Committee Blog: An Introduction to HVACD for Indoor Plant Environments – Why We Should Include a “D” for Dehumidification

by NCIA’s Facilities Design Committee

Transpiration and VPD are two fundamental components of plant vitality, and heating, ventilation, and air conditioning (HVAC) systems are one of the most critical considerations for an indoor cannabis cultivator. HVAC alone doesn’t tell the full story of environmental control for cultivation facilities. The term HVAC is typically used to refer to the cooling, heating, or ventilation systems in a building, and while it technically includes dehumidification in most forms, it does not directly highlight the significant dehumidification requirements necessary to maintain optimum plant health inside indoor cultivation spaces. In order to emphasize the importance of dehumidification in the mechanical equipment sizing and selection process, the controlled environment horticulture industry would benefit from moving toward the concept of heating, ventilation, air conditioning, and dehumidification (HVACD) as the common term for these systems.

“HVAC” Challenges

The term HVAC is typically used to reference conventional air conditioning and heating systems designed for temperature control to provide a comfortable environment for people. This is clearly demonstrated in the very design of these systems – for example, sensible heat ratios of commercial HVAC equipment are pretty high in order to meet the loads generated by people, lighting, and miscellaneous equipment found in offices. Plants grown in enclosed spaces have different needs than people do. Of particular interest is the large amount of dehumidification that needs to be performed on a daily basis to maintain an optimum vapor pressure deficit (VPD). 

When you apply a standard HVAC system to indoor horticulture, the instant the sensible load is removed from the space (i.e. the lights turn off), the air conditioning unit reaches the lower deadband of the specified temperature set point and shuts off. In the process of bringing the temperature down, we have raised the relative humidity to the detriment of the plants. Further, despite being mostly sensible cooling machines, conventional HVAC systems provide most of the dehumidification capacity in an indoor cultivation space, and that capacity is now inactive during the dark period.

Traditionally, growers would install stand-alone, pocket dehumidifiers to handle the moisture removal requirements that the air conditioning units cannot meet. There are a handful of challenges with this approach that can negatively impact plant health when scaling into industrial-scale operations. Most standalone dehumidifiers dry the air with mechanical refrigeration and in the process add hot air into the room, which then needs to be cooled by additional cooling equipment to maintain temperature. Another challenge is the numerous condensate drains throughout the growing space that are high risk for clogging and quickly leading to pest and pathogen proliferation which are potential GMP and GFSI compliance risks. The separate cooling and dehumidification systems typically do not have communication and control amongst them and ultimately “fight” against each other for temperature and relative humidity setpoints. Additional pest vectors can come into play when the HVAC contractor enters the cultivation or curing space to make repairs on mechanical equipment that is mounted above plants. Above all, maintaining cleanliness in the space can be challenging with many mechanical units perched above a dynamic plant canopy.

Dehumidification, or removing humidity from a room that is filled with water vapor as a result of plant transpiration, is arguably the biggest environmental challenge in controlled environment horticulture. When you size an HVAC system for human comfort or server rooms, the primary focus is temperature control (or sensible load). When selecting and sizing an HVACD system for plants to thrive, it’s all about the latent load, plant transpiration and VPD. Excess humidity is roughly twice as difficult to remove as excess heat from lights, so an effective system needs to be designed as a dehumidifier first and an air conditioner second. Integrated dehumidification needs to be at the beginning of every HVAC conversation, and a primary focus of every system.

The benefits of including dehumidification as a critical component

To maximize plant vitality, two fundamental components to understand are transpiration and VPD. Put simply, VPD is the humidity difference (or deficit) between the inside of a leaf and the environmental conditions surrounding that leaf. It is this humidity difference that draws water from the roots of a plant, through the stem and out of the leaf tissue, otherwise known as transpiration. This process is critical to photosynthesis and optimizing plant production, and it’s all directly related to the levels of humidity in a given room. As humidity is drawn out of the leaf, dehumidification must be used to remove the humidity from the environment and maintain appropriate VPD levels. Without dehumidification, humidity builds inside the room, plant growth and plant health are negatively impacted, and conditions become ideal for pests and pathogens. 

