Hi,

I have a 30 by 100 foot greenhouse. I have two heaters, forced air heaters,
that burn oil. One heater alone is enough to heat the greenhouse (on all
but the coldest nights). They hang on opposite side of the long greenhouse
and the un-used one is a back up incase the primary one dies. Two
thermostats hang in the middle of the space, 50 feet from the heaters about
midway way from the floor to the roof. One thermostat controls one heater
each and they are independent of each other. There are four temperature
monitoring stations in the greenhouse that are independent of the on/off
thermostats that are situated to read the two coldest spots and the two
warmest spots. The four spots are gathered from plant level around the
bench levels, (not extremely close to heaters or tucked down in cold
corners)

I can run the south wall heater with its thermostat set at 65 (in the middle
of the greenhouse) and the north end of the greenhouse will register around
63 and the heated side will register around 68 but the air is circulated by
LOTS of fans and the center air mass where the thermostat is, is 65 degrees.
The four temperature sensors around the greenhouse average 65. The same
pattern emerges no matter which heater I use; the far side is cool and the
heater side is hotter but the middle is 65.

If I set *both* heaters to come on at about 64, both sides of the greenhouse
heat to around 68 and the middle of the air mass turns on and off both
heaters at 64. And they don't seem to cycle on and off as often, and it
seems there is probably more radiant heat available from two heaters but
that may be an illusion because the outside night temperature varies a lot
from night to night and this has to effect how the heaters cycle on and off.

.....anyway the average temperature based on the 4 monitoring stations now
comes to just under 66.... I have to set the thermostats just about 63.5 to
get an average air temperature of 65, so it matches the average air
temperature of the single heater number.

Here's my question, so by running two identical heaters at 63.5 instead of
one heater at 65 am I using more oil, less oil, or the same amount of oil to
heat the same space to the same average temperature? there has to be an
equation that will answer this question. Anybody know how to figure it?