Airflow: Think Positive

Photo Source

Greenhouse (GH) cooling and ventilation systems have a direct influence on what enters and exits the GH. This is especially prevalent in warmer climates where a large volume of air is consistently being moved throughout the grow area. Whether you are pulling or pushing this air through the structure is important in not only temperature control but also in pest management (PM). There are three basic greenhouse-cooling systems: natural ventilation, negative pressure, and positive pressure. If your crop is one that thrives in an insect free environment, one of these systems is the better choice, providing superior temp control and PM.

Photo Source

Natural ventilation can be very effective in controlling air temperature, however, it’s often at the cost of giving up insect exclusion. Insect screens for passively or naturally ventilated greenhouses generally result in undesirably high air temperatures as they restrict natural airflow. In warmer climates where temperature control and PM is important to the survival of the crop, a negative or positive pressure cooling system is usually the better choice.

Negative pressure.png
Negative Pressure: Photo Source


A negative pressure cooling system focuses on pulling air through the greenhouse with exhaust fans. The process pulls outside air through cooling pads located on the opposite wall of the exhaust fans. What happens during this process is that the cool air traveling from one end of the GH to the other, mixes with the warm air in the house. The heat from the warm air then transfers into the cool air resulting in fluctuating air temperatures throughout the structure. The coldest air is experienced closest to the pad wall, while temperatures continually rise as the air approaches the exhaust wall. This can be a substantial change in air temperature from one end of the GH to the other depending on sq. footage, climate, etc. Furthermore, as the air is pulled into the greenhouse, insects can also be pulled in through any opening in the structure (doors, vents, tears, cracks.) Fortunately, this infiltration can be prevented up to a point with the proper insect screens in proper locations, usually both at air inlets and outlets.


positive pressure.jpg
Positive Pressure: Photo Source


A positive pressure cooling system forcefully pushes cool air through ducting into the GH. The air can escape through any openings (doors, vents, tears, cracks) but it is at a pressure high enough to exceed the flying speed of insects, making it impossible for them to enter into the GH. This is beneficial for temperature control as the warm air in the GH rises to the ceiling and out roof vents with the cool air residing near the grow area. A system like this creates a uniform temperature from wall to wall at +/- 2 °F. To avoid pushing insects into the GH, this type of system does require an insect screen at the air inlet. For example, a positive pressure cooling system like Agra Tech’s Agra Kool II in combination with Svensson’s ECONET 1515 can offer superior cooling while also excluding pests even down to the size of a thrip.

Overall, depending on your crop and PM goals, the proper cooling system can have a profound impact on more than just temperature as it also presents the opportunity to reduce pesticide use. Not to mention, the process of pushing air through the GH (positive pressure) has been shown to be more effective in excluding insects than an exhaust system (negative pressure) which tends to pull insects into the GH. Depending on your crop, location, cooling and PM goals, a positive pressure cooling system may be an appealing investment.

Do you have any experience with this topic? How do you cool your greenhouse? Do you have any recommendations? Please leave your thoughts in the comment section below.


Leave a Reply