AIR TEMPERATURE, HUMIDITY & QUALITIES

With contributions from Joey Ereñeta, Autumn Karcey, and Brandy Keen

Cannabis and the air that surrounds it have an intimate relationship. The stem and leaves are bathed in it and are affected by its temperature and humidity. Air temperature plays a major role in determining leaf temperature, and relative humidity affects how easily plants can absorb CO₂, which is required for photosynthesis. The effects of climatic conditions on the plant are found in its relationship to vapor pressure.

Ideal temperature is tied to light and humidity conditions. The combination of temperature and humidity is used to calculate the vapor pressure deficit (VPD). As more light is available, the ideal temperature for normal plant growth increases.

Cannabis grows well in moderate temperatures, between 70 and 85°F (21-29°C). Both high and low temperatures slow the rate of metabolism and growth. Plants grow best when the temperature at the leaf surface during the lighted period is kept between 72 and 79°F (22-26°C). When CO₂ augmentation is used, plants function better when the temperature is a few degrees warmer, between 79 and 85°F (26-29°C). Individual cannabis varieties differ in their temperature preferences by a few degrees, so some experimentation is required to find the ideal temperature and humidity for the specific cultivar.

Plants growing under moderate intensity lighting should be kept on the low side of the recommended temperature range. Plants growing under higher-intensity lighting should be kept on the warmer end of the scale. Strong light and low temperatures slow growth and decrease stem elongation. Conversely, when plants are given high temperatures and only moderate light, the stems elongate.

During dark periods, the temperature can be kept as much as 10°F (5°C) cooler than the lit period without any negative effects. Wider temperature differences cause slower growth, stem elongation, and delayed flower ripening. Plants that experience a large diurnal shift, the differential between day and night temperatures, suffer from stretching and slowed growth rates.

Plants that are kept at a constant temperature day and night grow stouter, sturdier stems and have denser bud growth. It lowers the chances of botrytis and powdery mildew infections, which prefer cooler temperatures. Maintaining temperature also eliminates higher humidity created by falling temperature.

At temperatures below 60°F (15°C), photosynthesis and plant metabolism slow, stunting growth until conditions improve. As soon as the temperature rises, the plant resumes full functioning. When the temperature falls below 40°F (4°C), cannabis plants experience tissue damage and require about 24 hours of warmer conditions to resume growth. However, young plants are tolerant of low temperatures; when outdoors, seedlings may pierce snow cover without ill effect. Low temperatures during ripening, even just overnight, delay or prevent bud maturation. Some equatorial varieties stop growth after a few nights when the temperature dips below 40°F (4°C). Wind exacerbates these conditions.

Ideal Temperature Ranges by Growth Cycle

Germination: 70-78°F (21-25°C)

Vegetative: 68-85°F (20-30°C)

Flowering: 68-85°F (20-30°C)

Cloning: 75-85°F (24-30°C)

Plants photosynthesize more rapidly in high light and high CO₂ conditions when the temperature is around 85°F (30°C)

When using vapor pressure deficit (VPD) to guide the climate, “finite control” is essential. VPD parameters are incredibly tight. The difference of one degree Fahrenheit (half a degree Celsius) at the leaf, two degrees Fahrenheit (~one degree Celsius) of the air, or 5% relative humidity drastically changes a targeted VPD range.

Most high-end instruments have an accuracy range of +/- one degree Fahrenheit (half a degree Celsius) and 1.5% relative humidity. At temperate conditions, this can be challenging when calculating multiple differentials around a target VPD. A good solution is to use a quality hygrometer such as a digital sling psychrometer to measure relative humidity. Walk around the garden, focusing first on room and canopy consistency, then compare with the other sensor’s readings. Stationary hygrometers can be used; however, they must be calibrated to provide accurate results.

One way to control humidity is to use a fogger and dehumidifier both controlled by the same sensor to keep temperature and humidity within the dead band. If they are on separate sensors, they must be coordinated so that they don’t fight each other.

Air Circulation

The steady movement of air in the garden provides many essential benefits for growing plants. Airflow homogenizes the temperature and humidity in the space, ensuring that all plants experience the same conditions. As heat builds up underneath cultivation lights, and moisture accumulates in the plant canopy, good airflow circulates cool, dry air from the air conditioner and dehumidifier. This air movement strengthens plant stems and vascular tissues. Plants that sway from optimal airflow respond by creating thicker stems and increased vascular pathways for the uptake and translocation of water and other valuable resources. This makes the plant more stress tolerant. Later, thicker-stemmed plants will be able to support the increased flower weight accumulated during the reproductive stage.

Airflow in the room eliminates “dead” zones where insects and fungus are more likely to create an infection. Airflow across the plant canopy also boosts vapor pressure deficit, providing additional protection from mold and fungus and driving transpiration.

Carbon dioxide is heavier than air and concentrates closer to the floor of the grow room. However, airflow keeps it moving through the plant canopy. Wall and ceiling-mounted fans can be used to move cooler, CO₂-rich air out from underneath tables and racks and back into the parts of the room where it can be used by the plants. Horizontally mounted fans circulate air through the canopy. As the CO₂ is used up by leaves and flowers, this air is blown away, replacing it with CO₂-enriched air.

Biosecurity

Climate control conditions affect the overall biosecurity of the garden in a number of ways:

• Humidity fluctuations invite fungal growth such as bud mold and powdery mildew.
• Plants expend energy responding to or recovering from poor climate conditions. This energy is diverted from growth-reducing yield and quality.
• Under poor environmental conditions, plants’ natural defenses are compromised, leaving them more vulnerable to infestation or infection from fungus or pathogens.
• Unsanitary ventilation systems can be a source of pest infiltration and infection colonies.
• Filtration and air sterilization systems are often incorporated into the HVAC system.

Besides temperature and CO₂ content, other things to consider about the air in the garden include dust content, electrical charge, and humidity.

Dust

The dust content of the air affects the efficiency of the plant’s ability to photosynthesize. Although floating dust blocks only a small amount of light, dust accumulated on leaves blocks large amounts of light.

“Dust” is actually composed of many different-sized solid and liquid particles that float in the gaseous soup of the atmosphere. The particles include organic fibers, hair, other animal and vegetable particles, bacteria, viruses, smoke, and such odoriferous liquid particles as essential oils and water-soluble condensates. Virtually all dust particles have a positive electrical charge because they are missing an electron, which causes them to float in gasses such as air.

They are captured in HEPA and carbon filters, both of which can be used in all garden spaces.

Negative ion generators charge oxygen molecules with an extra ion, which is unstable and easily jumps from the O₂ to a positively charged molecule floating in the air. The neutralized dust particle precipitates to a surface rather than continuing to float. One way negative ions are created is by crashing water such as fountains or waves at a beach. Negative ion generators produce them in copious amounts and use an extremely small amount of electricity. They are considered healthful and stress reducing for both plants and animals, including humans, and are sometimes used in schools and offices.

Ions can be used in rooms adjacent to rooms where ripening or mature buds are being grown or stored, but not inside them because the ions will interact with the terpenes. Ion generators can be used in spaces with plants in vegetative growth and during the first few weeks of flowering.

Ozone generators create ozone (O₃), which has a unique odor. As a gas, oxygen forms a molecule composed of two O atoms (O₂). Using an electrical charge, three O₂ molecules are combined into two O₃ molecules.

Ozone is unstable: the third atom jumps from the molecule to dust particles, spores, and bacteria. It is not safe to be around, either for plants or for humans, but degrades into O₂ over time. It can be used to clean uninhabited spaces but should not be used around living or cut plants or humans. Ozone generators are often used in air exhaust systems because of their effectiveness in eliminating odors.

