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Greenhouse Ventilation Strategies
Using Solar Powered Fans

Greenhouse ventilation is the movement of air from inside a greenhouse and replacing it with outside air to control high temperatures in the summer, to maintain relative humidity at acceptable levels during the winter and maintain acceptable carbon dioxide concentration levels.

Air movement also helps to improve plant respiration.  Solar powered fans such as Snap-Fan and Vari-Cyclone Ceiling Fans are the most cost effective way to ventilate greenhouses with no utility expense.

Summer Ventilation = Beat the Heat

Solar radiation through the greenhouse glazing raises the inside air temperature within the greenhouse. Summer greenhouse ventilation prevents the indoor temperature from rising too high above the outside air temperature.

The ventilation system must effectively move air directly through the crop and over the soil to prevent excessive temperature buildups around the plants. A generally accepted minimum ventilation rate for temperature control in the summer is one air change per minute.

Ventilation rates range from one air change every three minutes to three air changes per minute. As ventilation rates increase, the temperature difference between inside and outside air decreases.

Regardless of how high the ventilation rate in a greenhouse is in the summer, the inside air temperature during the day will never be as low as the outside air temperature. To maintain an inside air temperature below outside air temperature, evaporative cooling or some other means of conditioning the air must be used.

Winter Ventilation = Beat the Humidity

Winter greenhouse use may require a heating system or a passive heat sink like large volumes of water in storage. Regardless of the heating method, some ventilation will be required to keep relative humidity below 70 percent.

Two to three air changes per hour are typically adequate. One good way to provide the air movement needed to control humidity is to use the passive ventilation and a solar powered ceiling fan.

Air changes greater than 3 to 4 per hour will likely cause an increase in heating requirements.

Spring-Fall Ventilation

The varying weather conditions during spring and fall require a combination of occasional temperature control and occasional humidity control. A ventilating fan system with multiple fans allow a better method of being able to control the ventilation according to the varying requirements.

Air Distribution Within Greenhouses = HAF Fans

Horizontal Air Flow fans maintain good air circulation within a greenhouse. In larger greenhouses, HAF fans should be mounted 10 to 15 feet from the greenhouse end walls  with additional fans spaced 30 to 50 feet apart.

The HAF idea keeps air moving just enough to provide a more uniform temperature throughout the greenhouse to control humidity as well as replace air at the plant level with air that has a higher carbon dioxide level.

For an efficient system, the total fan capacity should be about two times the floor area. For example, in a 20 by 50 foot greenhouse, the fan capacity should be 20 by 50 by 2 = 2,000 cubic feet of air movement per minute. The number of fans needed can be calculated by dividing the capacity needed by the output of one fan.

Two 12 inch Snap-Fans would meet this requirement.

Learn more about solar fans for greenhouse ventilation.

 

   

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