Thomas,
All greenhouses should have a way to ventilate excessive heat out. The best
way to do this is with automated exhaust fans and power intake vents of the
proper size. It is usually recommended that the exhaust fan(s) should be
able to make a complete change in the greenhouse air every 60 to 120
seconds. Your 14 ft x 32 ft greenhouse, if it has a 10 foot roof, probably
holds close to 4,000 cubic feet of space. Therefore your exhaust fan(s)
should have a total exhaust capacity of about 3,000 to 4,000 cubic feet per
minute. Your intake vents should equal roughly twice the area of your
exhaust fan(s). (One 24 inch fan = 48 inch vent(s).) Most orchid
greenhouses should also have an evaporative cooler or wet pad system to cool
and humidify the greenhouse.
During the winter and transitional seasons when the sun warms the greenhouse
too much on cold days, an exhaust fan and vents are still necessary. Due to
the cold outdoor temperatures, plants should not be exposed to cold breezes
coming in through the intake vents. You can either move the plants, or
provide a second set of vents that does not open directly onto the plants.
Some growers have been known to place a blower fan (100 to 900 cfm) at roof
height, opposite of the intake vents, and a tube system that pumps the heat
down and out, gradually making a change of air over a longer period of time
on those cold sunny days.
In addition, an orchid greenhouse should have constant air movement
throughout by using several fans running 24 hours a day. These fans should
constantly mix the air so that temperatures at the roof are not too
different than those near the floor. Even in this kind of situation you
should still be able to find micro-climates for the cool and warm growing
orchids.
For my own 20 x 50 foot greenhouse here in Nashville, Tennessee, I have a
10,000 cubic feet per minute exhaust fan set on a thermostat that turns it
on at 78°F. I have a bank of vents that measures 2 x 16 ft, and two 36 inch
vents, that open when the fan comes on. I have a wet pad (Stuppy) measuring
4 x 16 ft. that runs during warm weather. I also have four 1500 cfm fans
inside the greenhouse that run 24 hours a day...probably could use a couple
more. My object is to provide enough air movement that the leaves and
flowers are constantly moving in a breeze nearly everywhere in the
greenhouse.
I hope this helps, and I'm sure others may have additional ideas.
Ed Merkle
Merkle's Orchids
1008 Maplewood Place
Nashville, Tennessee 37216
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