Horizontal airflow (HAF) fans are used to mix warm air near the roof of the greenhouse with the cooler air at ground level, providing a more uniform growing environment
Often referred to as mixing or circulation fans, HAF fans are not the same as ventilation fans and should not be run at the same time. It is also important to remember that despite many of the benefits of using HAF fans, improper installation and maintenance can greatly reduce these benefits.
Uniform Temperature Is A Big Benefit
One of the greatest benefits of using HAF fans is increased temperature uniformity. As air moves from one side and back up the other in a horizontal pattern, mixing occurs from side to side and from floor to ceiling.
Temperature variation, however, can also occur vertically within a greenhouse from floor to ceiling with or without HAF fans.
In order to quantify this phenomenon, researchers at Michigan State University measured air temperature at four different heights above the greenhouse floor. At each height, air temperature was measured with a thermocouple that had 2-inch diameter fans blowing into them.
The results of the study found that HAF fans were most important at night. During the night, the greenhouse floor and bench height air temperatures were similar. The temperature at 9.5 feet clocked in at a mere 1.9°F warmer.
When the HAF fans were turned off at night, average air flow temperature near the floor was 1.2°F cooler than the temperature at bench height and 3.3°F cooler than at 9.5 feet above ground. These studies concluded that HAF fans should be used at night, especially when growing plants on the ground.
HAF Fans Increase CO2 Levels
Additional benefits of HAF fans include higher carbon dioxide levels for plants. As plants photosynthesize, they deplete the CO2 in the boundary layer around the leaves. By turning on a HAF fan, air movement will increase, replacing the CO2-depleated air with air that has a higher concentration of CO2.
In addition to mixing water vapor and carbon dioxide, air movement from HAF fans helps to dry foliage more quickly than in a still environment. This can
reduce disease incidence as well as help distribute pesticide sprays to leaf surfaces.
HAF Fan Guidelines
To reduce greenhouse energy costs, be sure to use high-efficiency, low horsepower fans. Also, the fans should not be hung from chains, as they will move horizontally, preventing uniform air movement. The blade and guards should be cleaned routinely to improve efficiency.
As illustrated with the study from Michigan State University, HAF fan use is especially important at night. HAF fans should not be used in propagation houses where calm air is desirable. For proper air circulation, mount the fans below shade curtains and 4 feet above the crop. This ensures air movement is straight and parallel to the ground and flows downward and upward. Also place fans above or below hanging baskets to avoid blocking airflow.
For growers that don’t have time to ensure the fans are properly mounted and maintained, the use of HAF fans will lead to a less uniform airflow than not using the fans at all. For example, if HAF fans are positioned downward, toward the ground, this can dry out plants at variable rates, which can make uniform water management more difficult.