On 9 Mar 2006 10:21:06 -0800, "legin"
wrote:

I went to the recent Homebuilding show at the Nec. The reason was that
having googled and followed recent threads I was thinking that the best
probable solution would indeed be to fit a heatbank in my self build. I
went with the specific question in mind. How do you keep the
condednsing boiler condensing if a temperature of circa 75 degrees is
required for the heatbank. I even suggested mixing the return from the
heatbank with the return from the underfloor heating to get the return
temp down enough to get lull the boiler into condensing mode. Apart
from some exhibitors being unsure of how a system could be piped, the
overall conclusion was to fit an unvented cylinder. This would take
better advantage of a condensing boiler.
Anyone any better advice to offer.
Regards
Legin


One thing to bear in mind is that a boiler does not reach Nirvana or
orgasm when it "goes into condensing mode".


What actually happens for boilers designed for condensing is that
there is an efficient heat exchanger able to work with a temperature
differential between return and flow of 20 degrees and possible more.

The lower the return temperature, the more efficiently the boiler is
running. This happens regardless of whether it is condensing or not.

What happens at the dew point - around 54 degrees - below which
condensing takes place, is that the *rate of change* of efficiency
with return temperature increases below this point.

This is as a result of the contribution of the latent heat that is
released from the phase change of water from gas (i.e. steam from
combustion) to liquid. Note that I am talking about steam in the
true and invisible sense here and not "steam" that is visible and is
in fact, water vapour.

There isn't a step change of efficiency at the dew point - just a knee
in the curve.


In terms of optimising overall efficiency, anything that can be done
to reduce the temperature of the return water to the boiler will help.
UFH is possibly one aspect for this. Another, if you are going to
use radiators, is to make sure that these give the heat required when
you run them at 70 degree flow and 50 return. Conventionally,
radiators are sized based on 82 and 70. To achieve 70/50 requires
them to be approximately a third higher nominal output when choosing
from the manufacturer datasheet. The effect of doing this is to run
the boiler at a lower temperature overall for more of the heating
season.

Where a heatbank is used for water heating, normally the objective is
to maintain it at as high a temperature as is reasonably practicable -
i.e. 82 degrees. THe implication is that the return would be at
around 60 degrees and you are out of condensing range.

This is not really a problem for several reasons:

- The cylinder will cool as HW is run with cool water replacing hot at
the bottom. The plate heat exchangers used are able to trasnfer heat
very efficently to the cold water (100-200kW equivalent is typical),
so quite a drop in temperature in water returned to the cylinder.

- The point at which the boiler fires can be arranged to be when the
water is quite a bit cooler such that in any case there is a low
return temperature to the boiler. It is only going to reach the 62
degree point relatively late in the reheat cycle if adjusted
correctly.

There is not really a lot of point, therefore, in striving to achieve
55 return by reducing the flow to 75 max. because also you will lose
heat storage capacity in the heatbank

--

..andy