Properly Sizing Your Grow Room HVAC

Editor’s Note: This article is an extract taken from the Getting Grow Rooms Rightbook, published by AGronomic IQ. The book can be downloaded in its entirety at AGronomicIQ.com.

Ensuring that HVAC equipment is properly sized for its grow room application is critical for growers, as it will help define the success and profitability of your operation.

Units that are too big cost more to buy and use far more energy than necessary and could possibly result in an overcooled or overdried space. Similarly, equipment that is too small will be unable to keep up with demand and result in space conditions such as overheating or excess humidity, and these issues could potentially reduce crop quality or put plants in danger.

Given the many variables at play in grow rooms, significantly more than in standard environments, and considering most engineers are unfamiliar with this specific application, sizing calculations for grow room HVAC has been a very challenging task. This is largely due to misinformation and a lack of knowledge among those involved in the facility design industry. Even the most technically qualified engineering firms have had to guess at a considerable amount of the science and mechanics behind creating optimal grow environments due to a lack of HVAC design standards for engineers, and because the cannabis industry is so secretive about what works and what does not. In this industry, engineering jobs are extremely complex and involve dozens more variables than designing traditional HVAC systems for commercial buildings.

Getting grow room specifications right requires careful modeling of grow cycles, transpiration rates, temperature and humidity specifications, lighting loads, and irrigation loads, just to name a few. It also requires careful modeling of external factors, including annual temperature and humidity ranges, type of building structure, solar impacts of mixed-light sources, etc. Some of these factors may vary by type of plant and strain, which only further complicates the matter.

The Unitary Advantage

Unitary solutions can provide all the systems you need in one. Air conditioning with dehumidification priority offers superior functionality and advantages for growers compared to simple air conditioners that can’t typically meet the moisture-removal demands of the growing environment. Separate cooling and dehumidification systems, regardless of size, may be difficult to tie together and counteract each other and can be very inefficient, while also delivering inferior results and increased costs for the grower.

One advantage of unitary equipment is that you can use a different unit for each grow room, which may be used for different strains or stages and thus likely have varying needs. This option provides great scalability and flexibility for the grower, while making it easier to properly size equipment to the needs of each individual room.

In order to properly model conditions, you need to have a clear idea of what each room needs: Is it a mother room? Or flowering? What kind of watering system will be in use? How does transpiration rate change throughout the day and throughout each grow stage?

Reducing energy usage as much as possible by investing in smart technologies will benefit you greatly over time. Because unitary systems roll everything into one, they can recycle heat created by their processes; for example, they can divert energy from hot compressor gas into preheating the cold air that comes off the dehumidifier coil before it goes back into the space.

Although selecting the equipment with a lower capital cost is tempting, this is not usually the best value. You must also consider installation costs when looking at the overall system. In addition, consider the money saved later in ongoing operating and maintenance costs. Often, chosen equipment gets pared down and its energy-saving features eliminated to reduce upfront costs. This “value engineering” should be approached cautiously and avoided if it increases the operating cost of the equipment.

Careful Cost-Benefit Modeling

Central chilled water plants are a tried-and-true solution for projects requiring large refrigeration capacity. They are found in college campuses, hospitals, office buildings, and other large facilities. But while central chillers are a good default for most large-scale applications, they may fall short for the cannabis industry. Central chillers need very careful consideration before being used for this application, because they probably are not the best solution for a grow facility.

If the grower knows the exact size of facility needed, and the required conditions match the capability of a chilled water system, then it could be a viable HVAC option. However, getting it right can be expensive.

Chillers are usually simple systems in large facilities that do not demand much dehumidification. For example, data centers are great candidates for chillers because they only need standard air conditioning with no need for reheating or auxiliary heat, so a simple two-pipe system can be used.

The challenge with a grow space is the humidity. To properly control the space conditions and dehumidify effectively, you need to cool the air down to much colder temperatures in order to more effectively condense the humidity out of the air. That means the chilled water temperature needs to be much colder than is traditional, which turns out to be very inefficient when using the typical chilled water air handlers and chillers mentioned above. Furthermore, in lights-out mode, you need to be able to not just dry the air (cool it off enough to condense moisture out), but you then must reheat it, so as to not overcool the grow room. Energy recovery can be used to provide the reheat function for free. If an HVAC system doesn’t do that, then operating costs will be higher.

As a result of the need to reheat air, a chiller must include a boiler, along with another set of pipes and a more complicated and expensive installation. The inexpensive two-pipe system now becomes a much more expensive and complicated four-pipe system. You also need to run your chiller well below its most efficient operating point, then add heat from an auxiliary source (such as a boiler), and then incorporate sophisticated systems to control all of it. There may also be a desire to add redundancy to your chiller, your pumps, and your boiler so that you have no single point of failure for your entire complex. Otherwise, the central plant is a major liability.

Some Factors to Consider

As noted, there are numerous influencers of proper HVAC equipment sizing. Because most engineers do not have the knowledge to model them and select the right size, they will rely on equipment manufacturers to provide accurate sizing. Manufacturers know their equipment and would be best placed to advise on its application.

Each manufacturer may have a customized form to fill out with key variables. Here are some examples:

• Room dimensions
• Purpose (mother, clone, etc.)
• Building material
• Temperature and RH at beginning and end of grow
• Lighting type and wattage per square foot of active grow
• Active grow area (as a percentage of room volume)
• Number of plants or plant density
• Watering rate per plant per day
• Type of watering system

For your own benefit, you should know all of these factors before beginning your HVAC equipment buying process. You should be able to provide this information, openly and honestly, to your manufacturer so the best solution for your needs is met.

Lighting intensity expressed in wattage per square foot is one key metric that growers often have not decided when they look into buying their HVAC system. Because the lighting wattage is the biggest source of energy into the space, it’s actually going to have a large impact on the size of the HVAC system; therefore, this is a detail that needs to be worked out before you approach an HVAC manufacturer. More details on lighting are found in Chapter 7.

Unusual for this application is the fact that the airflow volume is dictated by equipment capacity and not room volume. Clearly, HVAC manufacturers cannot determine the appropriate equipment capacity based only on floor space; how would they be able to account for heat load from the lights or humidity produced by plant transpiration? That’s why it’s so important for you to be open with your vendor and have a frank discussion about how you are going to be running the building.

Prudent decisions about every element of the grow room are essential to maximizing profit and product quality. One example is the lighting system. Utilizing LED lights is an effective way to lower your HVAC costs because they create less heat. You would be able to save 30% of electricity just running the lights, plus achieve a 30% reduction in sensible heat load on the HVAC, which by extension allows for smaller equipment that costs less to purchase and operate. The compounding savings may make it worthwhile to go for LED, despite the higher upfront cost.

Conclusion

Work closely with your HVAC design engineers to discuss and incorporate all factors that will affect temperature and humidity loads into your ideal specifications. Be certain that they work with the equipment manufacturer to properly model the dozens of load factors and variations that your plants and your grow cycles will require.

Before you connect with your manufacturer, it’s important to have a clear idea on how you are going to run the facility and have solid knowledge about key factors that influence HVAC sizing. When you do, be sure to provide all the information and specifications needed in order for your manufacturer to provide you with the best solution.

Seeking energy-efficient products is a good idea for positive long-term business outcomes. Especially since growing cannabis consumes so much energy, investing in some smart technologies can save you considerable costs over time.