ASHRAE Journal - April 2020 - 84

SOLVING PROBLEMS

the grower how much water they will use to irrigate
their plants.
"But don't stop there. Typically, about 30% of the irrigation water is drained away from the planting media.
Therefore, the plant transpiration rate will be about 70%
of the total irrigation water," she said.
Sabeh said it is important to select equipment that
operates to meet both a temperature and a humidity setpoint, which helps avoid loss of humidity control when
lights are off.
"In cases where humidity control drives operation of
the unit, it's important to have a reheat coil after the
cooling coil so that the room is not over-cooled. Also, we
don't really want to deliver sub-50°F (10°C) degree air to
the plants," she said.

Airflow Volume
Proper airflow volume is important in grow facilities
because cannabis plants need correct amounts of airflow
over the leaves to help induce evapotranspiration, said
Roth. He said designers should calculate airflow according to latent load first. Then, designers can size for a
specific cfm/ton since airflow quantity is important for
ensuring that plants do not get mildew and mold.
"Often, you'll size for around 350 cfm/ton (47 L/s per
kW) for flower rooms. This will then affect the coil leaving temperature which may not be at the conventional
50°F to 55°F (10°C to 13°C)," he said.
Designers need to consider mold and mildew. Not having enough airflow over the plants may cause mold and
mildew to form which will destroy the plants, said Roth.
Designers also need to pay attention to ventilation and
filtration strategies.
"If the facility needs to be ventilated, then there will
need to be some type of exhaust system (to maintain
equalized pressure or slightly negative pressure)," said
Roth. "Because cannabis is so pungent, the exhaust system requires some type of filtration so that the odors do
not escape the facility and cause complaints from neighboring businesses and residences."
To avoid introducing mold spores into the grow room
from the air, Sabeh says that MERV 11 and MERV 13 filters are enough. "We don't need to install HEPA filters to
remove the particles that growers care most about-dust,
pollen and mold." Not only are MERV filters enough for
grow operations, they also use require less fan energy
than using HEPA filters.
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Different Rooms
Grow facilities have different rooms for various stages of
the cannabis plant life cycle. These rooms include the flower
room, vegetative room, cloning room and mother room.
"One size does not fit all in grow facilities," said Roth.
"Each of those rooms requires different sensible and
latent setpoints so each room requires its own dedicated
piece of HVAC equipment. One rooftop unit cannot serve
all these different room types at once."
The conditions for each room differ based on the cannabis plants' needs. Younger flowers require a higher
relative humidity while older, flowering flowers require
a lower relative humidity, said Roth. Airflow levels could
also vary in the different rooms.
Designers need to understand the control sequences of
operation, said Roth.
"Different grow rooms require different sensible and
latent setpoints along with different lighting schedules.
All of this will affect how and when the HVAC equipment
manages cooling and heating," he said.

Lighting
Roth said designers should consider the types of lights
being used in the grow facilities. High-intensity discharge (HID) lights give off a lot more heat than LED
lights, he said, but designers should consider whether
lights will be changed from HID to LED down the road.
"If so, the HVAC equipment will need to be updated,"
Roth said.

Resources and Tips
As the legalization of marijuana grows in the U.S. and
around the world, the need for HVAC solutions for grow
houses will increase.
Sabeh said there is considerable activity in ASHRAE
and ASABE, the American Society of Agricultural and
Biological Engineers, to develop standards and guidelines to help growers, engineers and other industry professionals understand the unique characteristics of the
indoor plant environment. She is the co-chair of a new
standard-X653: HVAC for Indoor Plant Environments-
that ASHRAE and ASABE are co-sponsoring.
ASHRAE recently granted TC 2.2, Plant and Animal
Environments, approval for a research request to study the
energy impacts of indoor plant environments, said Sabeh.
"Other groups, too, are sponsoring or engaged in
research to understand the metrics, baselines, and

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