water that will only
condense out when the temperature drops further, either within the colder
flue (if it is long enough) or upon exposure to colder air (at the
terminal).

Any condensing on the flue or outside air is wasted energy. That is one
reason why greater efficiency occurs at lower return temperatures. More
condensing can then occur upon the heat exchanger itself, where the energy
can be usefully recovered. (There are also thermodynamic reasons for greater
efficiency at low temperatures that apply to all boilers, not just
condensing types).

Christian.


I don't think that that is completely it, Christian.

The major heat recovery in terms of delivering heat to the heat
exchanger water is from the change of phase of water from gas to
liquid - latent heat of condensation.

The lower flue gas temperatures result in less loss of heat outside in
the form of hot air, CO2, water vapour etc., and anything that can
help with that is a bonus as well in that the heat will dissipate
inside the property. For example, having a longer flue inside the
house on a condensing boiler will result in the flue gases being
cooler as they leave the building and more water will have been
collected. However, that will be heat transferred other than into
the water.

Obviously the heat exchanger temperature has an impact on that as
well.

I just took a look at the display on the front of my boiler. It's
currently showing 6 degrees outside, the flow temperature is 61
degrees, the return 45 degrees. It also gives me the fan rpm from
which I can look up the heat input - approx. 7.5kW - and the pump is
running at 35% of max. There is very little plume at all.

I can press a test button and deliberately force it to run at a 50% of
full power rate (approx. 14kW). There is still very little plume.

I only have a very short flue - literally the elbow above the boiler
and straight through the wall.

..andy

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