Growers in Search of HVACD Efficiency

May 2023


High efficiency air conditioning systems don’t turn into actual energy savings for growers because of moisture removal limitations.

As law makers and energy companies struggle to understand the loads associated with indoor cannabis cultivation, they often make equipment recommendations with good intentions that result in higher energy use. For example, there are several jurisdictions that are recommending high efficiency Variable Refrigerant Flow (VRF) air conditioning (AC) systems for indoor cultivation, which is simply not the right tool for the application. Why? These systems are not designed for moisture removal which is a huge part of the overall heating, ventilation, air conditioning, and dehumidification (HVACD) load in indoor cultivation facilities.

Cultivation HVACD System Comparison Study coverRecently Anvil Agronomics, Zartarian Engineering and Anderson Porter Design, three well respected groups within the indoor cultivation space, commissioned a study preforming a head-to-head comparison between VRF systems with supplemental standalone dehumidification and integrated HVACD systems designed specifically for indoor cultivation (Cultivation HVACD System Comparison Study)

(In addition, read the Cannabis Science and Technology article "Integrated HVAC Systems for Cannabis Cultivation Have the Lowest Life Cycle Cost" by Gretchen Schimelpfenig, James E. Megerson, Brian D. Anderson, Michael Zartarian about the analysis of the study.)

This Desert Aire blog discusses the findings of the study and the resulting implications for cultivation groups, facilities owners and growers looking to select HVACD equipment to efficiently control the temperature and humidity levels within their facilities to maximize crop quality, yields and profits.

Indoor Cannabis Water Transpiration & VPD 

Indoor cannabis plants transpire nearly all the water they take in through the photosynthesis process. In order for the plants to continue to grow this moisture must be removed from the space to enable the plants to continually transpire. The ability for the ambient air to support transpiration is regulated by the vapor pressure deficit (VPD) between the leaf and the air directly around the leaf. Basically the air must be drier (lower vapor pressure) than the plant for transpiration to occur. As the air becomes drier, the VPD increases and plants will be able to transpire more.

When plants give off moisture they need to replace it. The more moisture they give off the more water and nutrients they will be able to take up. When the optimal feed rate is combined with the light energy provided by the high intensity lighting system it turbocharges the photosynthesis process and increases yields and profits. It is typical for a grow room to require between 150-200 lb/hr. of moisture removal per 1,000 sq. ft to operate at maximum effectiveness.

Learn more about vapor pressure deficit and HVAC system design in Desert Aire’s Application Note #28 Vapor Pressure Deficit and HVAC System Design.

Quick Review of High Efficiency VRF AC Systems

High efficiency AC systems achieve their high efficiency ratings by focusing on cooling air without removing moisture. To do this they are designed to cool air as much as possible without achieving dew point. As soon as air temp reaches dew point, moisture must be removed in order to cool the air further, which takes significantly more energy. High efficiency AC systems are not set up with an efficient coil design for moisture removal so when they are pushed to do so they are out of their efficiency range and require substantial energy to remove small amounts of moisture. For this reason, independent standalone dehumidifiers must be added to address the moisture load.

Supplemental Dehumidification

All facilities that elect to go with a High Efficiency AC system will need substantial supplemental dehumidification to assist the AC system in moisture removal in order to maintain somewhat stable conditions. Supplemental dehumidifiers convert moisture into heat energy and reject it into the grow room in the form of hot dry air. This heat must then be removed by the AC systems to maintain a stable ambient temperature. In most cases, it is very difficult to connect the dehumidifiers and AC system via one control system due to limitations on the dehumidifier side. Standalone dehumidifiers were originally designed for other applications where the level of environmental control is not as crucial and therefore they are not equipped with the ability to tie into a building management system (BMS) or communicate with other pieces of equipment HVAC equipment in a sophisticated manner.

Quick Review of Integrated HVACD Systems

Well-designed, CEA purpose built, integrated HVACD systems have the ability to control both temperature and humidity together by varying their sensible and latent removal ratios to precisely match the load. In addition, their coil designs are fully optimized for the efficient moisture removal required to conserve energy in indoor cultivation facilities.  This type of product will include a modulating, full sized, hot gas reheat coil that will use waste heat from the dehumidification process to prevent the space from being over cooled.  A unit that uses new energy for reheating the air (for example an electric resistance heating coil) after dehumidification is not ideal as this type of unit is using new energy for the reheat process. 

