More often than not, growers tell me they want to fully recirculate the air in their production room or greenhouse. Almost every time, they give me the same three reasons: to prevent pests and pathogens from getting in; to prevent odors from getting out; and to keep in carbon dioxide.
Although I don’t totally agree with the first explanation, I will admit that the second two reasons have their merit. In states where cannabis cultivation is legal, many cities and jurisdictions have created rules or ordinances intended to curb the smell emitted from grow facilities. And in an effort to boost the production rate of their crop, many growers are choosing to enrich the growing environment with carbon dioxide.
But are there situations when bringing in outside air and exhausting room air is beneficial?
There are at least five reasons I can think of where using outside air is important, whether you are growing in a warehouse, a greenhouse or a container.
Meet Building Code Standards
If you’re growing in a greenhouse, you probably don’t need to worry about this requirement too much. But if you are growing in a warehouse, your operation is most likely going to be designated as an Occupancy F building (factory).
Unless your local jurisdiction gives you an exemption, as an Occupancy F building, you will be required to meet the minimum ventilation requirements for your governing building code. In most states, the International Mechanical Code (IMC) governs and is based on the ASHRAE 62.1 standard for ventilation of commercial buildings. Under this designation, you will be required to supply at least 0.06 cubic feet per minute per square foot (cfm/sf) of outside air whenever the production rooms are occupied by humans (the front-of-house office areas will have an even higher ventilation rate requirement). That means, if you are growing in 5,000 square feet of production space, you will be legally required to provide at least 300 cubic feet per minute of outside air. (Note: The California Mechanical Code requires 0.15 cfm/sf.)
Some jurisdictions require growers to provide a means for rapidly purging the production room air, in the event that chemical levels exceed a maximum threshold. These chemicals often include carbon monoxide produced by fuel-based CO2 generators, as well as other chemical gases from fertilizers and pesticides.
In Oakland, exhaust fans are required to be interlocked with a carbon monoxide meter, which will automatically engage the exhaust fans if maximum CO levels are reached. In this event, outside air will be needed to replenish the air being eliminated.
Growers enriching with CO2 from liquid tanks or canisters may not have to worry about those products of combustion, but may be interested to know that a recent Harvard study showed a 50% decline in worker productivity at CO2 levels as low as 700 parts per million. Therefore, some extra fresh air strategically timed during harvest, pruning or other labor-intensive activities may boost your overall bottom line.
Replenish Carbon Dioxide
I recently had a client who was struggling to grow lettuce in a warehouse using state-of-the-art lighting, carefully mixed fertilizers, and the prescribed temperature and humidity ranges produced by a recirculating air conditioning system.
After eliminating all the obvious reasons, I asked, “Are you enriching with carbon dioxide?” Their response: “No. Is that important?”
For those of you who are not enriching with carbon dioxide, it is vitally important to replenish the CO2 absorbed by plants during photosynthesis. If you do not, they literally will not have the fuel needed to produce their own sugars. First they will stop growing and then they will stop living.
It doesn’t take much outside air to replenish CO2. The minimum building code requirements for ventilation will certainly be enough.
Remove Volatile Organic Compounds
Most growers know that ethylene is bad for their crop. It causes premature flower development and senescence (biological aging), basically accelerating the age of the crop (ethylene is what causes fruit, like bananas, to ripen). But there are other volatile organic compounds (VOCs) that can be bad for plants too.
Formaldehyde, in particular, has been shown to cause abnormal growth and development in plants. (It’s bad for humans too.) Formaldehyde can be found in everything from furniture glues to flooring bonds and adhesives, and is most concentrated in new and recently renovated buildings. Flushing the building with lots of outside air before moving in can remove a lot of the formaldehyde. And continuously bringing in some outside air and exhausting a portion of the room air will help prevent VOC levels from climbing too high.
Depending on where your production facility is, there are likely many times during the year when the outside conditions are perfect for growing cannabis. Don’t you wish sometimes you could just bring that air in rather than using the air conditioning system? Well, you can. It’s called an airside economizer.
Many commercial HVAC systems come with the option for an economizer, which uses a damper to control the amount of outside air depending on the inside and outside air conditions. Depending on the project location, growers can expect HVAC energy savings in the range of 10% to 35% when an airside economizer is available.
Nadia Sabeh is the president and founder of Dr. Greenhouse, an engineering consulting firm focused on designing HVAC systems for indoor farms. She is a licensed mechanical engineer in California, received her Ph.D. in agricultural and biosystems engineering from the University of Arizona’s Controlled Environment Agriculture Center, and has nearly 20 years of experience helping clients maximize crop productivity by translating the plants’ needs into the design and operation of the facility. She can be reached at firstname.lastname@example.org.
This story was originally published in the November 2017 issue of Marijuana Venture, on sale now.[contextly_auto_sidebar]