On 18/01/2021 05:49, RJH wrote:
On 17 Jan 2021 at 20:40:34 GMT, "Roger Mills" wrote:


Yes, that's what I'm asking. It would make more sense to me the throttle back
the gas in the first place, rather than let it get hot to the point that it
needs to cool down by pumping the hot water away.


Irrespective of how intelligent your thermostat it is basically an
on/off switch. The boiler has no prior knowledge that the thermostat is
going to tell it to turn off and without this prior information cannot
throttle back in the way you suggest.

Whilst there is a call for heat the boiler is trying to almost instantly
raise the temperature of a very fast flow of water in a VERY short run
of pipe by 10C. A boiler may have some intelligence to increase the
burner when the incoming water is at 20C, with an expected output of 60C
which it cannot achieve instantly, and modulate the burner down when the
incoming water is 50C, again with an expected output of 60C. To raise
the water by 10C almost instantly the heat exchanger is at a much higher
temperature even when the burner is turned down to low.

The boiler may be operating at the low burner mode for, say, half an
hour before the room heats up enough for the room thermostat to cut in.
The boiler, without warning, is told to shut down. The boiler throttles
back the gas by the largest amount it can by turning the burners off.
Even with the burner turned off the heat exchanger still is hot enough
to raise a fast flowing steam of water by 10C+ and if the flow of water
is stopped by very much more than 10C.

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