"Martin Brown" wrote in message
...
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!

My first degree was an engineering degree. We learnt all about
Newtons law of cooling in the first year. I did a laboratory
experiment to verify it. It actually is an empirically verified law.
The exponential law which you quote is one of the many myths which
need to be exposed and expunged.

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.

Nope. The power required to heat a greenhouse does most certainly not
rise exponentially with the temperature differential.
If you looked up Newton's law of cooling, you would find that for
small differences, the power required is proportional to the
temperature difference between the object and the ambient temperature.
Newton's law of cooling has been tested quite extensively for
centuries.
Actually, if the tempersature difference becomes larger, the power
begins to rise roughly as the 4/3 power of the difference. It never,
never becomes an exponential function.
And internal convection does not have a threshold.
--
Franz
One Galileo in 2000 years is enough. Pope Pius XII