If the leaves are dusty, wash them off using a mist or “shower” setting on sprayer nozzles. Be careful when using water sprays around hot lights because the lamps will shatter if even a little water hits the hot glass. Before spraying, make sure the lights are higher than the spray range or shut them off and let them cool down.

Temperature

Cannabis plants are hardy and survive outdoors in a wide range of temperatures. As the temperature rises from the high 70s into the 80s Fahrenheit (20-25°C), plants spend more energy staying cool and maintaining faster cell metabolism. Under high light conditions, photosynthesis increases as the temperature rises, resulting in a net gain in plant growth. Photosynthesis reaches its apex at about 85°F (30°C) and about 1,500 PPFD (Chandra et al. 2009).

As the temperature rises past 85°F (30°C), photosynthesis slows until it stops at between 90-95°F (32-35°C). At this point, plants go into preservation mode; photosynthesis stops as the plants spend energy acquiring water and transpiring it through the stomata in order to keep cool.

Outdoor plants facing long, hot spells should be encouraged to develop deep roots by providing deep penetration irrigation followed by moderate soil drying. Later in the season, when the roots are stressed to supply water, they will be able to draw on adequate supplies with less effort from the deeper level. As soil dries out, water tension increases, and the soil holds the remaining water tighter. Keeping the soil moist makes it easier for the plant to draw it from the medium.

Many or most cultivars withstand extremely hot weather, up to 120°F (49°C), for short periods, as long as they have adequate supplies of water and a large root system. When plants wilt in hot weather, either the medium is drying or the root system is not robust enough to supply the amount of water the plants are transpiring to keep cool. The plants can be cooled using cooling misters or a misting fan. If the temperature cannot be controlled directly, place a 20-30% shade cloth over the plants to reduce stress. Another solution is to spray a nontoxic antitranspirant, which will slow transpiration by blocking the stomatal openings somewhat and slow their release of water.

Excessive heat can be a problem indoors, too, though it is easier to control. Gardens using high wattage lamps generate a lot of heat; an unprotected 1,000-watt lamp emits about 3,412 Btus. During the winter, the heat produced may keep the garden space comfortable, but during the summer, the space may get too hot, particularly if there are several lights heating up the space.

The temperature of the uppermost foliage is the specific area of interest. The space in the aisles or the floor may be cool, but that doesn’t matter to the plants. What is important is the temperature of the canopy under the lights where the plants are producing new growth. Even when the overall temperature of the room is in the optimal range, the areas directly under the bulbs can be very hot.

Different lighting technologies produce different amounts of radiant heat, which means the ambient temperature and the leaf temperature can be dramatically different between different lighting technologies.

Each type of lighting technology produces direct waste heat (the electricity that isn’t converted to light) and radiant heat (the heat that is created when light strikes a surface and is converted from light energy to thermal energy). The result is that the leaf surface temperature beneath a high-intensity discharge (HID) light is higher than the leaf surface temperature beneath an LED, even in the same room with the same PPFD and air temperature.

Because of this, accurate air temperature measurements cannot be made in direct light when temperature readings are taken. Measurements taken in direct light are actually surface temperatures, not air temperatures. For best accuracy, temperature probes controlling the heating, ventilation, and air conditioning (HVAC) system should be out of direct light, and temperatures under direct light should also be taken periodically to understand the differential between the air temperature and the leaf temperature of the plants. An infrared thermometer provides the most accurate reading. Other sensors such as direct light or wet bulb thermometers are not as accurate.

An inexpensive way to get a temperature reading in direct light is to hang the temperature sensor at canopy level, but shaded. A white paper cup is very effective at providing that shade. Put a hole in the bottom of the cup so that the temperature sensor can be pushed through. Pull the wired temperature sensor through the hole until the sensor itself is almost to the rim of the cup. Now turn the cup upside down at the level of the plant canopy. This creates a shaded temperature sensor that can constantly take measurements even in direct light. It is important to use a white or other reflective color cup. A dark-colored cup will absorb more heat and give a false reading.

Although the air temperature affects leaf temperature, taking the measurement of air temperature is an indirect method of determining canopy conditions. A more accurate method, though not widely used, is to measure the leaf temperature at the top of the canopy, where most of the photosynthesis is taking place, using an infrared (surface temperature) thermometer, and regulating the air conditions based on this reading. The reasoning is that the general environmental conditions are not as relevant as the environment that the plant is experiencing. Under this regimen the air temperature is controlled based on leaf temperature measurements.

Relative Humidity (RH)

The ideal humidity for a cultivation space varies based on plant maturity level, temperature selection, and VPD goals. Maintaining proper humidity is a critical consideration, particularly during mid-to-late-stage flowering when the plant is especially vulnerable to mold.

In general, RH should be about 70% in the pre-rooted propagation stage. This is easily achieved using a humidity dome. The vegetative growth stage should be 55-65%. While this is a basic overview and general range, using VPD calculations is more precise. However, excessively low humidity creates an overly rapid transpiration of moisture, resulting in stunted growth and a poor-quality harvest.

Vapor Pressure Deficit (VPD)

The primary benefit of VPD is optimizing transpiration and gas exchange for photosynthesis and increasing nutrient uptake. Two other factors make VPD important to any indoor garden: producing high-quality cannabis without the use of harmful pesticides, and lowering production costs.

Plants require a flow of water in order to bring nutrients up from the roots and maintain temperature through transpiration, as well as for photosynthetic and metabolic processes. The rate that the water can evaporate from the leaves depends on the air’s temperature, humidity, and the difference between how much water the air is holding and how much water it is capable of holding. This is called vapor pressure deficit. For example, at the same temperature, a given amount of water will evaporate more quickly when the air is drier than when it is more humid. If the air is fully saturated and no longer has the capacity for more moisture, the plant cannot transpire.

Temperature plays an important role as well. When the water is warmer, in this case in the plant, it is closer to its point of “phase change” from liquid to vapor. This relationship is represented in the evaporation point and boiling point of an element or compound. The connection is transversely represented in dew point and freezing point.

The leaves’ surface area to air is also a factor in determining the exchange rate. The plant’s evaporation rate is called transpiration; the climate is responsible for its transpiration speed. VPD becomes the plant’s circulatory system in which photosynthesis is the driver. Vapor pressure is the measurement of the plant’s water pressure to transpire.

Instead of measuring and calculating it repeatedly, a chart is much easier to follow, as some values remain relatively stable. For example, a properly transpiring plant’s leaf temperature in a balanced, stable environment will remain stable.

Calculating Vapor Pressure Deficit (VPD)

By Pulse Grow

Air is made up of many gasses. It is about 78% nitrogen, 21% oxygen, and much smaller parts of other gases. Water vapor, the gaseous form of water, is one of those other gases. The amount of water vapor in the air (expressed as pressure) is called “vapor pressure.”

Air can hold only a certain amount of water vapor at a given temperature before it starts condensing back to liquid water (in forms such as dew or rain). The maximum amount of water vapor that air can hold at a certain temperature is called “saturation vapor pressure,” (SVP).

As the air gets hotter, the amount of water that the air can hold (its SVP) increases. As air cools down, the SVP decreases, meaning that the air can’t hold as much water vapor. That is why there is dew all over everything after a cool morning. The air just gets too full of water, and the water condenses out.

Similarly, the current actual amount of water vapor in the air is called the “actual vapor pressure,” or AVP.

Some key points:

AVP / SVP × 100 = RH%

RH is just the proportion of water the air is currently holding versus its maximum capacity. That’s why it’s called “relative” humidity.

