‘Digging Deep’ Into Greenhouse HVAC

Features - Cultivation

Part II of a two-part series on making the most of an efficient HVAC system.

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August 3, 2016
Kyle Brown
Photo: Courtesy Arleigh Kraus

A greenhouse brings a grow closer to the elements, but the same sun that feeds the plants can also push the temperature far past the comfortable range for cannabis.

One main consideration is whether the greenhouse will be “open” or “closed,” says Nadia Sabeh, agricultural and mechanical engineer for consulting/engineering firm Guttmann & Blaevoet. An open greenhouse has some form of air flow from outside the structure, while a closed greenhouse is structured more like an indoor grow and mostly sealed. But even though an open greenhouse has more interaction with outside air, it doesn’t mean the cooling strategy is ... just to open a window, she says.

Depending on the location of the greenhouse, natural, passive ventilation is an option with ridge vents or open side walls, which can be manual or automated, says Sabeh.

Open Greenhouses

An evaporative cooling pad, or pad and fan cooling, is a common setup for an open greenhouse, says Jeff Lloyd, owner of Emerald Kingdom Greenhouse, a greenhouse supply company in Weaverville, Calif. A wet membrane is hung on one end of the greenhouse with fans drawing outside air through, dropping the temperature. Evaporative cooling also runs with low energy requirements.

“We’ve found that these water walls, accompanied with shade cloth or ground cloth, really can be efficient at lowering [temperature] between 10 and 15 degrees,” he says.

Keith Sprau, founder at Colorado Leaf, LLC, in Pueblo, Colo., relies on a 25-foot by 6-foot cooling pad on the south wall in each of the four greenhouses of his 17,000-square-foot grow, paired off with American Coolair exhaust fans (two 48-inch fans in the veg greenhouses and three 54-inch fans in each of the flower houses).

“We draw the air through there, and our exhaust fans are on the north end,” Sprau says.

The placement and orientation (in relation to the sun) of a greenhouse can make a big difference in keeping cool, especially in a high-temperature climate, says Jeff Lloyd, owner of Emerald Kingdom Greenhouse.
Photo: Courtesy John Sakun

The pad and fan keeps cool air moving even at high temperatures, and when Sprau got his original quote for the installation, he doubled the size of the cooling setup to make sure he had the capacity to fight the summer heat.

“It’s been 90 all week, and we’re holding at 76 in all the greenhouses, which is nice,” he says.

Another passive way to regulate temperature in the greenhouse is to just dig deep, says Lloyd. Trenching the greenhouse 36 inches beneath ground level reaches a place in the earth where the temperature is roughly always 60 to 70 degrees. “So instead of trying to heat or cool air that may be 100 degrees or freezing, you can get that ambient underground air temperature of 60, and you’re using a lot less energy to convert to make that 60 degrees hotter or colder,” says Lloyd.

Insulation and ground covering can go a long way toward helping manage a particular temperature regardless of the season, he says, “especially if they’re using a reflective material for the heat.” Placing a reflective ground covering down keeps the heat from being absorbed into the base of the greenhouse in the winter, and ground cloth insulates the cool air inside the greenhouse when used along with an evaporative pad system in the summer.

Just filling the greenhouse with plants makes the temperature more manageable in the summer for John Sakun, owner at Southern Colorado Growers in Rye, Colo. Between harvest and moving new plants in, a section of the concrete floor is exposed, and there’s a noticeable difference, he says.

“There’s still about 30 percent of the concrete flooring that’s definitely absorbing heat,” he says. “Once the room is completely full, it’ll be a lot easier to manage the temperatures because there’s not so much exposed concrete sucking up all that heat. It’s literally hot to the touch.”

Closed Greenhouses

A closed greenhouse offers more control over conditions than an open greenhouse, but usually a little less than an actual indoor grow, says Sabeh. In general, however, the closed system runs similarly to an indoor grow.

Beyond packaged DX or split air conditioning units (more on this can be found in Part I of this series in the May/June issue of Cannabis Business Times; see box below), she suggests choosing better glass, double polycarbonate or other materials that better insulate.

Part of that insulation process for Sprau meant teaming up with his father and brother as the greenhouse construction was finishing and using, “no exaggeration, about 200 tubes of silicone,” to seal the grow as airtight as possible, he says.

“We did every seam, every frame,” says Sprau. “We really went overboard. But there’s a fantastic negative pressure anytime you try to open the greenhouse doors, and that’s what you want.”

Drying It Out

Though the evaporative cooling pad is effective at bringing the temperature down in the greenhouse, it floods the environment with humidity. Sprau uses a Wadsworth controller to manage the grow’s levels and relies mostly on those exhaust fans to pull in drier air from outside.

“As long as it’s not damp outside, it does a great job of holding within about two percent [humidity],” says Sprau.

If exterior humidity is a factor, a desiccant-based system can help, says Kurt Parbst, director of business development for Envirotech Greenhouse Solutions. (Desiccant is a media that absorbs moisture from the air; that media can then be recycled in a desiccant wheel, which collects moisture, then evaporates it with heat as it passes around the wheel. More on this also can be found in Part I.)

When growers irrigate, the sun goes down or supplemental lights are turned off, growers can lose control of relative humidity. “They’re afraid of getting into fungal pressure,” he says.

After the lights go out, air conditioning units can level out temperature quickly, but plants continue transpiring water into the room for some time after, raising the relative humidity. When facing those high pressures, a desiccant system running parallel to augment dehumidification could make the difference, says Parbst.

