"Al" wrote in message
...
ps. logically I come up with the same btu's must be needed to reach the
same average temperature, but is this the truth?


Nope. Nothing in this game is that easy. In determining BTUs, the temp
difference between the outside and the inside is critical. The BTUs required
to maintain a difference is not linear with this temp difference. If I
understand all of this right, it would take fewer BTUs to keep a greenhouse
at a uniform 65.5 then it would be to maintain a greenhouse that is 68 one
side linearly dropping to 63 on the other. The extra BTUs required to
maintain the part of the greenhouse 2.5 degrees above normal would not be
made up with the savings of only heating the other side to 2.5 degrees below
normal.

Pat