ASHRAE Journal - April 2020 - 83

SOLVING PROBLEMS

be higher when plants are smaller. When the plants get
bigger, relative humidity setpoints will decrease," he
said. "Different grow room types will require their own
individual HVAC systems too."
This among other grow facility characteristics affect
the buildings' loads.

Sabeh said some growers target low air temperatures
and/or low relative humidities that result in a high VPD
condition, but this could be detrimental to the crop.
"They're just so afraid of humidity that they think that
the drier the environment is, the better it is, the more
unlikely they are to get mildews and mold," she said.
"But actually it can really stress the plant."

Sensible and Latent Loads
Adding the sensible and latent loads together is a common error designers make when calculating loads for
cannabis grow facilities, said Sabeh.
Engineers rarely consider the fact that the plants often
act as their own mini-evaporative coolers, said Roth.
This means the plants themselves cool the air, so the
total calculated sensible load for sizing equipment must
be reduced by the amount of sensible cooling the plants
provide, he said.
"So all that water gets turned into water vapor, which
are those latent loads that are so challenging for HVAC
equipment to handle," Sabeh said. "Plant transpiration
is an evaporative cooling process, so sensible heat gets
converted to latent, reducing the overall sensible load of
the room."
Roth said designers need to accurately know the quantity of plants. This determines how much water is being
used, which will then determine the latent load.
"Make sure to consider 'net water,' which is the
watering rate minus any wastewater (usually between
10-30%)," he said.
Plants generate a lot of moisture in the room. When
growers irrigate their plants, the plants' root absorb
anywhere between 60% to 90% of the water delivered
to them, depending on the grow media and irrigation
frequency. That water then moves to the stem, and then
exits the leaves through transpiration, said Sabeh.

Vapor Pressure Differential
Designers need to understand the vapor pressure differential (VPD), which describes the pressure difference
between at the surface of the leaf and the vapor pressure
in the air, said Roth.
"VPD signiﬁcantly affects the plant, just like temperature affects the plant," said Sabeh.
Research has shown for various crops that reducing
VPD increases yield and quality of plants, which she said
is good news for HVAC engineers because higher VPDs
are hard on HVAC equipment.

Undersizing and Oversizing Equipment
Because plant transpiration is an evaporative cooling process, sensible heat gets converted to latent heat,
said Sabeh. This reduces the overall sensible load of the
room. Not understanding this could result in oversized
HVAC equipment, which increases both capital and
operating costs. Oversizing equipment can cause HVAC
equipment to short cycle, which affects its life expectancy. It can also cause unnecessary energy usage, which
will increase utility costs, said Roth.
Sabeh said she has also seen a lot of equipment undersized for latent loads. Plants generate a lot of moisture in
the room, and if not controlled, the humidity will continue to rise, creating risks for mold growth and condensation on cool surfaces.
Roth said web-based HVAC sizing software tools for
cannabis grow facilities can be good resources for calculating required HVAC cooling and heating loads and sizing equipment and getting pricing. He advised designers
to use software that is speciﬁcally designed for calculating grow facility cooling and heating loads.
"Conventional HVAC load calculation software may
return incorrect results since they solve for sensible loads
to calculate airﬂows. Software speciﬁc to grow facilities
solves for latent loads to calculate airﬂows," he said.

Nighttime Conditions
Sabeh said most of the water is generated when the
lights are on when the plants are actively photosynthesizing. Plants continue to transpire when the lights are
off, but at lower rates.
"This nighttime condition has caused a lot of challenges for growers. When the lights are on, the AC unit
operates trying to meet the temperature setpoint and
subsequently dehumidiﬁes the air. But when lights shut
off, the sensible load goes away, the thermostat setting
is satisﬁed and the AC unit shuts off, subsequently shutting down dehumidiﬁcation," she said.
To avoid these errors, Sabeh advised designers to ask
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