The maximum the AVP can be is the current SVP.

That means RH = 100%. If AVP reaches SVP, any additional moisture will precipitate out of the air as liquid water (dew, etc).

The full vapor pressure deficit (VPD) formula is saturation vapor (SVP) pressure minus actual vapor pressure (AVP), or:

VPD = SVP - AVP

This chart is in Fahrenheit and designed for the vegetative growth stage. The band in blue/green is the ideal range. Visit PulseGrow.com to use the VPD calculator or make a custom chart.

To achieve a target vapor pressure deficit (VPD), “finite control” is essential. VPD parameters are incredibly tight. The difference of one degree Fahrenheit (half a degree Celsius) at the leaf, two degrees Fahrenheit (~one degree Celsius) of the air, or 5% relative humidity, drastically changes a targeted VPD range.

The “optimal” VPD has been the subject of much debate, but anecdotal evidence points to somewhere between 1.2 and 1.6 kilopascals (kPa) in flowering plants. As the same VPD can be established at various temperature and humidity selections, it’s important to consider each variable independently and not just VPD as a target.

For instance, a VPD of approximately 1.33 kPa can be achieved at temperature-humidity pairings of 82°F (28°C) and 64% RH, and 70°F (21°C) and 47% RH. Even though each of these selections results in the same VPD, the higher temperature and humidity ranges result in faster growth and require less cooling than the lower temperature and humidity ranges.

It’s also important to note that although VPD is a significant consideration, it’s not the only consideration. Temperature and humidity must also be considered as independent metrics.

While understanding the simple principles of VPD is crucial, it is critical to observe the plants in their environment. Due to the variations between cultivars, the chart will get the grower close, but the plants will ultimately guide the way. Through combining useful data logging and consistent genetics, multiple cycles can be analyzed for beneficial trends and “best seasons” for repeatable optimal conditions in greenhouses and indoor environments.

While the fundamental laws of VPD are universal in principle to outdoor, indoor, and greenhouse cultivation, the control and manipulation humans have over VPD are vastly different between all three methodologies.

Climate Controls

Humidity Control

The moisture in the garden air affects the vapor pressure deficit and the rate at which plants transpire and perform gas exchange. If humidity is too high, plant transpiration is stifled and fungal pathogens can thrive. Humidity that is too low leads to excessive transpiration and stress responses such as wilting and stomatal closure. Optimal humidity and VPD allow the plants to transpire and uptake carbon dioxide at highly efficient rates, which can significantly increase photosynthesis and rates of growth.

The primary source of grow room humidity is plant transpiration, followed by evaporation of moisture from the planting medium and irrigation system. During periods of optimal growth, plants release almost as much moisture as the grower provides to them on a daily basis. To counteract this, dehumidifiers are regularly used to remove excess water from the air.

In other cases, humidification is used to elevate RH. This can occur when growing in a particularly dry climate and is commonly used in propagation rooms for rooting cuttings. Both humidifiers and dehumidifiers are typically equipped with a hygrometer for measuring RH and a hydrostat for setting the desired humidity levels in each zone.

The Growlink Platform virtually connects all the technology in the cultivation space. Growlink captures massive amounts of valuable crop-level data using custom wireless mesh canopy sensors to drive yields, reduce losses, optimize irrigation, prevent disease, and reduce energy consumption. The system comes with the 24/7 support, on-site commissioning, and training.

Sizing Dehumidifiers

Dehumidifiers are rated by the amount of moisture they can remove from the surrounding air on a daily basis. The dehumidification capacity required can be calculated based on the total wattage of lighting and the lighting type used. The power of the lights to drive photosynthesis determines the plants’ rate of transpiration.

The more powerful the lighting, the faster plants transpire. The more transpiration, the greater the dehumidification capacity needed. Lighting type plays a big factor in this calculation because different light technologies have different levels of efficacy. For example, LED lights have a different efficacy factor than high-intensity discharge (HID) lights, in that they produce more photons per watt of electricity consumed. A 600-watt LED produces more light and drives more photosynthesis and transpiration than a 600-watt HID light. Therefore, there are slightly different rules of thumb for sizing dehumidifiers depending on the light technology used.

On average, 0.02 pints (0.095 l) of dehumidification is needed per watt of HID light, or 0.03 pints (0.14 l) of dehumidification per watt of LED light.

To properly calculate the dehumidification capacity required, use the equation above and follow these steps:

1. Add up the total wattage of lighting in the garden.
2. Based on lighting type, multiply the total lighting wattage by 0.02 pints (0.095 l) for HID or by 0.03 pints (0.14 l) for LED.

For example, using 3,000 watts of HID light in the space: 3,000w × 0.02 pints (0.095 l) = 60 pints (28 l)

In this room, a dehumidifier that has a minimum rating of 60 pints (28.4 l) per day is required.

Using 3,000 watts of LED light in the space: 3,000w × 0.03 pints (0.14 l) = 90 pints (42.6 l)

In this room, a dehumidifier that has a minimum rating of 90 pints (42.6 l) per day is required.

Cooling Indoors

There are several ways to manage heat in indoor gardens:

• Don’t create a hot environment. In small gardens using HIDs, air-cooled lights prevent heat from being trapped in the garden space.
• Keep heat-producing ballasts outside the growing area.
• Run lights at night. If the room is lit exclusively by lamps, the day/night cycle can be reversed so heat is generated at night, when it is cooler outside. It’s also a financial savings because electricity is cheaper at night.
• Ventilate the garden area with filtered air. During the winter and in the evening, outdoor air may be cool enough to lower the temperature of the garden. Don’t ventilate if the outside air is too hot or cold.
• Install an air conditioner. Air conditioners can be set up to exchange the heat in the room without venting telltale odors. Portable air conditioners work well. Window models can be installed in a window or an internal wall to vent the heat into a central area.
• Use an evaporative or “swamp” cooler. Warm dry spaces can be cooled using portable air coolers. These appliances cool the air using evaporation. Air is drawn through a wet filter; as the water evaporates, it cools the air. This works best in low-humidity situations.

Even without lights, an enclosed space such as a greenhouse can get hot rapidly when sunlight is illuminating it or when outdoor temperatures rise. Sunlight is converted to heat energy when it hits an opaque surface in the greenhouse. The heat is trapped, so the temperature increases throughout the day. Some greenhouses have roofs that open to let hot air escape and draw cooler air in.

For ventilated greenhouses, swamp coolers are very effective. Water runs through fibrous plastic mats as fans blow air, lowering the greenhouse temperature. Because swamp coolers work by evaporation, they are most effective in hot, dry areas.

Another evaporation technique uses five-micron spray nozzles, or cooling fans, to pulverize water into small particles less than five microns in diameter. The water pieces are so small that they immediately evaporate as they are sprayed into the hot air, lowering the ambient temperature. Spray coolers and water misters are available through nursery and patio supply houses.

Air Conditioning

For proper cooling, air conditioning (AC) is the most common and effective choice. Air conditioners pass air across coils filled with a refrigerant. Heat from the air is transferred to this fluid, and the air is blown back into the garden at a lower temperature. All air conditioners produce condensate—moisture collected by the coils in the air handler—that must drain from the unit. Many ACs have a condensate tube, which can be sent to a drain. Some cultivators capture the condensate from their ACs and dehumidifiers for recycling. (See Sustainability.) If so, this water must be regularly tested and filtered to ensure that it is safe for reuse in the garden.

