roger wrote:
The message
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.

Scalding radiatators happen when any system that is not modulated, goes
into heat mode.

It doesn;t matter whethert tu re one degree or 100 degrees below TRV
temp, thermostat temp. The rads and the boiler will be working full on.

What modulates the radiator surface temperature is in fact the time
they are on 'heat' because they have thermal mass.

Thats teh whole point.

If you had e.g. a house made of polystrene with hot air heating,
ultimately probably te lowest theral mas possible, then I would almost
agree with your perspective.

But the more mass there is in the building, or the heating system, the
more a high heat regime will tend to overheat parts of it with respect
to other parts. And leave the possibility of higher heat loss from those
parts during the transient phase.


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.


Well the figures and experiences of people in high insulated houses tend
to bear out the proposition that it makes very little difference on a
modern house whether its run 24x7 or timed.

The worse the insulation and the lower the thermal mass, the more the
timed solution saves you. Simply because it both loses far more heat
(and gets icy cold when the heating is off) and because its not got much
to heat up mass wise, so it comes up to temp quickly.

Houses that keep heat in them well, don't benefit much, and as I have
been trying to say, the heatloss from parts of the system that dont heat
the house as such at all, can be a lot higher when their average
temperature is now at 65C-70C for prolonged periods, I.e heating pipes
in the loft or walls. If you have to use high boiler temps to get the
house to heat up, you WILL be losing more heat during that phase.

A pipe, even in a couple of inches of foam, is nowhere near as well
insulated as the rest of the house should be.

This is not a simple analysis.

Boilers modulate as well, because it is both better to run for longer
periods than to run at lower power levels. The reasons are simple: high
power means less efficiency as the condensers - if fitted - work less
well, and also the alternative to modulation - cycling - leads to start
up losses when the boiler fires.

And if you use the most efficient heater there is - a heat pump - the
efficiency severely degrades when called for high output temperatures.

I m not saying that its always better to run a system this way or that
way, just trying to point out that the simple analysis runs into trouble
the more modern the house is.

There is a law of diminishing returns. And sometimes the transient
behaviour of the system makes it a net loss.