It’s easy for cultivators to focus on lighting and nutrients. But a well-planned HVAC system not only keeps plants growing comfortably, it helps protect against threats like downy mildew.
It can be tough to figure out what HVAC equipment is needed, and the range of options can leave growers feeling overwhelmed, says Nadia Sabeh, agricultural and mechanical engineer for consulting/engineering firm Guttmann & Blaevoet.
“There [are] so many systems, so many moving parts,” she says. “But what I find is that [cannabis growers] are just trying to grow, and get growing as soon as possible. These guys just want something that works.”
In an indoor facility, the goal is to take both the heat and moisture from the recirculating inside air, says Sabeh.
She starts with refrigerant-based cooling and dehumidification usually called “mechanical cooling,” which is an air conditioner that comes in two main forms: a packaged DX (direct expansion) unit or a split unit.
A packaged DX unit works by flowing air across a cooling coil past the dew point, which pulls moisture out of the air as it chills. Once the air is dry, a reheater brings the temperature to a healthier target for the plants.
Denny Dikeman, head grower at Colorado-based B*GOOD, runs four 10-ton Carrier packaged DX units, one for each of his grow rooms, totaling about 5,000 square feet. He supplements that with three 5-ton units that are used to help balance temperature and humidity swings.
“That main unit cools, reheats and dehumidifies probably 80 percent of the air,” says Dikeman.
To help with air circulation, Dikeman uses 16- to 18-inch wall-hung circular fans, about 25 per grow room. They move air through the canopy, while six ceiling-mounted Big Ass Fans in each of his bloom rooms disperse hot air rising from his high-pressure sodium (HPS) lights.
A split unit is similar to a residential-use air conditioner, which runs in two parts: a condenser that rejects heat (possibly outside the building), and fan coil units or air-handling units inside the building. Refrigerant runs between the outside and inside units, pulling heat out of the air inside the building.
Rafael Chavez, consultant for Advantage Consulting Group Inc., relies on a custom system of two split 30-ton units running in 12-hour rotation for his 9,500-square-foot grow (four flowering rooms and one veg room) in Los Angeles.
“Each one of the four flowering rooms has about 30 tons of AC going into it. We place the air handlers inside the rooms, and run the Freon copper and all that to the roof, where the condensers are,” he says. He also runs a 260-pint HI-E Dry dehumidifier system in each room.
With 40 dual-ended HPS lights in each flower room and a 12-foot ceiling, the system generates a lot of heat. Part of the reason he maintains the split units is a habit of wanting to build a discreet grow, and packaged DX units tend to be much larger and attention-grabbing, he says.
Between the two, Sabeh prefers a packaged DX unit. A split unit usually means more indoor units to reach temperature goals. Plus, more units also means more parts that could break down. A split unit doesn’t remove enough moisture from the room as it cools, instead trapping it in that space, she says, because they are sized for temperature rather than humidity. That means it can hit a temperature goal, but could change the relative humidity in the grow room with less control.
When Dan Grace, president of Dark Heart Nursery in Oakland, Calif., planned a 7,000-square-foot addition to his nursery operation, he wanted a system that would not only keep the plants cool, but also manage humidity, he says.
“Although we were able to tightly control most of the variables in our system including temperature, watering and fertilizing, we were not able to tightly control humidity,” says Grace.
They measured evapotranspiration rates of the grow and approached Sabeh with the data: The nursery actually had a substantially greater dehumidification load than temperature load, he says. The 70-ton AAON packaged DX unit he decided on keeps control of the temperature. But when it reaches the correct temperature, it can also separately dehumidify the air by cooling, then reheating it to maintain both targets.
A third option for an indoor facility is a chilled water unit, which uses a chiller to cool a water reservoir. The system pumps that chilled water to air handlers (built with fans and a cooling coil to dehumidify) throughout the grow area, pulling heat and moisture out of the air. A chilled water unit is meant for a larger grow, probably more than an acre, says Sabeh.
“But chillers are complex pieces of equipment,” she says. “You need a pretty savvy maintenance staff. Or you might want to set up a service agreement with the local maintenance contractor.”
Chavez is in the process of building a new grow in Desert Hot Springs, Calif., in an 80,000-square-foot warehouse. There, he’s looking to place two 300-ton chilled water units to manage the heat. The grow uses LED lighting, which runs cooler, despite the hot environment.
