The ultimate in greenhouse ventilation is now available with climate control systems. Regardless of the weather, you need not lift a finger to control ventilation, humidity, or summer cooling. One system utilizes a 12-inch exhaust fan mounted in one end or in one side of the greenhouse in a clear plastic panel. The fan is equipped with a built-in shutter and safety backguard, and is automatically controlled by a thermostat. When temperature rises above the setting, the fan operates thermostatically to force out hot air. Cool air is pulled in and through the greenhouse by means of a large jalousie installed in the opposite end or side. This system provides one complete change of air per minute.
For semi-automatic operation, the volume of incoming air can be hand controlled: the jalousie is fully opened in summer, only partly opened in cool weather. For complete automatic ventilation, you simply install a thermostatically activated motor to open the jalousie. When the proper temperature is achieved, the motor will shut off the fan and close the jalousie.
A more complete climate-control system utilizes two thermostatically controlled jalousies at one end of the greenhouse, one jalousie above the other. They open and close automatically as temperature fluctuates. Cool air, drawn through the lower jalousie by a two-speed 16-inch fan, circulates through the greenhouse and is forced out through the upper jalousie. An aluminum air-control cabinet is placed in front of the lower jalousie to serve as a mixing chamber, wanning the cool outside air by mixing it with warm greenhouse air before it is circulated, to prevent sudden temperature changes harmful to plants.
For maximum summer ventilation, you set the fan at high speed and adjust the control damper in the cabinet to bring in enough outside air to provide up to one-and-a-half complete air changes per minute. For winter operation, you close the lower jalousie and set the fan at slow speed to recirculate the warm greenhouse air without introducing cold outside air.
To air condition your greenhouse in summer, and to provide year-round humidity control, a built-in mist spray attachment is included. An electric valve regulated by a humidistat controls the flow of water to a mist-fog nozzle mounted on the front of the fan cabinet. When the greenhouse air gets too dry, the fog nozzle produces a fine mist that is circulated throughout the greenhouse by the air fan. Thus humidity can be automatically regulated and maintained throughout the year.
My first greenhouse was thus climate-controlled and my snapdragons were the finest I have ever grown. My second greenhouse did not have a complete-control system, but it did have automatic ventilation. Night temperature was maintained at 55 F, and after some experimenting with manual operation of the vents in very cold weather, I found that setting the vent thermostat 20 degrees higher than the heat thermostat (instead of the usual 10 degree differential) kept the greenhouse adequately supplied with fresh air without harm to plants. Unless the day was extremely warm and sunny, the thermostat did not open vents often enough to cause excessive changes in temperature. Since I had to leave my greenhouse unattended during the day, this proved a satisfactory arrangement. In warm weather, the differential between heat and vent thermostat settings was returned to 10 degrees, since more circulation was required and the warmer outside air was no cause for concern.
My present greenhouse has modified climate control. It is equipped with air-control cabinet, fan, and jalousies, but no mist nozzle. Instead, a galvanized tray 20 by 24 by 1 ½ inches deep, placed in front of the air-control cabinet, is kept filled with water. This evaporates and is circulated through the air by the fan. Considerable humidity is thus provided and it is augmented by an occasional hose spraying of walks to produce lovely cymbidium and cattleya orchids. In extremely warm weather, additional ventilation can always be provided by replacing the solid glass panel in the door with jalousie or screen or both.