"roger" wrote in message
...
The message
from Mike Clarke contains these words:

Sorry but that does not make sense.
If its off for 8 hours out of 24 you will save nearly one third.
You may save more as it usually colder at night and will use more
energy
than in the day.
It will take slightly more energy to raise the temp back to the norm in
the morning but it shouldn't take much.

I agree with your sound logic, in theory the heat losses (and hence
energy
costs) through the night will be higher if the house is heated but a
recent
check on meter readings suggest that the case might not be quite so
straightforward.

Hardly sound logic to suggest that the heat needed to rewarm the house
is minimal.

We have the heating set to run from 08:00 to 23:30. The overnight set
point
is 13C so it's effectively turned off overnight unless the weather gets
very cold. The programmer uses optimum start so I don't know exactly what
time it normally starts up but the maximum advance is 2 hours so I'll
assume it starts up on average a bit less than this at 06:30 (I certainly
don't intend waking up at 06:00 to find out). Average gas consumption for
the last week has been about 300 ft^3 per day but extra meter readings
last
thing at night and at about 08:00 for a couple of nights show that about
100 ft^3 are consumed each morning in bringing the house back up to
temperature leaving 200ft^3 for the remaining 15.5 hours, i.e. 200/15.5 =
12.9 ft^3 per hour to maintain a steady temperature. So if we left the
heating on all the time the daily consumption would be 12.9x24 = 309.6
ft^3 - remarkably close to what we're using already.

O.K. the losses through the night would be a bit higher due to the
outside
temperature being lower but I don't expect it would make a huge
difference.
I don't intend to put it to the test though because we don't like
sleeping
in a hot bedroom but don't want to turn the bedroom TRV down because we
want the room to have warmed up by the time we have to get up in the
morning.

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.

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.

In the real world you have to take into account the heat capacity to work
out how long it takes to reheat the house, not the heat loss. I.e. your
simple model doesn't work.