Overview

Growing plants indoors presents a unique challenge for heating ventilation air conditioning dehumidification (HVACD) systems that is unlike any other application. There are exceptionally large heat and moisture loads that change constantly throughout the grow cycle as the plants mature, increase in size and release more water. In addition, the “lights on” loads are dramatically different than the “lights off” loads. To make matters worse, optimal yields require very tight control of both temperature and relative humidity.  All this adds up to a complex task for the HVACD system and frankly the entire success of the grow operation will hinge on its ability to perform. When HVACD systems in cultivation facilities cannot control the indoor environment tightly or go offline due to a component failure, cultivators experience substantial crop damage or crop loss within hours.

In many cases when a new state launches a cannabis program, there is a rush to quickly build grow facilities to grab market share. Unfortunately, these compressed timelines often result in poor HVACD equipment choices that diminish the cultivator’s level of control over the indoor environment, blocking them from unlocking their facility's full potential.

In its most simple form, a grow facility’s HVACD system is responsible for maintaining temperature and relative humidity levels within the grow space to a tight tolerance. By maintaining both temperature and relative humidity cultivators can achieve a specific Vapor Pressure Deficit or VPD. Without getting too technical, VPD is a measurement of the difference in vapor pressure between the leaf, which is fully saturated, and the surrounding air. If the surrounding air is near saturation, meaning it does not have much additional capacity to take on more water from the plant, the VPD will be low, and the plant will struggle to transpire. When plants cannot give off enough moisture, they are then unable to take in more water and nutrients through their root system and growth slows or stops. Conversely, if the air surrounding the leaf is very dry (low vapor pressure) it will be much easier for the plant to give off moisture to the surrounding air as it has ample capacity to take on the additional moisture (high VPD).

While these examples represent a dramatic oversimplification of the process, the key take away is clear, by controlling VPD cultivators can control how much moisture plants can give off via transpiration and in turn how much new water and nutrients they can uptake through their root systems to fuel new growth. Therefore, by controlling VPD cultivators can manipulate the plant’s metabolic rate. For this reason, there is a very tight correlation between the level of VPD control and overall crop quality and yield quantities.

Considerations for an Indoor Grow Room HVACD System

To select the right HVACD system for an indoor grow facility the following must be considered.

Full load profile for the temp and moisture loads inside the grow facility

• It is important to have a deep understanding of the actual sensible and latent loads in the grow space throughout the cultivation cycle. This will show how much cooling and or dehumidification is required in each room throughout the grow cycle.

• Many grow facilities have historically been built out without fully understanding these highly dynamic loads and as a result they often are not sized correctly from a capacity standpoint.

Outdoor ambient temperature ranges

• Outdoor ambient temperatures have a dramatic effect on the HVACD system’s ability to operate efficiently and effectively. 

Cost and availability of electrical service

• Indoor grow facilities require a HUGE amount of power, in some situations it may be best to go with a chilled water system, which can be operated by a natural gas chiller, and dramatically reduce the overall power needed at the facility.

Facility size and layout

• Differences in facility size, number of facilities per site, and other factors can help inform equipment selection.

Desired level of control and set points

• Tighter control requires more sophisticated purpose-built and controlled HVACD equipment

• Lower temperatures and relative humidity levels present an especially large challenge to non-purpose-built HVACD equipment.

Any way you slice it, achieving tight, energy efficient, reliable control of indoor environments requires specialty knowledge and specialty equipment. Unfortunately, many cultivation facilities are built without a full understanding of the loads once plants are in the facility and utilize standard comfort cooling HVAC systems with supplemental dehumidification to control the space the best they can. 

For a head-to-head comparison between VRF systems with supplemental standalone dehumidification and integrated HVACD systems designed specifically for indoor cultivation read our blog: Growers in Search of Efficiency.

• From a reliability standpoint this is a perfect storm. The decoupled standard HVAC system is not designed for the loads or duty cycles of indoor agriculture and will burn out quickly.

• From an energy standpoint it is wasteful as the non-purpose-built HVACD system is not optimized for efficient moisture removal, but rather for comfort cooling, resulting in significantly higher energy usage as it works to control the grow space.