For a small garden, a portable or window AC unit may be the easiest option to provide sufficient cooling. They are inexpensive to purchase; however, they are less energy efficient and more costly to operate than higher-efficiency commercial units. Because the air handler, coils, and condenser are all combined into one packaged unit, some portable and window air conditioners release unfiltered odor with the warm air they exhaust. ACs that exchange heat but not air are preferable.

Commercial facilities commonly use a split or ducted air conditioning system. Split ACs have separate air handlers and condenser units. The air handler sits inside the grow room, while the compressor sits outside the facility. Although split systems are more expensive to purchase than packaged window and portable ACs, they are more efficient at converting power into cooling capacity. Additionally, split ACs are better than some single-unit ACs at odor mitigation because all the air that is pulled into the air handler is cooled and blown right back into the grow room. The refrigerant used for heat transfer is the only thing that travels between the indoor and outdoor environments.

A ducted air conditioner utilizes commercial cooling and air handling equipment, which should be specifically sized for the application. If installing such a system, or retrofitting an existing one, it’s best to seek out a professional who specializes in cannabis projects. It’s easiest to use products from HVAC manufacturers that manufacture equipment designed for the indoor horticulture industry. They offer units that provide cooling, heating, dehumidification, and CO₂ supplementation.

Traditional HVAC systems designed for comfort cooling can lower yields, contaminate crops, damage plants, and increase energy consumption and costs. Surna offers integrated and stand-alone climate control systems. Integrated systems coordinate the cooling and dehumidification functions, usually with the same pieces of equipment and provide far greater precision and more energy-saving options.

Selecting and Sizing HVACs

The amount of heat produced by grow room equipment (heat load) and the cooling capacity of an air conditioner are generally rated in British thermal units (Btus). AC capacity is also often rated in units called “tons.” One ton of air conditioning is equal to 12,000 Btus of cooling capacity. When the heat loads in the room are known, such as lights, fans, and other electronic equipment, calculations can be made to determine how many Btus (or tons) of cooling power are needed to maintain temperatures within the optimal range.

All electrical devices produce 3.412 Btus of heat per watt of energy consumed. Because grow lights are the greatest heat load in an indoor garden, air conditioning needs are estimated based on the amount of lighting in each space. The rule of thumb is between four and six Btus of AC for every watt of lighting. Since lights only produce 3.412 Btus per watt, this range factors in a buffer of additional cooling to account for all other potential heat loads in the grow room. Additional heat loads include dehumidifiers, CO₂ generators, pumps, fans, and other electrical devices, as well as people. Using this equation, the range of cooling required for any grow room can be calculated by simply adding up the total lighting wattage and multiplying by four to six Btus.

For example, a grow room with 3,000 watts of lighting will need:

3,000w × 4 Btus = 12,000 Btus (minimum AC capacity)

3,000w × 6 Btus = 18,000 Btus (maximum AC capacity)

At 12,000 to 18,000 Btus, a 1 ton to 1.5 ton air conditioner is required.

The air conditioning system can be a significant capital expense for any indoor garden, so it should not be overengineered. Conversely, an undersized AC is forced to run constantly to combat the heat from lights and other equipment. A properly sized air conditioner will be somewhat stronger than the heat loads in a given room. As the temperature rises, the air conditioner will be able to quickly cool the room to the lower set point of the optimal range and then shut down until it is needed again. This will prolong the life of the air conditioner, because it will not have to work as hard or as often to provide adequate temperature control.

Types of HVAC Systems

Stand-alone Systems

“Stand-alone” describes climate control systems where the cooling function is decoupled from the dehumidification function, and integrated systems describe climate control systems where the cooling and dehumidification functions are coordinated, usually with the same pieces of equipment. In general, stand-alone systems have lower upfront costs but provide less precision and fewer energy-related benefits, and integrated systems may be more expensive, but with far greater precision and more energy-saving options.

Surna’s grow room climate controller solutions have been developed specifically to address the unique requirements of growing cannabis, including direct temperature and humidity control, CO₂ supplementation, and more. Sensors placed around the canopy provide accurate readings of these crucial limiting factors and maintain the levels set by the grower. All the data is captured and used to optimize the facility and cultivar-specific climate conditions. The SentrylQ system includes the Surna SentrylQ^® Central Plant Controller, Room Controller, and Facility Supervisor.

In DX systems with stand-alone dehumidification, the cooling and heating unit is separate from the dehumidification unit, which is usually located in the space it serves. The most common types of standalone DX systems are split systems, mini-splits, and packaged rooftop units (RTUs). Common types of dehumidifiers are stand-alone electric, or desiccant units used in low-humidity applications.

Split DX with Stand-alone Dehumidification

Split systems are made up of two parts: an indoor fan coil unit (FCU) or small air handler (AHU), and an outside condensing unit. The FCU or AHU passes the warm air from inside the cultivation space over cold evaporator coils, which contain refrigerant, absorbs the heat from the air inside the space, and transfers it to the condensing unit outside the grow to be rejected.

Variable refrigerant flow (VRF) systems also fall under this category. While VRF systems are generally very energy efficient in comfort-cooling applications, in process-cooling applications such as cultivation facilities, much of what makes them energy efficient in comfort cooling does not apply. However, they do offer greater redundancy than other stand-alone systems, and there may be some limited energy benefit in smaller facilities compared with standard split systems.

These systems typically supply cooled or heated air to a single zone in grow applications. The typical capacity is from 1 to 10 tons, but they can be larger in some applications.

Pros:

• Low cost.
• Smaller size (split units) common.
• Installation requirements well understood.
• Common, don’t require significant building alterations.
• Short lead times.

Cons:

• Lower life expectancy (3-10 years depending on selection), as they are designed for light commercial or residential versus industrial applications.
• Higher maintenance costs, as they are not intended for heavy-duty operation.
• Consistent dehumidification is not available. Requires additional stand-alone dehumidifiers.
• Temperature and humidity fluctuations due to staged on/off operation and tendency for the DX units and DHU to “fight” each other and operate independently.
• Usually less energy efficient than other options
• Many points of electrical connection required.
• Electrical infrastructure costs may be greater due to large HVAC loads being spread throughout the building as opposed to being centralized at the plant.
• Limited free cooling or economization options.

2-Pipe Chilled Water (aka Hydronic Cooling & Dehumidification)

Hydronic cooling is simply the removal of heat and moisture from a space utilizing chilled water as the heat exchange medium. People sometimes confuse chilled water systems with evaporative cooling, which introduces humidity into the space and consumes water to cool, but they are quite different.

Hydronic cooling systems are completely closed loop, meaning no water is added to the space, much like the radiator in a car. In hydronic cooling, water is chilled by a chiller, dry cooler, or cooling tower and circulated via pump through the system into heat exchanger units in the space (air handlers or fan coils) and then back to the chiller to be recirculated again.

Air handlers and/or fan coil units utilize a fan to pull warm air in and over the heat exchanger inside. As warm air in the room moves over the heat exchangers, heat is transferred from the air into the cool water inside the coils, pulling heat and humidity from the room and returning cool, dry air to the space. Since pumps keep water inside the system constantly moving, the warm water leaving the heat exchanger is immediately returned to a chiller, dry cooler, or cooling tower.

These units may sit inside or outside the facility. While indoor chillers are more efficient, for the added cost, complexity, and floor space required, most facilities prefer them outside so that they don’t have to give up space inside for equipment.

One of the primary benefits of chilled water systems is that for a minor cost addition, N+1 redundancy in each space can be built into the design, offering complete redundancy without doubling the equipment cost. These systems utilize multiple chillers organized into a bank and multiple fan coils that are all tied back to the collective chiller bank. This allows for flexibility for the system to grow and cooling capacity to be added as needed.

