Even the best-designed and built greenhouses will lose heat through radiation, conduction, and convection through the glass, walls, roof, doors, vents, cracks, and even the floor.
Every greenhouse needs to have a ventilation system and circulation fans installed to produce optimal growing conditions. Small greenhouses can be heated economically with an electric space heater, propane gas heaters, or using a wood-burning stove.
Ventilation is the exchange of inside air for outside air to control temperature, remove moisture, or replenish carbon dioxide (CO2). Several ventilation systems can be used, but be careful when mixing parts of two (2) systems since natural ventilation uses roof vents on the ridgeline with side inlet vents (louvers).
Warm air rises on convective currents to escape through the top, drawing cool air in through the sides. Mechanical ventilation uses an exhaust fan to move air out one end of the greenhouse while outside air enters the other end through motorized inlet louvers.
Exhaust fans should be sized to exchange the total volume of air in the greenhouse each minute. The total volume of air in a medium to large greenhouse can be estimated by multiplying the floor area times 8.0 (the average height of a greenhouse).
A small greenhouse (less than 5,000 FT3 in air volume) should have an exhaust-fan capacity estimated by multiplying the floor area by twelve (12). The capacity of the exhaust fan should be selected at one-eighth (1/8) of an inch of static water pressure.
Greenhouse Air Volume
The static pressure rating accounts for air resistance through the louvers, fans, and greenhouse and is usually shown in the fan selection chart. Ventilation requirements vary with the weather and season.
One must decide how much the greenhouse will be used during the summer, 1-to-1 air volume changes per minute are needed while smaller greenhouses need a larger amount. In winter, 20% – 30% of one (1) air volume exchange per minute is sufficient for mixing in cool air without chilling the plants.
One (1) single-speed fan cannot meet these criteria so two (2) single-speed fans are better. A combination of a single-speed fan and a two-speed fan allows three (3) ventilation rates that best satisfy your year-round needs.
A single-stage and a two-stage thermostat are needed to control the operation so a two-speed motor at low speed delivers about 70% of its full capacity.
If the two (2) fans have the same capacity rating, then the low-speed fan supplies about 35% of the combined total. This rate of ventilation is reasonable for the winter, but in spring the fan operates at high speed and in summer both fans will operate at high speed.
A 16-foot wide by 24-foot long house would need an estimated FT3 CFM (cubic feet per minute) per minute total capacity; that is, 16 x 24 x 12 FT3 per minute. For use all year, select two (2) fans to deliver 2,300 FT3 per minute (1) each, and one fan to have two (2) speeds so that the high speed is 2,300 FT3 per minute.
Adding the second fan, the third ventilation rate is the sum of both fans at high speed or 4,600 FT3 per minute.
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