A closed greenhouse has many of the same HVAC options as an indoor grow, such as a system using a packaged DX or split air conditioning unit, in addition to more passive ventilation methods.
Photo: Courtesy John Sakun

Desiccant systems are useful in many situations (like drying in cooler temperatures), but it takes a lot of energy to heat the material, says Clif Tomasini, product manager for Quest Dehumidifiers.

“You’re paying the price every month in electricity,” says Tomasini.

He suggests keeping the greenhouse temperature a little warmer at night, around 70 degrees, and using a refrigerant-based dehumidifier system. That uses the same basic build as an air conditioner, but is geared toward drying air efficiently instead of cooling it. Compared to a desiccant-based system, it uses about a quarter of the energy per pint, he says.

Arleigh Kraus uses a dehumidifier in both of her 1,000 square-foot greenhouses in Knox County, Maine. Humidity is a constant problem when the wind carries in from the ocean, she says. In addition to ventilation and an industrial fan in each greenhouse, she uses a Quest Power Dry 4000 to manage the pressure.

“In California or Colorado, [ventilation] would definitely work, but here, you just need [the dehumidifier],” she says. “You need to pull moisture out of the air, especially at night.”

John Sakun, owner of Southern Colorado Growers, uses a radiant heat system to maintain a healthy temperature for his greenhouse during the winter. When weather is cold, a boiler feeds a mix of hot water and antifreeze through pipes under the concrete pad in his greenhouse.
Photo: Courtesy John Sakun

Though buying the separate dehumidification system was about 30 percent of the upfront cost of her budget, that money is made up elsewhere, she says.

“The thing is, you’re basically getting free light. [And] without using a dehumidifier, you’re looking at a minimum of 25-percent upwards to 50-percent loss of product just from mold,” she says. “The cost of purchasing [a dehumidifier], running your electric bill [up a bit] definitely outweighs the profit loss you’ll have if you don’t use it.”

Fighting the Cold

You have many options to heat a greenhouse, starting with the choice between hot air or hot water, says Sabeh. Either one can run through pipes placed near the grow or in the flooring to provide heat through convection.

Radiant floor heating and cooling is becoming a more common solution for greenhouses, because it places heat near the plants, says Lloyd.

Sakun keeps his grow warm during the winter with a radiant heat system from BioTherm, using a boiler that feeds a mix of water and antifreeze through pipes that run about three inches inside the concrete pad under his greenhouse. The system keeps heat right near the root zone.

Being able to control where the heat is placed is important for each stage of growth, says Parbst. During the vegetative stage of the plant, the root zone is the focus, which means under-crop heating. At the flower stage, “the root zone and the canopy should be in balance,” he says.

For him, that means some heat from the company’s Agam VLHC (ventilated latent heat converter) desiccant system, supplemented by hot water heating around the perimeter of the grow or overhead. The system serves mainly to dehumidify the grow, but also can feed some of that energy back to the grow as heat, he says.

Like an indoor grow, hot water also can heat a coil connected to a fan to blow the hot air out into a greenhouse, says Sabeh.

When it comes to hot air from a furnace running on natural gas, propane or other fossil fuels, there are energy caveats. Sakun uses a Lennox propane heater to assist his radiant heat setup, but he relies on radiant heat to get him through winter at a lower energy cost compared to his propane heater.

The choice of furnace fuel generally comes down to how tightly a grower wants to control humidity, says Lloyd.

“One thing about natural gas heaters, when they combust, they create a level of moisture in that process, so growers who are really worried about molds will sometimes stray away from natural gas heaters and go for electric. The electric is really dry,” he says.

He has also seen some clients go with a wood-burning option for greenhouses where that fuel is more available, he says.

Efficiency should be a primary concern, says Lloyd, and growers should look for high-efficiency models even if they cost more up front.

To boost that efficiency, insulation comes into play for Sprau. Whenever he knows a bad snowstorm is on the way, he’ll shut all the curtains in the greenhouse to help seal in heat, he says, using about 30 percent less energy for heat.

Another relatively inexpensive insulation trick for greenhouses in a cold environment Lloyd has seen is placing a double layer of film on the outside of a structure, then inflating that film with a blower.

Free Resources

The building itself is a factor in keeping cool. The structure’s placement relative to sun’s path determines how much direct sunlight it will get at particular points of the day, and how much shade, says Lloyd. If a greenhouse is in a high-temperature climate, orienting the greenhouse to provide some relief during the hottest part of the day will save in energy costs overall.

“Those guys [in hot climates] can orient their building to the point they’re getting some afternoon shade, after 3 p.m.,” he says. “Their building is just going to stay cooler. … Orienting can be a trick to help with your energy costs.”

In terms of overall cost, managing the goal temperature in a greenhouse isn’t as hefty as it is for an indoor grow, says Sakun, who also maintains a 4,000-square-foot warehouse grow. For his greenhouse grow, about 10 percent of his overall budget was spent on HVAC systems. But for his indoor grow, “it was almost double; about 20 percent of our overall cost went to AC,” he says.

Working with greenhouses to get the most out of potentially cheaper natural resources for heating and cooling is a necessary part of staying competitive in the industry, he says.

“Obviously we have supplemental lights in there because it’s a year-round room, but it’s utilizing our free, wonderful sun,” says Sakun. “If you want to [dedicate] time and effort and money toward your plants, you have to take money away from heating costs, cooling costs, lighting costs. If you can minimize those costs, you can put more effort into the actual plant itself, and that’s what we try to do.”