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

"Al" wrote in message
...
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?