On Sun, 26 Oct 2003 10:47:21 -0000, "mike.james"
wrote:


"IMM" wrote in message
...


6%?? look at http://www.sedbuk.com More like 12% according to sedbuk.
If
the system is engineered to take advantage of a condensing boiler, then
more
like 25%.


Now this brings me back to my original point.
What does "engineered to take advantage" mean - lower return temperatures?
Hence my "silly" idea of putting the pool's heat exchanger in the return
flow.
mikej



I think that you are missing the point.

It is the case that a lower return temperature improves the efficiency
of a condensing boiler, however this is not the complete story.

Any boiler heat exchanger will have a maximum temperature differential
that it can support - for a condesning boiler around 20-25 degrees,
for a conventional one around 10-12.
This is designed around firing and flow rates as well.

The heat transfer through emitters such as the radiators or the pool
heat exchanger is proportional to the temperature difference and the
flow rate. This is given by the equation energy = mass x specific
heat x temperature rise or fall.

If you connect the radiators, as you should, across the boiler flow
and return, they will receive the full temperature drop, as would the
pool heat exchanger. Typically, you would have a valve arrangement to
balance the flow between the two and hence share the heat.
If the requirement for the pool heat exchanger is large, one might
even have separate pumps for the heating and this; the effect of which
would be to increase flow through the boiler when there is more heat
demand. In a modulating boiler, this would cause the boiler to
modulate upwards while maintaining the same temperature drop.
Some condensing boilers with built in single pump even control the
pump speed as the power requirements change.

If you connect in series, the radiators and the heat exchanger will
get the same flow rate (by definition). The temperature drop will
be split between them and so each will only get a proportion and not
be able to run at full load. The boiler won't be able to increase its
tremperature difference to deal with this, and so the only way to get
more heat output would be to increase the flow. Unfortunately, this
will not usually be possible because the radiator and pool heat
exchanger will probably not be able to take that. Moreover, there
would be no way to balance heat between the two.

You have to look at the system as a whole - you can't just focus on
one aspect and assume that everything else falls into place.




..andy

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