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
APRI L 2020

ashrae.org

ASHRAE JOURNAL

83


http://www.ashrae.org

ASHRAE Journal - April 2020

Table of Contents for the Digital Edition of ASHRAE Journal - April 2020

Contents
ASHRAE Journal - April 2020 - Intro
ASHRAE Journal - April 2020 - Cover1
ASHRAE Journal - April 2020 - Cover2
ASHRAE Journal - April 2020 - 1
ASHRAE Journal - April 2020 - Contents
ASHRAE Journal - April 2020 - 3
ASHRAE Journal - April 2020 - 4
ASHRAE Journal - April 2020 - 5
ASHRAE Journal - April 2020 - 6
ASHRAE Journal - April 2020 - 7
ASHRAE Journal - April 2020 - 8
ASHRAE Journal - April 2020 - 9
ASHRAE Journal - April 2020 - 10
ASHRAE Journal - April 2020 - 11
ASHRAE Journal - April 2020 - 12
ASHRAE Journal - April 2020 - 13
ASHRAE Journal - April 2020 - 14
ASHRAE Journal - April 2020 - 15
ASHRAE Journal - April 2020 - 16
ASHRAE Journal - April 2020 - 17
ASHRAE Journal - April 2020 - 18
ASHRAE Journal - April 2020 - 19
ASHRAE Journal - April 2020 - 20
ASHRAE Journal - April 2020 - 21
ASHRAE Journal - April 2020 - 22
ASHRAE Journal - April 2020 - 23
ASHRAE Journal - April 2020 - 24
ASHRAE Journal - April 2020 - 25
ASHRAE Journal - April 2020 - 26
ASHRAE Journal - April 2020 - 27
ASHRAE Journal - April 2020 - 28
ASHRAE Journal - April 2020 - 29
ASHRAE Journal - April 2020 - 30
ASHRAE Journal - April 2020 - 31
ASHRAE Journal - April 2020 - 32
ASHRAE Journal - April 2020 - 33
ASHRAE Journal - April 2020 - 34
ASHRAE Journal - April 2020 - 35
ASHRAE Journal - April 2020 - 36
ASHRAE Journal - April 2020 - 37
ASHRAE Journal - April 2020 - 38
ASHRAE Journal - April 2020 - 39
ASHRAE Journal - April 2020 - 40
ASHRAE Journal - April 2020 - 41
ASHRAE Journal - April 2020 - 42
ASHRAE Journal - April 2020 - 43
ASHRAE Journal - April 2020 - 44
ASHRAE Journal - April 2020 - 45
ASHRAE Journal - April 2020 - 46
ASHRAE Journal - April 2020 - 47
ASHRAE Journal - April 2020 - 48
ASHRAE Journal - April 2020 - 49
ASHRAE Journal - April 2020 - 50
ASHRAE Journal - April 2020 - 51
ASHRAE Journal - April 2020 - 52
ASHRAE Journal - April 2020 - 53
ASHRAE Journal - April 2020 - 54
ASHRAE Journal - April 2020 - 55
ASHRAE Journal - April 2020 - 56
ASHRAE Journal - April 2020 - 57
ASHRAE Journal - April 2020 - 58
ASHRAE Journal - April 2020 - 59
ASHRAE Journal - April 2020 - 60
ASHRAE Journal - April 2020 - 61
ASHRAE Journal - April 2020 - 62
ASHRAE Journal - April 2020 - 63
ASHRAE Journal - April 2020 - 64
ASHRAE Journal - April 2020 - 65
ASHRAE Journal - April 2020 - 66
ASHRAE Journal - April 2020 - 67
ASHRAE Journal - April 2020 - 68
ASHRAE Journal - April 2020 - 69
ASHRAE Journal - April 2020 - 70
ASHRAE Journal - April 2020 - 71
ASHRAE Journal - April 2020 - 72
ASHRAE Journal - April 2020 - 73
ASHRAE Journal - April 2020 - 74
ASHRAE Journal - April 2020 - 75
ASHRAE Journal - April 2020 - 76
ASHRAE Journal - April 2020 - 77
ASHRAE Journal - April 2020 - 78
ASHRAE Journal - April 2020 - 79
ASHRAE Journal - April 2020 - 80
ASHRAE Journal - April 2020 - 81
ASHRAE Journal - April 2020 - 82
ASHRAE Journal - April 2020 - 83
ASHRAE Journal - April 2020 - 84
ASHRAE Journal - April 2020 - 85
ASHRAE Journal - April 2020 - 86
ASHRAE Journal - April 2020 - 87
ASHRAE Journal - April 2020 - 88
ASHRAE Journal - April 2020 - 89
ASHRAE Journal - April 2020 - 90
ASHRAE Journal - April 2020 - 91
ASHRAE Journal - April 2020 - 92
ASHRAE Journal - April 2020 - 93
ASHRAE Journal - April 2020 - 94
ASHRAE Journal - April 2020 - 95
ASHRAE Journal - April 2020 - 96
ASHRAE Journal - April 2020 - Cover3
ASHRAE Journal - April 2020 - Cover4
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_YELQLJ
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_QJLWMC
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_MCDEBX
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_WNYSQY
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_XATVOD
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_FJSHSS
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_CCBZDS
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_XDEFVG
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2023november_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2023november
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_VHGNBL
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_WPKBNJ
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_UUVCDE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_RTGDEW
https://www.nxtbook.com/nxtbooks/ashrae/ashraemexico_2023
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_LKRFXS
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_AZSOFG
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_ERCDBH
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_QWDFRV
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_JHGVDF
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_OPUYHG
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_SREIBM
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_LRTGLK
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_OKRFGH
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2022november_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2022november
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_TZSERA
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_LVRUIX
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_RPTVYZ
https://www.nxtbook.com/nxtbooks/ashrae/mini_pub_catalog
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_XIYTGD
https://www.nxtbook.com/nxtbooks/ashrae/ashraemexico_2022
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_RFGDOB
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_PABXNU
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_REMKLS
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_PICVBT
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_AOYTVW
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_JQOPLS