Built-in redundancy is inherent because indoor gardens use multiple chillers and air handlers or fan coils. Because no fan coil is tied to one specific chiller directly via the chilled water loop, any malfunction of either piece will not cause complete loss of cooling capacity. In this way, facilities can ensure they always have some manner of cooling or even full cooling if going with an N+1 design concept.

In a 2-pipe chilled water system, the fan coil or air handler has only one supply pipe and one return pipe, so the system uses the chiller loop for sensible cooling and passive assist to stand-alone dehumidifiers; the dehumidification is not integral, so there is no direct humidity control.

The 2-pipe chilled water systems provide the ability to economize via a dry cooler in colder climates, saving significant amounts of energy by turning off the compressors in the chiller, which are the single-largest energy consumer in the HVAC system.

Pros:

• Flexible air handling options (ducted, ductless, multiple sizes and configurations available).
• Minimal indoor space requirements.
• Usually results in lower overall electrical infrastructure than DX units.
• Flexible air handling options can result in better room air homogenization.
• N+1 redundancy possible at a minimal cost addition.
• Ability to easily expand operations if phasing plan is identified ahead of time.
• Flexible tonnage.
• Share equipment between rooms without sharing air between rooms (great for flips).
• Lower ongoing maintenance.
• High longevity (~20+ years).
• Significant economization (free cooling) options.

Cons:

• Initial expense can be slightly higher than packaged DX or split units.
• More engineering work is required to design the water piping loop properly than simpler systems.
• More complicated to install than most DX systems.
• Requires a mechanical room in the building.
• Stand-alone dehumidifiers are required.

4-Pipe Chilled Water

The 4-pipe chilled water system is also a hydronic cooling and dehumidification system, as previously described. As the name suggests, four pipe systems have four pipes going to multiple heat exchangers inside the building, with two supply and two return lines. One set is dedicated to chilled water, typically kept between 40 and 60°F (4 and 15°C). Another set of pipes is dedicated to hot water, generally kept between 130 and 200°F (55 and 93°C). The pipes run to terminal fan coils or air handlers, which use chilled or hot water to change the air temperature by cooling, heating, or dehumidifying. The main benefit of using a 4-pipe system over a 2-pipe system is that stand-alone dehumidifiers are not required, as the system provides temperature and humidity control simultaneously in a single unit when controlled properly.

Traditionally these systems utilize air handler units (AHUs) or fan coil units (FCUs), which contain a blower, heating or cooling elements, and control valves (and optionally, filter racks and modulating dampers). AHUs and FCUs can either

a. be connected to a ductwork system that distributes the conditioned air to the area served and reside outside the space, or

b. reside directly in and condition the space served without ductwork.

The 4-pipe chilled water systems provide the ability to economize via a dry cooler in colder climates, saving significant amounts of energy by turning off the compressors in the chiller, which are the single-largest energy consumer in the HVAC system. The most energy-efficient 4-pipe designs utilize heat recovery on the chiller plant to minimize or eliminate dehumidification reheat costs.

Pros:

• When connected to a heat recovery chiller plant, it can recover heat from that process, rather than generating new heat through the consumption of electricity or natural gas.
• Usually among the most energy-efficient options.
• Flexible air-handling options (ducted/ductless, with multiple sizes and configurations available).
• Long life expectancy if maintained properly (over 20 years).
• Very flexible biosecurity options (MERV/HEPA/UV/PCO).
• Ability to economize via a dry cooler in colder climates, saving significant amounts of energy by turning off the compressors in the system.
• When controls are properly applied, flexibility to adjust to changing room parameters and insight into system operation (for perfecting operation and troubleshooting where required) are unmatched.
• Stand-alone dehumidifiers not required.
• Greatest precision of all options in varying conditions.

Cons:

• Controls and installation are more complex than other systems and may increase initial cost.
• Longer lead times for engineering, equipment production, and installation due to the complexity and custom nature of the design and machines themselves.
• Not as common as DX units (usually utilized in true industrial applications).
• More complex maintenance requirements.

Water-Cooled Condensers

In both chilled water and DX systems, water-cooling condensers, either through a dry cooler or cooling tower set up, depending on climate, may offer significant energy benefits. Condensers, where heat removed from cultivation spaces is ejected to the outside, are most commonly air cooled. However, water has a higher heat capacity than air, as well as higher thermal conductivity, so water cooling offers energy benefits over air cooling. But water-cooled condensers also carry substantial upfront and ongoing maintenance costs that are generally only cost effective in large-tonnage systems. Consult with a mechanical engineer and maintenance team to understand the benefits and drawbacks of water-cooled systems.

Using Air-Cooled Reflectors

Because the primary heat load in any grow room is the lights, some growers who use HID lamps use air-cooled reflectors to capture and exhaust grow room heat right at its source. These specialty devices are designed to capture and exhaust much of the heat produced by the lights while allowing most of the light to radiate down into the canopy. Air-cooled reflectors create an enclosed chamber around each lamp using highly reflective metal above and a pane of glass below. They have two duct flanges mounted on either side for connecting to ducting and inline fans. These pressurized systems blow cool outside air through each lighting reflector, picking up heat from each light, which is exhausted back outside via exhaust ducting. Each inline fan can be ducted to a single reflector or to a row of air-cooled reflectors using round, metal ducting.

Sizing Air-Cooled Reflectors

Air-cooled reflector flanges come in four main sizes: 4 inch (10 cm), which has an area of about 12 square inches (77 cm²); 6 inch (15 cm), with an area of about 28 square inches (180 cm²); 8 inch (20 cm), with an area of 50 square inches (322 cm²); and 10 inch (25 cm), with an area of 78.5 square inches (506 cm²). Larger reflector flanges connect to bigger fans and ducting and have greater cooling capacity. When properly sized, air-cooled reflectors can significantly reduce the amount of air conditioning required to cool a grow room. For air-cooled reflectors to be effective, the reflector, fan, and ducting must be accurately sized. The size of the ducted system limits the volume of air that it can move. Larger fans and ducting cool faster.

Air volume is measured in cubic footage (or cubic meters), and fans and ducts are rated in cubic feet per minute (cfm) or cubic meters per hour (cmh), the volume of air they are rated to move each minute (or hour). Air-cooled reflector systems are designed based on the amount of heat they must combat. Because the heat from the lights is proportional to their wattage, as more and stronger lights are added, larger and stronger fans are required to cool them. Therefore, the rule of thumb when sizing the fan for an air-cooled reflector system is to use a fan with a rating of 0.2 to 0.3 cfm (or 0.34-0.51 cmh) per watt of lighting.

To successfully use this equation, start with the lighting design. Once the number and strength of the lights in the room is known, add up the total wattage of lights in each row. Then the size of fan needed to cool that row of lights can be calculated. Because air-cooled reflectors come with four main flange sizes, the equation is used to determine which of those four sizes will be used for fan, reflectors, and ducting.

For example, if the space is lit with two 1,000-watt lamps in one row of lights:

1. 2 × 1,000w lamps = 2,000w = total lighting wattage per row
2. 2,000w × 0.2 cfm = 400 cfm (680 cmh) = minimum fan rating

   2,000w × 0.3 cfm = 600 cfm (1,020 cmh) = optimal fan rating

   Between 400-600 cfm (or 680-1,020 cmh) of airflow is needed to cool that row of lights.

3. The average six inch (15 cm) inline fan is rated at approximately 435 cfm (740 cmh) and is perfectly sized within the calculated range from step two.

This equation should be performed for each row of lights in each room. If an inline fan is rated within the calculated cfm range, it can be successfully used. Once the fan size is determined, the reflectors and ducting for that row of lights should be sized accordingly.

