"Bob Eager" wrote in message
...
On Fri, 19 Dec 2008 15:41:51 UTC, wrote:

ISTR hearing that the story began with experiments on immersion
heaters and was erroneously extrapolated to central heating which is,
of course, completely different.

It's not valid for them anyway.

You have something which, left to its own devices, loses heat over time.
The rate of heat loss will reduce as its temperature gets nearer to that
of its surroundings (heat transfer rate depends on difference in
temperature).

If left to cool, there is reducing heat loss over whatever time elapses.
If kept at same temperature, there is heat loss over the same time, but
it's constant (-ish) because the temperature difference does not reduce
- i.e. more heat is lost over the same time.

In both cases, that heat loss has to be made up. That's how much
fuel/cost will be involved. More heat lost - more cost.

If you can do the maths you will find its always cheaper to turn it off and
reheat unless you have a very odd tariff.
As it cools it loses less heat so it will always lose less heat over the
same period when off than when on.
This is true for CH and immersion heaters.