Vivify your ventilation

As greenhouse growers look for ways to reduce energy usage and improve overall efficiencies, it stands to reason that they need to examine the way they are ventilating their houses to contain costs, provide maximum growth for their plants and keep their crops free of biotic and abiotic stressors.

To get the biggest bang for your buck, it comes down to how your greenhouse is designed and the type of fans and other equipment you choose to purchase.

“My advice is to not take anything for granted,” says Nadia Sabeh, president and founder of Dr. Greenhouse, a greenhouse design and consulting company based in California. Sabeh, who is an agricultural and mechanical engineer, says some of the design principles currently being implemented are based on concepts that are over 20 years old. For instance, she says during her doctoral research she found that under most conditions, .75 air changes per minute would be sufficient in a house, whereas the standard, or “rule of thumb,” has always been one air change per minute. The difference can mean a lot in terms of the size of fans required and the energy to run them.

“When I say don’t take it for granted, I mean that system might work for you, but you might be able to design a better system or select equipment that is more customized to your location and what you’re growing,” Sabeh says. “Ultimately, you save on costs of equipment because you don’t need as much as you think.”

Ventilation basics

While new technologies are making their way to the greenhouse — from artificial intelligence that can monitor pests and diseases, to data tracking software — the basics of the way and the reasons greenhouses need to be ventilated remain the same: to remove heat and moisture and provide an exchange of CO2 for photosynthesis. What is really at issue for growers is which system they should purchase to suit their needs.

Dr. John W. Worley is professor emeritus of poultry science at the University of Georgia. He was also an agricultural extension agent for several years and taught the university’s Master Gardener program. In addition, he was an extension engineer and performed energy audits of greenhouses.

“You have to look at what you’re going to grow in the greenhouse during what time of year,” Worley says. “In other words, there are some who put them up without heat because they’re just going to use it in the spring. No. 1 [is] look at the plants and what they need, how cold can they stand it without being damaged.” He suggests to also look at your climate and weather conditions and design a system that will give you protection against the coldest temperatures that occur in your region.

Here is some timely information that will help you design your next greenhouse or upgrade an existing one.

Natural ventilation

“A lot of people new to greenhouse production, whether they came from an indoor facility or a field grown operation, are unaware of the opportunities with natural ventilation,” Sabeh says. “When I talk to my clients, helping them evaluate greenhouse designs, I ask them if they’re interested in natural ventilation, and usually I get a response like, ‘What is that?’ and, ‘Tell me more.’”

Sabeh says a lot of prospective greenhouse growers orient their greenhouses to the sun but don’t think about how it is oriented to prevailing winds, the key ingredient in natural ventilation.

Ultimately, you save on costs of equipment because you don’t need as much as you think.” - Nadia Sabeh, president and founder, Dr. Greenhouse

Thermal gradients are created in the greenhouse on sunny days when the air becomes warmer and lighter. As it does so, it rises through the ridge vents, as cool air is replaced via the sidewalls. If the prevailing winds are out of the west, install side vents along the west side of the greenhouse. To achieve satisfactory ventilation, the sidewall and ridge vents must be properly sized.

The American Society of Agricultural and Biological Engineers recommends that the combined sidewall vent area should equal the combined ridge vent area and each should be 15 to 20 percent of the floor area.

Fan and louver ventilation

There is a lot to be said for the horizontal air flow (HAF) concept that draws air from one end of the greenhouse through the plant canopy and out the other end through mechanized louvers. This concept, which has been used for decades, has the benefit of cooling the house, removing moisture and humidity, and replenishing CO2, while offering good control over greenhouse temperatures. Worley says you can hardly beat this type of setup, but it is limited to smaller hobby and greenhouse operations that may be found at garden centers with 30-foot by 100-foot houses.  

Fan sizing and location are the keys to maximizing this method of ventilation. Small, 1/10 to 1/15 horsepower circulating fans with 12 to 18-inch blades will provide sufficient air movement while keeping energy costs at a minimum. Select fans that have a high ventilating efficiency ratio (VER), measured as cfm of output per watt of electricity input.

Locate fans to be in sync with the circular horizontal air mass that is being created by HAF ventilation. Place one fan 10 to 15 feet from one endwall to go with the flow of the air that is coming around the corner from the other side. Additional fans should be located 40 to 50 feet apart to keep the air mass moving. Spacing fans too far apart will create a dead space. A smoke bomb or incense stick can be used to determine airflow in different areas of the greenhouse.

The fan height should also be considered and is best if placed 4 to 5 feet above benches, and above or below hanging baskets to prevent excessive drying out of plants.

To get the maximum efficiency and cost savings from fans, Worley says it is best to purchase energy-efficient fans when you’re setting up a new greenhouse or are replacing worn-out ones. He says one source for energy ratings of fans is bess.illinois.edu, a site hosted by the University of Illinois.

Neil is a copywriter for the green industry and a frequent contributor to Greenhouse Management. greenindustrywriter.com