Root Temperature

The temperature of the plant canopy is critical, but it is not the only area to consider. Root temperature is also important. Cold floors lower the temperature of containers and the planting medium, slowing germination and growth. Cold temperatures also are implicated in encouraging more of the plants to develop as males when growing from seed. When a plant’s roots are kept warm, the rest of the plant can be kept cooler with no damage. Ideally, the medium temperature should be 65-70°F (18-20°C). With cold air temperatures, media temperature can be 5°F (~3°C) higher or more as long as roots have access to oxygen. There are several ways to warm the medium or protect it from cold surroundings:

• The best way to insulate a container from a cold floor is to raise it so there is air space between them, using a thin sheet of Styrofoam a half-inch thick (1.25 cm).
• The medium can be warmed using overhead fans to push warm air down from the top of the room, warming containers that are placed on pallets.
• Heat cables, or heat mats, apply heat directly to the root area.
• Heat the water in recirculating systems with an aquarium heater controlled by a thermostat. If the air is cool, from 45 to 60°F (7-15°C), the water can be heated to 80°F (27°C). At these high temperatures, hydroponic system water should be supplemented with oxygen using hydrogen peroxide or other methods such as nanobubbles, venturi systems, or vigorous agitation. Water holds little oxygen above 80°F (27°C).

In general, water temperature should be adjusted to balance out the air temperature. If the air is warm, over 75°F (22°C), the water should be no more than 70°F (20°C). Should air temperature rise above 90°F (30°C), lower the water temperature from 70 to 65°F (20 to 18°C) to help decrease canopy stress.

Air Filtration/Sanitation

When a closed-loop air system is being used, any infiltration of pests or diseases is brought into the space by humans or imported plants. This can mostly be prevented. The integrity of the structure enveloping the air space is responsible for keeping out climatic variables and airborne impurities such as ash, dust, pathogens, and pests. To prevent them from getting in, they have to be filtered out.

• Media filters slow the air but catch particles.
• Ultraviolet (UVC) lamps kill airborne fungal spores and bacteria as air passes through their field.
• Activated charcoal filters adsorb vapor so that they clear the air of odor. They can be used in the space to keep the air clean and decrease odors.
• Negative ion generators add electrons to the air. These negatively charged particles neutralize positively charged dust and odor particles. Then the particle precipitates out of the air. They should not be used in rooms during the last several weeks of flowering because they neutralize terpenes.
• Ozone generators create ozone (O₃), which is not considered healthy for humans and their pets. However, ozone cleans the air of odors and can be used to treat exhaust air and to sanitize spaces devoid of plants and animals.
• Oxidizers chemically break down compounds, including infectious agents.

Air filtration is used to maintain clean, pest- and pathogen-free air in greenhouse and indoor gardens. Incoming air can introduce contaminants to the growing environment if not properly filtered. For this reason, it is important to seal each grow room as much as possible so that where and how air enters the space is controlled. Similar to how odor mitigation devices are frequently connected to air exhaust systems, decontamination filters are often incorporated into the intake ventilation system. The most common choice is a high efficiency particulate air (HEPA) filter. HEPA filters function by forcing incoming air through a fine mesh that traps bugs, mold and mildew spores, and pollen. By mitigating the introduction of these harmful contaminants, intake air filters greatly reduce the risk of pests, pathogens, and unintentional pollination.

In-room, air filtration and sanitation technologies can also be used to decontaminate the air. These devices can filter and/or sanitize the pass-through air, further reducing the frequency and severity of pest and pathogen outbreaks. Each device will be rated for how much air it can sanitize or what size room it can treat. The required quantity and size of these air-sanitizing devices depends on the volume of air in the room and level of contamination to be treated. To improve in-room air sanitation, frequently replace filters used in air conditioning, dehumidification, and other mechanical devices.

Air Sniper is the industrial-scale solution to air purification. The Air Sniper uses cleanable pre-filters instead of disposable ones and UVC light intensity combined with the highest cfm of air to destroy mobile air pathogens, instead of just capturing them. Air Sniper also includes access to a dashboard system that allows clients to monitor and control the equipment from phones, tablets, and computers.

Odor Control

Cannabis odors come from the large volume of aromatic terpenes that make up the plant’s essential oil. Terpenes are volatile organic compounds (VOCs) that very easily convert from a liquid to a gas and can travel and permeate wherever air travels. The signature odor from cannabis, especially those that are flowering, creates both a risk and a nuisance that can be avoided with proper ventilation system design and technologies.

Negative Ions

Negative ions can precipitate dust, spores, and odors. The air’s electrical charge affects plant growth and animal behavior, as well as the strength of odors. The clean, fresh-smelling air that follows a rain is due to the extra negative ions left in the air by falling water. Air in verdant, non industrialized areas and near large bodies of water is also negatively charged; electrons float in the air loosely attached to oxygen molecules. In industrialized areas or very dry regions, the air is positively charged because molecules are missing electrons. Negative ions jump from oxygen carriers to electron-deficient molecules, neutralizing them and causing them to precipitate.

Negative ion enrichment creates a few readily observable effects:

• Plants in negative ion environments grow faster than those in positively charged ones.
• Negative ions precipitate dust particles, hair, and dander from the air, so there are fewer bacteria and fungus spores floating around.
• Negative ions eliminate unwanted odors, which are positively charged particles in the air. Increasing negative ions causes the odor-carrying particles to precipitate. With enough negative ions, even a room filled with pungent, flowering sinsemilla becomes odorless.

Negative ion generators, ionizers, or ion fountain units are inexpensive and safe, use minuscule amounts of electricity, and are recommended for vegetatively growing gardens during the first weeks of flowering. However, when they are used in the garden itself during the last weeks of flowering, they eliminate odors in the garden and the plants.

Most modern ion generators capture the particles they precipitate in a cleanable, reusable filter. A few cheaper models have no provision for capturing the greasy precipitate. Since the thick film of grime usually lands within a two-foot (60 cm) radius of the ion fountain, placing newspaper around the unit is a convenient way to collect the residue. A precipitator can be made by grounding a sheet of aluminum foil to a metal plumbing line or grounding box. Attach an alligator clip and a piece of wire to the foil and grounding source. When the foil gets soiled, replace it.

To preserve the aroma of the crop, don’t use negative ions in the flowering room during the last several weeks of flowering. Negative ions interact with the odor molecules not only in the air but also those present on the plant that are not fully protected by the trichome membrane. The negative ions neutralize them so they are odor-free. The terpenes inside the membrane are unaffected, so a pinched bud still releases a powerful aroma. For best results, use ion generators during the last several weeks of flowering only in the rooms surrounding the garden room, not inside.

Carbon Filters

For indoor and some greenhouse facilities, using activated charcoal filtration is the first line of defense against odor leaks. Carbon filtration captures the terpene VOCs emanating from the plants and significantly reduces the odor footprint of the pass-through air. Using an inline fan, air is drawn through a jacket of activated charcoal, trapping odor and releasing odor-reduced air. The size of the fan and carbon filter depends on the amount of airflow required.

Facilities may have a sizable exhaust system where odor-filled air is drawn through carbon filters before it is exhausted outside. When an active (fan-powered) exhaust system is implemented, a vacuum or negative pressure is created inside the facility. This ensures that odor-filled air cannot escape through door frames and other cracks or wall penetrations. The negative room pressurization creates a system where inside air can only escape through odor-mitigating technologies, like carbon filters.