“The [chilled water units are] a little more expensive, but they’re much more efficient,” says Chavez. “Since we’re growing in the desert, I need it to be reliable.”
If a cooling system doesn’t specifically handle dehumidification (or enough to achieve a facility’s goals), a second option would be desiccant.
Desiccant is a media that absorbs moisture from the air. That media can be recycled in a desiccant wheel, which collects moisture, then evaporates that moisture with heat as it passes around the wheel. It dries the air, but it needs to be cooled again to match the temperature goal for the grow. The temperature coming from a desiccant wheel can reach anywhere from 90 to 140 degrees, says Sabeh.
Heating Things Up
Heating options begin with the source: hot water or hot air.
Hot water can be run through pipes under a bench or on the ground to the media itself to provide heat directly to the root zone, says Sabeh. Hot water tubes also can be run along the base of the plant to provide some convection as well as heat radiation.
Hot water also can be run through a coil with a fan attached to push the heated air out into the grow. Though it’s not as common, some greenhouses still run with a traditional radiator, says Sabeh.
Hot air can come from a furnace that burns natural gas, propane, or other fossil fuels. They can be located either centrally in the air handling unit or as separate unit heaters mounted directly in the grow space. However, Sabeh has never seen a unit heater inside an indoor grow; they are more typical in greenhouses.
Electric heaters are available, but Sabeh doesn’t recommend them as an option because they heat inefficiently for the amount of energy used.
Energy is also a concern for a hot water system, since no matter how the heat is distributed, the water still has to be initially heated. In many cases, that means a hot water boiler, which will often be fueled by natural gas, says Sabeh. Electricity is also an option here as well, but depending on the energy codes in the state where the grow is based, the amount of electric heat in a facility could be limited.
If the grow is in a location with strong sunlight, solar radiation could be used to heat the water and store it in a tank until nighttime, when the heat is needed, supplemented by a boiler or water heater when necessary, Sabeh says.
Energy usage plays into HVAC system choices as well, mainly through utility rates. When designing an HVAC system, take local gas and electricity rates and availability into account, says Sabeh.
Considering Expense vs. the Cost of Choosing the Wrong Equipment
It can be tough to look past the initial cost of a system (the capital expenditure, or CAPEX) to how efficiently it can support the grow (operating expense, or OPEX), says Sabeh. And when growers get caught without an effective plan, sometimes they end up going to the nearest big-box store rather than to an engineer.
For Chavez’s new Desert Hot Springs grow, he’s planning on spending about $300,000 for his chilled water units, which plays out to about 10 percent of his total budget there. He’s doing the planning himself, with plenty of research to back up his decisions. His planned model runs the grow on about 80 percent of his HVAC, with about 20 as backup.
“It’s basically to be able to maximize what we can out of the equipment we have,” says Chavez. “The more research I do, cost has a lot to do with it.”
Although Chavez has mapped out most of the HVAC system (working with a dedicated HVAC employee), he cautions that if cultivators are going to go D.I.Y., they need to understand enough of the system to know how to check an employee or contractor’s work.
“I always do things myself. I always do research. The manufacturers have a lot of good data out there, and I like to make friends with smart people in the industry,” he says.
Dikeman estimates that he has spent more than $250,000 in air conditioning for his grow over the last five years, including the four rooftop units at about $30,000 each and $25,000 for the supplemental 5-ton units, he says.
Taking data to an engineer narrowed down the choices as to what would fit the required parameters for Grace’s grow.
“By the time we were done …, we had a really tight specification on the piece of equipment we were going to buy and install,” says Grace. “It costs something to do that, but much less than it costs if you realize down the road that you bought the wrong equipment.”
Cultivators could dedicate about 25 percent of their CAPEX to the HVAC system, and get a customized system that not only supports the grow, but could benefit the facility through high energy efficiency or other means, says Sabeh.
In addition to finding the right piece of equipment to achieve your goals, she says, it’s also important to “talk about everything else you weren’t thinking of. Let’s talk about noise, or maintenance or redundancy. So then you can play with temperature and humidity; you can decide what the best combination is. If you just pick an off-the-shelf piece of equipment, you get what you get.”