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from The Natural Philosopher contains these words:

roger wrote:

Consider a really simplified model in which the outside temperature is
constant and the house requires a single unit to keep it up to
temperature and loses one degree for every hour without heat. Heating on
for 24 hours would require 24 units. Heating on for 16 hours would
require 16 units plus whatever it takes to get it back up to
temperature. In our simplistic model the temperature loss is 8 degrees
which requires 8 units to reverse. 16 + 8 = 24. So no saving.

Now consider the actual boiler output. The plus whatever it takes to get
it back to temperature may push teh boiler into continuous flat out
mode. How much less efficient is it then?

You have your condensing boiler set so it always condenses. If it is a
non condensing boiler then the difference would not be significant.

Or you may have to raise the outlet temperature to get it to warm up
faster. More loss of efficiency.

Marginal.

Analsysing transient conditions using steady state analysis is a very
precarious exercise.

But unless you have the heating firing for 100% of the time you do not
have anything approaching a steady state in the first place. Having the
heating off for 8 hours at a time is closer to normal operation than
continuous firing, it is just that the hysterisis is larger.


In the real world there is a small saving because the heat loss is a
function of the temperature difference so during the time the heating is
off the rate of heat loss will decay.


Don't be too sure. Red hot rads heating up room against a cold wall,
lose a lot of heat..then the wall itself gets locally very hot leading
to more heat losses through that part of it. Until the room stabilises.


The more concentrated the heat and the higher temp the heat the more
there is a chance of excess loss over simple steady state analysis. what
about ultra hot feed pipes in the loft? sure they are insulated, but the
hotter they are, the more they lose..

The better your insulation the more likely you are to get little benefit
from time switching and actually stray into excess transient losses.

But the better the insulation the less the heating system has to do to
restore the status quo. Scalding radiators are not needed. It may not be
entirely impossible to engineer a heating system in an extremely well
insulated house to use more energy if it is switched off for a lengthy
period but, other things being equal, the only way that is going to
happen is for a single firing of the boiler to produce a larger
temperature overshoot on the heating phase than the temperature drop on
the non heating phase. ISTM that that is much more likely in normal
operation than it is when the temperature drop is significant.

Of course deliberately buggering up the operation of a condensing boiler
would make the task easier but even the 10% loss of efficiency may not
be enough even if the set-up could be designed to give maximum
efficiency in normal operation and maximum inefficiency in sustained
firing.

--
Roger Chapman