no continuous flame. The back of the heaters have about 3 feet space from
the outer wall. And they are not very efficient, I think the number was
somewhere around 75 to 85% efficient when I bought them.

"Doug Houseman" wrote in message
...
First it depends on the heaters - if they have standing pilot lights and
you normally have the backup off, you burn more.
Second if you really had a cold spot at the far end, then you were not
heating all the air to the same temp, so you are using more fuel
Third you put some BTU's up the stack, depending on the efficiency of
the heaters - more square inches of stack in operation, means more btu's
lost.

On the other hand if everything is perfectly efficient, then you would
use the same.
Also if the heaters are away from the wall of the greenhouse, and run
less, then less heat is radiated directly to the wall and lost that way,
meaning that you probably used less.

Doug

In article ,
"Al" wrote:

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?