Franz Heymann wrote:
"Martin Brown" wrote in message
...

A quick and dirty rule of thumb is that the cost to
maintain the temperature an extra 5C above ambient roughly doubles.

That does not make sense. What would it, according to that rule, cost
to keep the temperature just 5 deg C above ambient? Nothing?

It is an engineering rule of thumb, Franz, not a law of physics!

If it costs X to maintain 5C above ambient it costs 2X for 10C and 4X
for 15C. It climbs quite a bit faster than proportional to the
temperature difference because of convection kicking in.

In fact, the cost does not vary exponentially, but is is roughly
proportional to the desired temperature differential.

Only in the draft proof ideal world of the physics lab.

Regards,
Martin Brown