Properly-designed dehumidification creates consistent and precise environmental conditions across the plant canopy, mitigating risk against issues like powdery mildew and botrytis. Well-executed dehumidification allows growers to control their VPD and drive plant health. At the end of the day, a stable climate sets a strong foundation of cultural control for a facility to maximize quality biomass while limiting crop loss associated with pest and pathogen issues.

Why does this all matter? Because properly sized and commissioned HVACD systems empower the cultivator to maximize production efficiency, reduce the risk of production downtime, and promote growth. 

HVACD will change industry standards and best practices

Collectively updating the industry’s knowledge and understanding surrounding dehumidification highlights the true challenges of growing plants indoors, and the need for purpose-built equipment and controls that optimize the environment based on every stage of the plant life cycle. Plants are living organisms that have different needs at different times, and mechanical equipment should be designed and manufactured around this concept. 

We can draw a comparison to this concept by looking at indoor horticulture lighting systems. When the industry began to develop new terms like Photosynthetic Photon Flux Density (PPFD), the phrase “PPFD for plants because lumens are for humans” came to be. Meaning that if we are measuring lumens, we are prioritizing people in the space as opposed to the plants. This same mindset should apply to all mechanical equipment involved in a cultivation facility.

Now that more scientific studies and data are becoming available on topics like plant transpiration, dehumidification, and VPD as a driving force in plant vitality, it is clear that there is a better way to think about climate control when applied to controlled environment horticulture. HVAC focuses on the sensible (or temperature) cooling that keeps people happy indoors, while HVACD focuses on the latent cooling (or moisture removal) that keeps plants happy indoors. Both are critical concepts that must be considered during the design of an indoor cannabis cultivation facility to ensure both people and plants are happy indoors.

Critical Dehumidification Requirements for other Rooms in an Integrated Cannabis Grow/Manufacturing Facility

Extending the discussion downstream of horticulture to other areas of the facility, humidity control plays a critical role in profitability, food safety, asset utilization, and operating efficiency. 

Humidity control in the Curing Room and the awareness of any air exchange with building areas adjacent to Cure is important. More broadly, any room-to-room pressure differentials can transfer air with different humidity levels. 

Food safety is enhanced by considering humidity control, usually dehumidification. Any cold surfaces below the dew point of the room can cause condensation, which can lead to microbiological growth. Room environments controlled so that the water activity (aW) of the cannabis is maintained between 0.55 and 0.65, will also help limit mold growth and the associated mycotoxins. 

In many cases, there are special dehumidification requirements, such as in an equipment drying room after warewashing, or to dry out a room and return it to operation after washing and sanitizing. HVACD designers need to coordinate closely with process or manufacturing specialists to be aware of any process exhausts, combustion air requirements, or high outside air exchanges. That additional outside air and the humidity carried in with it, must be considered. 

And for Marijuana Infused Products (MIPs), specifically gummies and chocolates, the primary food safety control is low water activity, rather than a robust kill step, so precise humidity control is again a critical issue.

Even further, packaging machinery operates more efficiently if the flow characteristics of the cannabis are a key operating parameter. Moist cannabis will adhere to machinery and create other problems that slow run rates and cause downtime. This problem can occur with any weight fillers or the pre-roll machines. 

All told, moisture can be both your friend and foe in a wide variety of cannabis endeavors. The ability to maintain the appropriate relative humidity, in addition to temperature, in each different type of room in a cannabis production facility is a key factor in a successful operation.

The Facilities Design Committee (FDC) focuses on providing NCIA members and regulators a framework and information about facilities design options through which legal producers can plan for GMP level production as the market transitions from a state to a federally regulated industry.