Legal marijuana within the state of Colorado continues to be a booming industry with many unique design considerations for the HVAC professional. As the market continues to become ever more competitive, cultivators are realizing the importance of having properly designed grow rooms and the positive impacts it can have on the quality and quantity of their final product.
Since large amounts of money are invested in each harvest, cultivators must be cognizant of every minute detail within their grow rooms, especially air quality. That means thousands of dollars, per harvest, can be lost due to improper environmental control in the grow room.
With so much on the line, it is of the utmost importance for the HVAC designer to work closely with the cultivator to ensure long-term success of the grow room.
When marijuana was first legalized within the state of Colorado, most cultivators used ducted lighting systems, with each hood having its own respective piece of glass. These systems allowed for a large portion of the sensible heat load to be removed through convection and then exhausted out of the building. Often these systems were capable of having the ballast mounted external to the grow room, further negating the sensible load within the room. Today, the most common lighting system we see in use is non-ducted, with electronic ballast mounted directly to the hood. The use of electronic ballast over the “old-school” magnetic ballast dramatically reduces the heating load caused by the ballast itself, but does not totally negate it.
The two most common light bulb sizes we see in use today are 1000W and 600W bulbs. To calculate the sensible heat load caused by these light bulbs is relatively straightforward. Essentially each light bulb is counted, and assumed to contribute 100% of its heat as a sensible load. In addition to accounting for each light bulb, 200W per hood is also used to account for the hood’s integral ballast. This means for each hood in the room, 1200W is assumed to be contributed to the sensible heating load.
The next step is to determine the latent load within the grow room. Determining the latent load is a much more involved task as there are many factors (temperature, plant strain, airflow, etc.) which can affect plant transpiration. Working with the cultivator to determine watering times, and quantities can be a useful first step in determining the latent load. Additionally, direct data collection (through the use of temperature and humidity probes) informs our analysis by providing the exact humidity levels that occur within the room.
Using psychometric analysis, it is possible to determine the properties of the air within the room; knowing the air properties, it is possible to determine the latent load within the room. The following table is what we saw from our own data collection:
It has been our recommendation that it is most beneficial to properly size the HVAC equipment within the space so that additional dehumidification (through the use of a dehumidifier) is not needed. By using only one properly sized piece of equipment, the total equipment footprint within the room can be minimized. In addition, it is much more straightforward to control a single piece of equipment, and can eliminate potential conflicts between the HVAC system and the dehumidifier. Finally, the use of a dehumidifier will increase the sensible load within the room—possibly to a point the HVAC system cannot handle—and may adversely impacts the grow room environment. In our industry, the initial rule of thumb for cooling has been to provide 1 ton of cooling per 3 hoods. Over time, this number has been found to be inadequate for optimal grow room control: As each grower has their own unique needs, the latent load must be considered in greater detail to maintain optimal conditions.
An additional consideration of the HVAC designer is the use of CO2 within the grow room. It is common practice for cultivators to enrich the indoor environment with CO2 up to concentrations of 1500ppm. To meet code requirements, and save on energy costs, economizer cooling presents challenges when an incorrect HVAC system is chosen. Choosing the correct system allows for code compliance and saves operating energy consumption. Ventilation strategies for the room must be carefully considered as over-ventilation can cause significant CO2 loss.
The increased requirements of Colorado’s Marijuana Enforcement Division now require Odor Control Plans for all marijuana facilities within Denver’s jurisdiction. While an Odor Control Plan is relatively easy to fill out, the implications on fan sizing must be noted by the HVAC designer. To mitigate odor originating from grow facilities, the industry standard is to use charcoal filters. Those charcoal filters cause a substantial static pressure drop (determined and provided by the filter manufacturer) which must be accounted for when selecting any exhaust fan for a marijuana cultivation facility.
The final step prior to marijuana sales is the curing and drying process. As the industry has become more competitive, more and more growers are seeking total control of their drying rooms. Although each master grower will have their own individual humidity preferences for their drying environment, the ability to control a range of humidity set points from 30%-70% is standard. In addition, it is important to maintain the temperature within the drying room with set points ranging anywhere from 65⁰F to 75⁰F.
As the marijuana industry continues to mature and become more competitive there is ample opportunity for the HVAC designer to aid in complete environmental control. A grow room environment offers many unique challenges, allowing the creative HVAC designer to provide optimum solutions for the cultivator. Through proper design of a grow room environment, cultivators can achieve the highest possible yield for their space, while also increasing product quality and reducing operating costs.