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?

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

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


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.