VRF & Integrated HVACD Systems Compared

The Cultivation HVACD System Comparison Study compared the energy efficiency and level of control, temperature and RH%, between high efficiency VRF systems (both ductless and ducted) and an Integrated HVACD system. This blog discusses the performance between the ducted VRF system and the Integrated system.

  • System 1: Four 8-ton VRF Split Fan coils with matching remote condensers for the sensible load and seven 710-pint dehumidifiers to control the latent load.
  • System 2: Two 20-ton package Integrated HVACD units with full reheat coils and the ability to vary the sensible heat ratio.

Energy Consumption

The study found that the VRF systems consumed 15% more energy than the integrated HVAC system.

VRF Split System and Integrated HVACD System Type Energy Usage kWh per year
The wasted energy is the direct result of the inherent limitations of the two separate systems to work in unison. Instead of efficiently removing moisture and heat simultaneously with one integrated system, two separate systems are toggling on and off trying to get the right mix of cooling and moisture removal. In this scenario the equipment will run longer than necessary due to the heat added by the standalone dehumidifiers as they work to remove the moisture. Once the moisture is removed, the AC units then need to turn on to remove the additional heat. Often times the AC will over shoot the temp set point and overcool the space which drives up the RH% causing the dehumidifier to turn on which adds more heat restarting the process.

Level of Control 

The VRF system with standalone dehumidifiers were unable to manage temp and RH set points within a tight bAmbient Temperature and Relative Humidity Over Time VRF Systemand throughout the grow cycle. Instead, there are constant swings in temp and RH% of roughly 10% occurring multiple times per hour. The charts below depict these constant fluctuations graphically showing that the VPD is moving constantly which stresses the plants and harms yields and overall crop quality. As VPD fluctuates the plants’ stomata must open and close in order to regulate transpiration. These constant swings signal trouble to the plant, and can cause the plant to shut down or slowdown transpiration which dramatically affects yields.Grow Room with Standalone Dehumidifier Units

Another contributing factor to the instability of control has to do with the individual micro climates each piece of non-integrated equipment produces. For example, each of the seven installed dehumidifiers is exhausting hot dry air in the immediate vicinity of unit placement (20 degrees above ambient temperature with low RH%). This air must then mix with the ambient air which takes space before a stable condition is achieved. The AC system produces a similar problem in reverse, by blowing cool, high RH% air which again needs space to dissipate.  Thus plants that are closest to the standalone dehumidifiers will see a warmer and dryer ambient condition while plants located close to the AC system diffusers will see a cooler temperature.   Ambient Temperature and Relative Humidity Over Time Integrated HVACD System

The Integrated HVACD system did a much better job maintaining a stable environment in the grow room with only slight temperature and RH variations throughout the grow cycle.   Side Note: Desert Aire provides remote tuning of our operating controller in order to provide more precise control than the integrated system depicted in this study.  

Not Just Wasting Energy, Wasting Full Crop Potential 

Beyond the wasted energy dollars there is another negative impact to instable environmental control that is perhaps more consequential to the bottom line, crop quality and total yield. As previously discussed, the wide variation in VPD stresses the plants as their stomata are constantly opening and closing to regulate transpiration rates. In addition, the ever persistent microclimates generated by the nonintegrated equipment make it hard to achieve room level consistency and produces a breeding ground for issues.  In a world where the price delta between top quality flower and average quality flower is the difference between a successful operation and going bust, more attention needs to be placed on making smart HVACD system choices when setting up new or improving existing indoor grow facilities.

What About the Upfront Equipment Costs?

There is a market perception that purpose built Integrated HVACD systems cost more than off the shelf AC systems with standalone dehumidifiers. The comparison study did not find this to be the case. In fact, the integrated HVACD system was the lowest total cost.

Upfront HVACD equipment costs

The market perception of AC systems with supplemental dehumidification is skewed because most of these systems are undersized and do not have the required capacity to reliably control the internal loads. Cultivators will not learn this until it is too late and all the equipment has been installed, only to need to purchase and install substantially more equipment to stabilize the environment. For example, installing multiple standalone dehumidifiers per room is not a cheap or easy task. These units often need to be hung from the ceiling which typically requires two people and a lift at minimum. In addition, each unit requires a dedicated 30 amp 240V connection along with a trap and drain line for the condensate. Finally, the additional heat created by the dehumidifiers needs to be considered. Often when dehumidifiers are added, the AC system will need to be upsized to handle the additional sensible load.