Carbon filters can also be implemented as in-room “scrubbers” to reduce the odor footprint inside the garden space and in adjacent, enclosed areas. A scrubber system consists of an inline fan outfitted with a carbon filter and is freestanding or mounted to the wall or ceiling. The fan pulls high-odor air through activated charcoal filtration and exhausts the reduced-odor air back into the room. Multiple scrubber systems can be used in targeted areas and can operate 24 hours a day for round-the-clock odor reduction. Over time and use, and depending on air and filter quality, the carbon inside will eventually become ineffective and the filter will need replacement. Higher humidity, above 65%, lowers the filter’s efficiency.

Using Ozone Generators and Air Ionizers

While activated charcoal can capture a lot of VOCs and reduce the intensity of odor from the pass-through air, these filters are not 100% effective on their own. Many cultivators add an additional technology to the exhaust system that can neutralize any residual odors that make it past the carbon filter. Ozone generators produce ozone (O₃), a volatile gas that oxidizes and neutralizes VOCs. An inline, ozone generator installed in the exhaust ducting can inject O₃ into the air that has already passed through carbon filtration. This ozone gas oxidizes and neutralizes terpenes, rendering them odorless.

Exposure to ozone can damage the eyes, ears, nose, throat, and lungs and should only be used as part of an enclosed exhaust system or in an empty room. When injecting ozone in an exhaust system, it is recommended that there be a run of ducting at least 15 feet (4.6 m) long post-ozone injection, so that the ozone can fully mix with the air and neutralize any odors before it is exhausted outside.

Ionizers purify and reduce the odor of the air that is treated by adding electrons to air molecules and creating negatively charged particles that attach to free-floating, positively charged contaminants. This neutralizes their charge and causes them to precipitate out of the air. In the process, cleaner, odor-reduced air is created.

Ionizers are safe to use around people and are recommended for use in hallways and other areas adjacent to indoor gardens. While both ozone generators and ionizers can eliminate odors from the facility, they should not be placed inside flowering rooms during the last several weeks of flowering, as they also neutralize the odor of the plants.

Ventilation

Carbon Dioxide Replenishment

During the lights-on period of the day, plants absorb CO₂ from the air surrounding their leaves and flowers. Replenishing CO₂ is one of the most important functions of a grow room ventilation system, ensuring that the plants have access to the levels they need throughout the day and during each stage of development. There are two primary approaches to providing CO₂ in closed-environment agriculture (CEA): air exchange and CO₂ augmentation. Grow room air exchange provides for atmospheric levels of carbon dioxide, while supplementation systems often elevate CO₂ concentrations to much higher than atmospheric levels, to facilitate more rapid photosynthesis and growth.

Open-Loop Ventilation (Atmospheric CO₂)

Atmospheric levels of carbon dioxide are adequate for healthy plant growth during all stages of development. Although it varies depending on geography, planetary CO₂ averages about 420 parts per million (ppm). In CEA rooms, close-to-atmospheric levels can be maintained using what is referred to as “open-loop” ventilation. In an open-loop ventilation system, grow room air is exchanged with outside air to replenish carbon dioxide. Using fans, filters, and ducting, low-CO₂ air from inside the grow room is exhausted and replaced with fresh, outside air that is rich in atmospheric CO₂.

The exhaust components of the open-loop ventilation system are typically mounted high in the room in order to draw on warm, CO₂-depleted air. The active (fan-powered) exhaust is generally hooked up to an odor mitigation system and ducted to the highest exterior point of exhaust in order to achieve the greatest reduction in the garden’s odor footprint. As air is drawn out of the room, it creates a vacuum, or negative pressure, inside the room. This is what draws in fresh air, often via a passive (fan-free) intake. This is often a simple wall penetration, mounted on the side of the room opposite the exhaust, and is typically outfitted with a high efficiency particulate air (HEPA) filter.

Exchanging the air at the appropriate rate ensures that as plants use it up, carbon dioxide is replenished and readily available for continued uptake. To maintain close to 420 ppm CO₂ inside the garden, the air in an open-loop ventilated grow room should be exchanged every 5 to 10 minutes. This is achieved using a properly sized exhaust fan, along with intake and exhaust filtration and ducting.

If the exhaust system is undersized, a full turnover of air in the room will take longer than 10 minutes. As a result, the plants may not have adequate access to CO₂ to achieve healthy growth. If the exhaust fan is oversized and exchanges the air faster than every 5 minutes, the in-room climate control equipment, such as air conditioners and dehumidifiers, may not have enough time to function effectively. Incoming air may, at times, be hotter, colder, drier, or more humid than is desired, and HVAC equipment may not be able to keep up and adequately temper this rapidly exchanging air. By consistently turning over all the air in the grow room at the optimal 5-10-minute rate, the plants are ensured to receive adequate carbon dioxide, and the mechanical equipment can, at all times, maintain optimal temperature and humidity.

Closed Loop Ventilation Space

The garden is subject to the vagaries of nature when using ventilation. The temperature of the incoming air is always changing, both daily and seasonally, and can reach such extremes that the stream is harmful to plants. Rapid changes in outdoor temperature and humidity as well as temporary changes in air quality make it difficult to closely regulate air inside a ventilated space. A closed-loop ventilation system is sealed from the rest of the environment. Instead of using air from outside to modify temperature and humidity, the air in the room is constantly adjusted and modified. With a climate-controlled, sealed space, the environment is always conducive to plant health and to optimizing growth and yield.

Since the space is receiving no air from outside, equipment such as heaters, air conditioners, humidifiers, and dehumidifiers are used to maintain the room’s air conditions. Carbon dioxide enhancement is used to keep levels high to promote rapid growth. To keep the air sanitized, a carbon filter can constantly filter air in the room. An enclosed UVC light, such as a model often used in restaurants, cleans the space of airborne pathogens.

The advantages are:

• Insects and disease are less of a threat because there is no exchange of air.
• No matter the conditions outside, a closed-loop space creates its own controlled atmosphere.
• With a properly sized conditioning system, there is complete control of air conditions: temperature, humidity, VPD, and CO₂.
• Poor, outside air quality, such as from dust, smoke, and noxious chemicals, is less likely to affect the space.

The disadvantages are:

• The system and installation are more expensive than other systems.
• There is a limited selection of greenhouse manufacturers, builders, and installers who are experienced in closed-loop systems.
• It may take a little longer to dial in the best conditions, especially if they aren’t regulated automatically.
• Odor control can become more challenging with the loss of an odor-mitigated exhaust system.

Although closed-loop systems cost more both to install and operate, they can control the air with more precision than a ventilated system. This can increase both yield and quality.

Hybrid Loop Ventilation

For CO₂ enrichment to be successful, the ventilation system must allow for CO₂ to be pumped into the grow room without significant dilution. The rapid air exchange of an open-loop ventilation system dilutes the grow room’s CO₂ enriched air with lower-CO₂ air from outside. Instead, cultivators using CO₂ supplementation typically switch to either a closed- or hybrid-loop ventilation system, stopping or slowing air exchange to allow CO₂ to concentrate to more optimal levels.

Traditionally, a closed-loop ventilation system has been used in CO₂-enriched indoor gardens. These “sealed” grow rooms completely eliminate the exhaust ventilation system, allowing CO₂ to be pumped into and saturate each cultivation zone, using a tank or generator. When eliminating the exhaust portion of the ventilation system, the aforementioned negative pressurization, often used for odor control, is lost. The “zero pressure” environment of a sealed room allows odor-filled air to travel freely and potentially escape unfiltered. This could create a significant risk of odor leaks. Using in-room carbon filter scrubbers and negative ion generators in the surrounding nonflowering rooms and hallways reduces but does not completely remove the odors in closed-loop gardens.

