Greenhouse Ventilation Strategies
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
Vari-Cyclone Ceiling Fans are the most cost effective
way to ventilate greenhouses with no utility expense.
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
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.
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.
Two 12 inch Snap-Fans would meet this requirement.
more about solar
fans for greenhouse ventilation.
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