• Lastly and most importantly, an improperly sized decoupled HVACD system will not be able to deliver stable tight control of both temp and RH throughout the grow cycle. This means the cultivator will be unable to hit the necessary VPD targets to fully maximize their crops yield and quality potential.

Why do cultivation groups keep making the same HVACD system mistakes?

The problem is threefold:

Awareness

Many groups are simply unaware that purpose-built, integrated HVACD systems designed specifically for indoor grow rooms are available. In addition, manufacturers like Desert Aire with a dedicated CEA division, offer in-depth application and sizing support including a grow room questionnaire that incorporates all the necessary variables, including the plant’s transpiration rate, into one model for accurate load calculations. 

Cost Perception

There is a market perception that purpose-built equipment is more expensive than standard comfort cooling equipment and supplemental dehumidifiers. A recent third-party comparison study demonstrates that this is not the case. In addition, as discussed earlier, equipment that is not designed for a CEA application will not last nearly as long or preform nearly as reliably or efficiently as a purpose-built integrated HVACD system.

Longer Lead Times

Often times cultivation groups are under a time crunch and as a result off the shelf non-purpose-built equipment appears attractive because it is in stock and ready right away. In the case of indoor agriculture, not waiting a few extra weeks for the right equipment can be a catastrophic business mistake. As mentioned earlier, choosing the wrong equipment will result in higher operational expense (lower energy efficiency), and diminished crop value as a result of unstable control of VPD. 

Designing an efficient and fully optimized indoor grow facility requires substantial planning, research, and forethought. All too often, while ample attention is paid upfront to lighting and racking decisions, HVACD system decisions are left to the last minute, so they do not receive the necessary attention. The most successful grow operations design their HVACD and lighting systems at the same time. It makes sense to evaluate lighting and HVACD together because the two systems work hand in hand. In an indoor grow, facility lights provide the energy for photosynthesis and the HVACD system maintains the environment such that conditions are optimized for efficient plant transpiration (VPD targets). If the plants cannot transpire efficiently because the ambient air is too humid (VPD too low) they will not be able to take full advantage of the available light energy for productive growth.

By selecting the HVACD system early in the process along with the lighting, cultivation groups can ensure that there is enough time to secure the right, purpose-built, integrated HVACD system to deliver reliable, efficient, and tight environmental control. The right HVACD systems will give cultivators a leg up on the competition both in terms of revenue generating potential and energy related cost savings. These systems provide:

• The control to hit and tightly hold to VPD targets which can dramatically increase crop yield and overall flower quality.

• Significantly lower amounts energy use to control the indoor environment due to their full optimization for efficient temperature and moisture control in an indoor cultivation application.

• Specific design for the rigors of indoor agriculture and that last far longer than standard HVAC systems

Desert Aire are industry experts when it comes to indoor grow room heating, ventilation, air conditioning and dehumidification. Learn more at our Why Desert Aire? page, or view more info on GreenAire, GrowAire and DriCure, Desert Aire’s state-of-the-art dehumidification systems for grow rooms and indoor farming. We provide full HVACD Systems for indoor grow rooms, and can help select the perfect fit for you. 

For more information, help determining the right dehumidification system for your indoor grow room, or to discuss a grow room application HVACD system selection, Contact us today. A grow room dehumidification expert will get back to you shortly.

Learn more in our blogs (www.desert-aire.com/blog):

Growers in Search of Efficiency

An Insider's View of 8 Indoor Grow Room Design Factors

GrowAire VPDsync Masters 4 Elements of CEA Control

The Ultimate Indoor Grow Room Climate Control Showdown

and application notes:

Application Note #25: Grow Room Load Determination

Application Note #26: Grow Room Environmental Control

Application Note #27: HVAC Systems and Grow Room Energy Usage

Application Note #28: Vapor Pressure Deficit and HVAC System Design

Application Note #30: Structure Design in Grow Room Applications

Application Note #33: Impact of Design Conditions on Grow Facility Equipment Performance