It is less-efficient to run two.

When a heater comes on, you pay to heat the heater in addition to heating
the air. If you plotted a curve of the percentage of energy spent heating
the hardware versus heating the air, it gets more efficient with longer
individual run times. The longer the burn time per start, the more
efficient it is. That's why it doesn't pay to buy a 100K BTU heater when
your need is for a 50K unit - it may heat the air really fast, but it has to
do so more often. The ideal would be a heater that generated heat
continuously at the exact rate the greenhouse lost it.

With two heaters, you are heating double the mass of steel and running each
for half the time for each burn cycle.

--

Ray Barkalow - First Rays Orchids - www.firstrays.com
Plants, Supplies, Artwork, Books and Lots of Free Info!


"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?

"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?