Air is the source of gases that cacti and other plants use to thrive and grow. It is at night that cacti, with their crassu-lacean acid metabolism, take in carbon dioxide through the stomata in their stems, storing it for use in photosynthesis during the day. Yet it is at night that a greenhouse may be shut tight, and air circulation or ventilation may be needed.
Air circulation is simply the movement of air within a plant growing area. Any sort of fan will circulate air. Ventilation introduces new air and exhausts old air. Greenhouses also use ventilation as a method of cooling. Ventilation fans should exhaust enough air to provide one volume of air exchange per minute to a height of 8 ft (2.4 m). For example, a greenhouse 20 x 50 ft (6 x 15 m) in area would require fans with a combined rated capacity of 20 x 50 x 8 = 8000 cubic feet (6 x 15 x 2.4 ~ 220 cubic meters) per minute.
Greenhouses benefit from horizontal airflow fans, which are designed to move large volumes of air rapidly. Horizontal airflow fans are expensive, however, and generally are recommended only for greenhouses larger than 600 square feet (56 square meters). Typical household fans will be adequate in smaller greenhouses. Air circulation fans are generally left on 24 hours a day.
Smaller greenhouses will save energy by using two-speed fans, and larger ones will need multifan installations. During winter months only one-fourth of normal air-exchange volume is required. Locate the fan on one wall and air-intake louvers on the opposite wall. Air-intake louvers should be at least 1.25 times larger than the fan area. The bottoms of the fans and louvers should be about 3 ft (1 m) above the floor, allowing air to flow over the plants on benches. Electronic thermostats, with intervals of about 1°F (0.6°C), are preferable to mechanical ones, usually with intervals of 5°F (2.8°C). If a mechanical one were set at 80°F (27°C), it would run the fans until air temperature reached 75°F (24°C), wasting electricity and money.
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