"Andy Hall" wrote in message
...
On Thu, 2 Sep 2004 15:05:07 +0100, "Christian McArdle"
wrote:

You can't eliminate boiler cycling with a simple thermostat, or even
two of them. They have hysteresis.

Indeed. The idea is to promote slow cycling, perhaps using even a third
or
half the heat bank capacity before attempting recovery. The questions is
whether the thermodynamic advantage of running at full pelt outweigh that
of
the lower return temperature at the less efficient low burner setting.


True. The other factor is how to detect the difference between the
continuous and relatively low load of the radiators vs. the large
demand for DHW.

Remember that with TRVs, the radiators also present a variable load
anyway - or to some extent with zones.


We are considering two systems he

1. Radiators, low hysterysis single thermostat, modulating condensing
boiler.

Kind of. If you have TRVs or zones then the heat load varies and the
return temperature from the radiators will change accordingly (for a
given flow temperature to them). A modulating boiler will respond
to this and adjust power level to match.


2. Heat bank, v.high hysterysis double thermostat, unmodulating
condensing
boiler.

The high hysterysis would lead to a modulating boiler become effectively
non-modulating, as the heat bank will be able to absorb whatever heat is
thrown at it until the off thermostat fires.

Which is better, running:

1. a burner at a low modulation rate, but with a lower return temperature
to
promote condensing.
2. a burner at its full rated value, with long cycling (i.e. 10-15
minutes)
but with a higher return temperature.

This is all based on the understanding that modulating boilers are
actually
more efficient at full rated value, than at their minimum level.

This is not the case for condensing boilers. If you look at the
immediate gross and net figures for efficiency for condensing boilers
(not SEDBUK weighted, which confuses the issue here) you will find
that efficiency figures increase at lower running temperatures as well
a with matching heat output of the boiler to the load. Partial load
efficency, normally measured at 30% also increases with reducing
temperatures.

Take a look at http://www.eduvinet.de/servitec/brenne.htm
which is quite a useful German tutorial on condensing boiler
technology. Pages 35-41 are pertinent.
Keep in mind that some of the points about condensate and chimnies
have to do with local German regulations, and they talk about
"forerun" and "reverse run" temperatures - means flow and return.

For a DHW system, the objective is to get as much heat as possible in
the shortest time possible into the store. The heatbank does this
well and runs the boiler at full tilt for this short time. That is
certainly more efficient than going via an inadequate coil which
results in slower transfer of heat and cycling. The objective is
different, to CH though.

With CH, the objective is to match the load to the boiler,

Which a weather compensator does with a heat bank between rads and boiler.
he large volume of water being heated makes it more efficient.