chris French wrote:
There seems to be floating about among some people I know online the
idea that keeping the central heating on all the time is cheaper than
turning it off at night. Not sure where it comes from, but seeing as
they all frequent the same/ related e-mail lists, maybe from there.

The basic idea seems to be that it costs more to heat the house back
up in the morning, than to keep it at the same temp.

now I know it depends on things like internal and external temp (we
have a programmable stat set at 12-13 C, it hardly seems to turn on
except when down to about freezing outside from what I can tell),

But in principle it seems rubbish, it must use more energy to keep
something hot, than to let it cool and then reheat., but a couple of
people had been hard to convince, esp. as a 'plumber told me it)

Or is my brain addled?? :-)

The rate of heat loss from a system always increases as the temperature
difference between inside and outside increases. In fact it's pretty much
proportional to that difference. You therefore MUST always use more energy
keeping that temperature difference constantly high than letting it cool for
a while and reheating it again as needed.

However with a reasonably well insulated house the net extra cost is very
low. If we assume in cold weather an 18 degree C difference between inside
and outside when the heat is on, 16 hours of heat on during the day and 8
with it off overnight and a 4 degree total drop in temp of the house
overnight (i.e. an average of 2 degrees) the net extra cost of keeping the
heating on all the time is only about 4%. That's less than you'd save just
turning the thermostat temperature down by 1 degree.

For a poorly insulated house the savings would be greater but still not very
much. For the saving to be as much as 10% the house temperature would have
to drop by 10 degrees C overnight for which you'd probably have to leave all
the windows open.

As with most things where some people think one thing and some another the
reason is simple. There's not much difference in the two approaches.

The two biggest influences on heating costs are the temperature you set and
the insulation of the house. Turning the heat off at night is negligible in
comparison to either of those.
--
Dave Baker