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_IOYTBC
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_SGAJJF
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_IGHYER
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_PDRKLS
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2021november
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2021november_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_XCODFR
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_QSLFGO
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_ILKVNM
https://www.nxtbook.com/nxtbooks/ashrae/ashraemexico_2021
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_OPDJKD
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_VJKSRY
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_SDHUTC
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_JPPKRR
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_SDLTTH
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_CKLLES
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_SLDOX
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_HJETUK
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_OLUHGE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2020october
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2020october_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_ZERDGH
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_QVMNEO
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_RTPOKE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_BBATRE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_STUBMW
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_TPEMPE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_JNMKDS
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_FBTTPA
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_WQMMNE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_TVBRYN
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_showguide2020
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_KTUZMA
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_ABEDGD
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201910
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201909
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2019septmeber_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2019september
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201908
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201907
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201906
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201905
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201904
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2019april
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201903
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2019march
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201902
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201901
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_showguide2019
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2018december
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2018november
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2018fall_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2018fall
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2018october
https://www.nxtbook.com/nxtbooks/ashrae/ashraemexico_2018
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201810
https://www.nxtbook.com/nxtbooks/ashrae/ashraeinsights_201806
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201805
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201804
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201803
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201712
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201711
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201710
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2017fall_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2017fall
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201709
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201705
https://www.nxtbook.com/nxtbooks/ashrae/ashrae_meetinginsert_201610
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2016fall_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2016fall
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_acrexindia
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2015summer_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2015summer
https://www.nxtbook.com/nxtbooks/amca/2014summer2
https://www.nxtbook.com/nxtbooks/amca/2014summer
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_acma_2014summer
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201311
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201309
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_acmasupp_2013fall
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201305
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201303
https://www.nxtbook.com/nxtbooks/ashrae/pubcatalog_2013winter
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201211
https://www.nxtbook.com/nxtbooks/ashrae/achr_expo_mexico2012
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201209
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201208_v3
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201208_v2
https://www.nxtbook.com/nxtbooks/ashrae/pubcatalog_2012summer
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201205
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201203
https://www.nxtbook.com/nxtbooks/ashrae/pubcatalog_2012winter
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201111_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201109_v2
https://www.nxtbook.com/nxtbooks/ashrae/pubcatalog_2011summer
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201105
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201103
https://www.nxtbookmedia.com