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?? :-)

I've heard one reasonable idea for when this might be true - in obscure
circumstances.

If when cool the walls drop below the dewpoint they will start soaking
up water. Wet walls are not as good an insulator as dry ones.

So if when the heating is off your walls start getting damp, then *and
only then* it might be worth leaving the heating on. Or more likely,
buying a dehumidifier to keep the mould under control!

Andy