"Phil Addison" wrote in message
...
On Sat, 18 Sep 2004 16:17:55 +0100, "Tom"
wrote:

Most manufacturers seem to recommend setting flow rates in CH systems to
achieve a rise of 11deg across boiler, why is this? is it the most
efficient
setting for heat transfer?
Tom

The straightforward answer to Tom's question, "11degC across Boiler,
why?" is that the 11 degree C is not a requirement at all.

Read the installation instructions. That is what they say.

It is merely the temperature rise from return
to flow that the particular boiler can,
and does, achieve at the specified water flowrate when it is burning
flat out. Simple physics says that "Flow Rate X Temp Rise" is
proportional to "Heat Input X Boiler Efficiency".

The boiler is a heat source and so (obviously) generates a temperature
rise in the water flowing through it. The remainder of the system is a
heat dissipator and the temperature drop of the returning water is
determined by radiator sizes, room temperatures, air changes and so on.

A boiler can equally well operate at full output with a smaller rise, if
the system pumps at faster than the quoted rate, and conversely a lower
flow leads to a greater rise, possibly causing the limit stat to turn
off the burner, or turn it down if a modulator.

This tends to be the case with combi's in small flats. Most combi's can heat
a 5 bedroomed house. The modulation really helps. Early non-modulating
combi's cycled like crazy as the temp rise was far too great.

Radiators are also spec'd to give out their stated output at a certain
temperature drop and average temperature. This is commonly 11 degree C,
and is possibly the reason that so many believe that the drop is
'supposed to be' exactly that.

As has been mentioned in the thread above, systems can and are designed
for different flow/return temperatures to exploit the efficiency
characteristics of different systems. The actual temperature difference
the system uses depends on the c/h system designer's preferences as
admirably discussed above, and refers mainly to the case when the system
is going flat out in winter.

Phil
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