System Reliability

Indoor cultivation facilities have very large HVAC loads that require near constant operation of equipment to ensure a stable environment. - AC Systems and standalone dehumidifiers are not designed for this rigorous duty cycle that includes year round operation. For this reason, it is typical in the industry to see frequent failures and dramatically shorter total life associated with such HVAC equipment. Purpose built integrated HVACD equipment that is designed for the demands of indoor cultivation will last far longer and deliver significantly more reliable service.

So Why the misleading information?

Frankly, many cultivators, owners and design engineers are not familiar enough with the available integrated HVACD systems to make an informed choice. Also they may lack the knowledge to fully understand the Latent and Sensible loads created by this application.  A lack of knowledge on the Evapotranspiration rate of a grow room may prevent the decision maker to fully evaluate the proper equipment requirements to achieve optimal growth with repeatable results.  In addition, many manufacturers of partial solutions (comfort cooling and standalone dehumidifiers) are very aggressive at pushing their equipment even though it costs more to operate and does not provide nearly as tight control.  

The indoor cannabis industry is relatively new and best practices are still being established. Many in the space do not fully understand the loads at play and simply throw more equipment at problems rather than taking a step back to analyze the true loads. There are also a lot of groups that had success with their historic equipment mix when cannabis prices were sky high and wasted energy costs were not nearly as painful. As the industry matures and competition increases there will be less margin for error and efficiencies will become increasingly important. This will lead to the increased adoption of smart integrated HVACD systems specifically designed for the challenges of indoor cultivation.

Are all Integrated HVACD Systems Alike? 

The answer to this question is “no”. In fact, there are wide differences in available systems. For, example, most integrated systems do not have the ability to fully recapture reheat for efficient dehumidification. This leaves operators either adding supplemental dehumidification or running auxiliary gas or electric heaters to provide the necessary reheat.  Potential buyers should seek systems like those offered by Desert Aire, a major industry player with thousands of successful installations and happy customers. Buyers should also ask to tour the manufacturing facilities of their HVACD equipment manufacturer to see the designing, manufacturing, and testing capabilities firsthand.  Equipment providers that utilize contract manufacturing likely do not have control over functions such as testing, continuous quality improvement, component sourcing, or even order prioritization. 

Discover the advantages of Desert Aire's cannabis indoor grow room and farming solutions by visiting our Cannabis Indoor Grow Rooms and Farming page.

Manufacturer Sizing Support

Grow room sizing and load determination is the most important aspect of getting environmental control right. Many manufacturers do not offer much sizing support; this is always a red flag. Before any equipment is selected Desert Aire always performs an in-depth proprietary sizing analysis to fully understand the sensible and latent loads at all points of the grow cycle. Once the loads are understood, the indoor grow HVACD experts at Desert Aire can present customers with several different equipment options based on their goals, budget, geographic location, facility type, and desired level of energy efficiency. Finally, all Desert Aire equipment is run tested in the factory before it ships and Desert Aire engineers will be onsite for the initial startup to ensure everything goes smoothly.

Learn more about Desert Aire's innovative GreenAire™ System by exploring our GreenAire™ System page. GreenAire chilled water HVACD system

Indoor cannabis cultivation is a very specialized type of agriculture that requires purpose built equipment to efficiently and effectively manage the indoor climate. There is a lot of confusing and misleading information in the market that can make selecting the ideal equipment challenging. A true HVACD equipment partner can make all the difference. If you are considering HVACD equipment for your next indoor grow project, please reach out to Desert Aire. We are excited to leverage our equipment and application knowledge to help you succeed. We have been assisting customers since 1978 with moisture removal applications that require temperature control. 


This blog was written by Chip Seidel, CEA Brand Manager, Desert Aire


Download a copy of the Cultivation System Comparison Study by Anderson Porter Design

For more information, see the following Desert Aire 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 ApplicationsA Guide to a Controlled Environment Agriculture Chilled Water HVACD System Design

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

Application Note #35: Guide to Optimal Cannabis Drying

A Guide to a Controlled Environment Agriculture Chilled Water HVACD System Design