The endwalls are twinwall lexan. The design resembles a wide low pentagon
shape more than a hoop. The average wall height is 5 feet. The average
center ridge height is 14 feet. (The greenhouse is built on a slope.) The
roof area and wall area are covered in inflated double poly 6 mill plastic.

The heaters are rated at 210,000 BTU, (new, with original nozzles. I can't
find the efficiency rating but I think it is 85%.
http://www.aaagreenhouse.com/quantumoil.htm (Siebring Quantum 210), the
size of refrigerators. The pictures make them look tiny.

I am not sure I see what effect most of the variables like greenhouse
covering, outside night temps, etc, have on the question but I think you
answered it. I think changing the heater efficiency would effect the
answer.

I think you are telling me: Assuming all other variables remain equal I use
a bit more oil with the two heater method I described ...but you can't say
exactly how much more unless you know more specifics about my greenhouse,
and my local temps and wind patterns etc.

I have a calculation on a spread sheet that has pretty closely predicted how
much oil my greenhouse will use over the course of the last 5 winters. For
instance, last years gallons used were 3186 and the spread sheet, based on
average mean temps in my area came up with 3325. I was not able to look at
these calculations and see how to adjust them to answer my one heater or two
heater question.

"Doug Houseman" wrote in message
...
Al -

At 85% the furnaces use about 4% more fuel to operate at 1.5 degrees
lower with both running, instead of one running at the higher temp.
(assuming 100L BTU and a hoop house design with a 2% per hour air loss
and double poly inflated plastic as the covering for the top and
assuming R-9 in the end walls). according to the model that I have and
ran. I made a lot of assumptions.

Doug

Hope this helps.

In article ,
"Pat Brennan" wrote:

Hey Al,

I did not say which option would use less oil. It is a hard question and
I
do not have enough information or remember the calculus. There are two
factors at play, the heat generation and the heat loss. On the
generation
side it is more efficient to run one heater due to start up efficiency
and
the loss of heat up the stacks. Heating the mass of steel is no big deal
since that is in the greenhouse and losses are into the greenhouse.
Barometric dampers will reduce the losses up the stack, but they never
work
right for me and have lead to soot problems. On the heat loss side it is
better to have uniform heat for a given average. As a greenhouse gets
longer, at some length it would be better to have heaters on both end
than a
single heater and a wide range of temperatures. I have no idea what the
length is, but one furnace is not always the best answer.

If you opt for two heaters you might consider reducing the burner nozzle
size. Nozzles are measured in gals per hour and only cost a couple of
dollars. This will reduce the BTU output of the furnaces and they will
not
cycle on and off as much. If you change the nozzle, remember to reset
the
air intake. Also make sure your nozzles are large enough for the coldest
nights. (we use big nozzles that are not always stocked on repair
trucks--always keep an extra one on hand.)

I have seen the long plastic tubes used even when they are not connected
to
the furnace directly. This would help level out the temps (saving oil)
but
would add another fan to your electric bill. Watch out for HAF fans
blowing
against the plastic as this leads to greater temp and humidity loss as
the
warm air moves down the greenhouse I have reduced the size of my HAF
fans
and been more careful adjusting where they blow. I assume it has lead to
oil savings, I know it has help in growing.

If you think your greenhouses grow better with two heaters; increased
sales
from better plants or faster growing plants will most likely more than
pay
for the additional oil. Humidity will be more uniform in a greenhouse
that
has uniform temps. How many blasted buds does it take to cost more than
the
oil savings?

I like the temp difference across the greenhouse; oncs, zygos, and dens
go
in the cold side, seedlings in the warm section, and mature phals in the
'average' section.

Sorry I can not help, I have no idea what is best.

Pat

"Al" wrote in message
...
So you all are saying that I am burring more oil by what I am doing:
by running two identical heaters at 63.5 instead of one heater at 65 to
maintain an average of 65 in the space I will spend more money?

"heating double the mass of steel and running each for half the time"
uses more oil?

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" so I will use more oil?

Lacking some calculus equation I could not understand anyway, I'll take
your word for it. :-) I do kind of see what you are saying. So the
smartest thing I can do (since I was dumb enough to buy these
inefficient
heaters anyway) is to set one heater at 65 and one at 63 so the second
unit would only come on if the outside temps/winds stressed the first
unit's ability to maintain the temperature or failed to start at all.
My
goal is to use the least amount of oil while knocking the fewest buds
off
the phals.

I do like the nice even heat distribution I get with both heaters. I
kind
of think some of the bud loss I got last year was from marauding cold
dry
drafts in the circulating pattern. I will have to look into a way to
distribute the heat from one unit more evenly. Those clear plastic
tubes
that hook in front of some heater brands and run down the length of the
greenhouse dispensing heated air though little holes will not work
because
they can not be attached to the front of my heaters. The manufactures
told me so last year. If these things would just hurry up and rust
away,
I would invest in a better method of heating; probably a boiler with
fin-tubes of circulating water running under the benches.

"Ray" wrote in message
...
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?