In favor of fans

In greenhouses without air circulation, it’s common to find 10-15 F difference in the nighttime temperature between different growing areas. Plant growth and quality often reflect this temperature swing. A properly installed horizontal air flow (HAF) system can reduce this to no more than a 2 F difference anywhere in the greenhouse. It can also reduce the incidence of disease and provide more carbon dioxide to the leaf area.

Air is heavy

The air over every square foot of floor area in a greenhouse weighs about 1 pound, which works out to about 11/2 tons in a 30’ x 100’ greenhouse. Once the air is moving in a coherent pattern, it will coast along like a car on a level road. This is why HAF is so efficient. It takes only four small fans to keep the air moving at 50 to 100 feet per minute in the above greenhouse.

Horizontal circular pattern

Locate fans 40 to 50 feet apart to create a horizontal pattern down one side and back up the other in a free-standing greenhouse or down one bay and back in an adjacent bay in a gutter-connected house, as air mixing occurs from side to side and floor to ceiling.

Use circulation fans

1/10 to 1/15 horsepower circulating fans with blades that are designed to operate against zero static pressure are more efficient than exhaust fans that are designed to force air through louvers. Compare fans on an energy efficiency rating (EER), cfm output/watt of electricity input. Generally, permanent split capacitor (PSC) motors have a higher efficiency than shaded pole motors.

Low energy use

To keep the air mass moving requires only enough energy to overcome turbulence and friction losses. A rule of thumb based on greenhouse trials and smoke bomb tests is 2 cubic feet/minute (cfm) of fan capacity for each square foot of floor area. It may need to be increased slightly in houses with tall crops or hanging baskets.

Control

Up to 15 fans can be placed on one 20-amp circuit. During the fall, winter and spring, the HAF system should run continuously. The fan circuit can be controlled by an on-off switch. A power relay could be placed in the circuit so that the HAF system is turned off if the exhaust fans are activated or vents opened. If the environment is being controlled by a controller/computer, the HAF fans should be operated during heating, setpoint and low-stage ventilation modes.