To simultaneously minimize the dilution of CO₂ enrichment and maintain negative pressure odor mitigation, a hybrid-loop ventilation system is recommended. In a hybrid-loop room, exhaust ventilation is significantly reduced, but not eliminated. An exhaust system consisting of a fan, carbon filter, ozone generator, and exhaust ducting can be used, but it must be much smaller than the one used for open-loop air exchange. Using the minimum rate of exhaust needed to maintain net negative pressurization in the grow room, control of where and how grow room air is exhausted (and odor mitigated) from the facility is regained.

Hybrid-loop ventilation uses a 20-to-30-minute air exchange rate. Although this is much slower than the open-loop, atmospheric CO₂ approach to air exchange, it is just fast enough to maintain negative pressure for odor control and just slow enough to support CO₂-enrichment efforts. Compared with closed-loop “sealed” grow rooms, using hybrid-loop ventilation will cost a little more for CO₂. However, the cost is generally negligible and well worth the benefits of using CO₂ enrichment while also maintaining 24/7 odor control.

Sizing Exhaust Systems

1. Measure and multiply the grow room’s length, width, and height (in feet or meters) to calculate the total volume of air the room can hold (in cubic feet or cubic meters).

   a. L × W × H = volume (in ft³ or m³)

2. Divide this volume of air by the range of time in which a total turnover of air should be achieved, in order to calculate the optimal rate of air exchange. This will provide an optimal range in cubic feet per minute (cfm) or cubic meters per hour (cmh).

   a. For open-loop (atmospheric CO₂) ventilation systems, a 5-10-minute turnover is required (0.083-0.167 hour) for air exchange.

   b. For hybrid-loop (supplemented CO₂) ventilation systems, a 20-30-minute (0.333-0.5 hour) air exchange range is required.

3. Find the properly sized and rated fan that fits within the optimal cfm or cmh range calculated in step two.

Example 1: Open-loop ventilation in a 10 × 20 × 12 foot (3 × 6 × 3.6 m) garden space.

1. 10 × 20 × 12 feet = 2,400 ft³ (3 × 6 × 3.6 m = 68 m³) = volume of air in the room.
2. 2,400 ft³ ÷ 10 min = 240 cfm (68 m³ ÷ 0.166 hr = 409 cmh) = minimum fan rating.

   2,400 ft³ ÷ 5 min = 480 cfm (68 m³ ÷ 0.083 hr = 819 cmh) = maximum fan rating.

3. The average 6-inch (15 cm) inline fan is rated at approximately 435 cfm (740 cmh) and is perfectly sized within the calculated range from step two.

Therefore, to achieve a 5-to-10-minute air exchange using open-loop ventilation in this garden space, a 6-inch (150 mm) exhaust fan needs to be installed. For odor control, odor-mitigating devices (carbon filter, inline ozone generator) and ducting that are also sized at 6-inch (15 cm) in diameter should be installed.

Example 2: Hybrid-loop ventilation in a 10 × 20 × 12 foot (3 × 6 × 3.6 m) garden space.

1. 10 × 20 × 12 feet = 2,400 ft³ (3 × 6 × 3.6 m = 68 m³) = volume of air in the room.
2. 2,400 ft³ ÷ 30 min = 80 cfm (68 m³ ÷ 0.5 hr = 136 cmh) = minimum fan rating

   2,400 ft³ ÷ 20 min = 120 cfm (68 m³ ÷ 0.33 hr = 206 cmh) = maximum fan rating

3. The average 4 inch (10 cm) inline fan is rated at approximately 200 cfm (340 cmh), which is oversized for the calculated range from step two. However, 4 inch (10 cm) inline fans are typically the smallest available option. Since this fan is oversized for the calculated range for hybrid-loop air exchange, if left running at full strength, it would begin depleting CO₂ from the room too rapidly. To avoid this, connect the 4 inch exhaust fan to a fan-speed controller, a device that can adjust the speed of the fan and reduce its cfm (cmh) airflow. In this case, a 50% reduction would put it back in the optimal range.

Therefore, to achieve a 20-to-30-minute air exchange using hybrid-loop, CO₂-enriched ventilation in this garden space, a 4-inch (10 cm) exhaust fan dialed down to 50% strength by a fan-speed controller is required. For odor control, connect an exhaust fan or odor-mitigating devices (carbon filter, inline ozone generator) and ducting that are also sized at 4 inches (10 cm) in diameter.

Outdoor Temperature & Air Control

Outdoor Temperature

Having evolved outdoors, cannabis appreciates temperate climates and tolerates the extremes of the geographic conditions in which particular cultivars have evolved. For example, tropical sativas can withstand high humidity and are less susceptible to molds and mildews. In contrast, high-altitude indicas cannot and are more vulnerable to molds and mildews than high-humidity varieties. However, both groups can thrive around 75 to 85°F (24-30°C) with relative humidity no higher than 60%. When cultivating outdoors, each cultivar is best suited to climates replicating its geographic origins or appellations. Most cultivars in commerce are not associated with a specific appellation, so it may take a few rotations before the right conditions are dialed in.

Cooling Outdoors

Photosynthesis crawls to a halt when the temperature gets close to 90°F (32°C). In hot weather outdoor plants may be photosynthesizing efficiently during only part of the day. For instance, if the temperature climbs to 90°F (32°C) at 11 a.m. and does not drop below that level until 4 p.m., the plant will utilize only the morning and late afternoon light. High temperatures outdoors during flowering interferes with bud development, making maturing buds airy and lanky. Temperature is a factor when plants are forced to flower in hot areas during the summer.

• Misters that use 5-micron sprayers evaporate water in the air, cooling the space without getting the plants wet. Some systems use rows of emitters, high-speed fans, or a combination of both to create a superfine mist or fog that evaporates instantly, lowering the temperature as much as 30°F (16°C). They work best in dry areas.
• A fine spray of water on the plants keeps them cooler during the hot period. The evaporating water absorbs heat from both the leaf and the air in the micro-space at the leaf surface. Use a water spray only during vegetative growth and the earliest stages of flowering.
• Use cool water to cool the roots. Positive effects will travel up the stem.
• When using planting containers, place a piece of Styrofoam between the container and the soil to stop heat transfer from the ground.
• During the summer when it’s important to keep the containers cool, use white or light-colored containers or wrap the containers in white plastic or sturdy paper to reflect the light.
• Cover the ground or raised bed with white plastic to keep the soil cool and slow water evaporation.
• Use a thick layer of organic mulch. It keeps the soil warm when it’s cool and cool when it’s warm. At the same time, it releases CO₂ to the air and nutrients to the soil as it decomposes.
• Use a 10-20% removable shade cloth to block light during the brightest part of the day. This helps keep the plant canopy temperature lower.

Warming Outdoors

Cultivars differ in their tolerance to cold weather. Most can tolerate low temperatures into the low 50s to high 40s F (8-12°C), and many can tolerate even colder nights. However, lower temperatures set the plants back, and it sometimes takes them several days to continue the growth or ripening.

It’s not just the cold that sometimes damages the plants. It’s a combination of cold and wind, especially if there is low humidity. The wind causes evaporation, which lowers the temperature of solid objects such as leaves below the air’s ambient reading.

When plant roots are kept warm, the plant canopy can withstand cold temperatures more easily and with less damage

Here are a few solutions to cold temperatures and wind:

• Move container plants to a sheltered location such as a shed or other structure. Small plants can be carried. Larger plants can be placed on a cart or dolly. This technique can also be used for light deprivation to induce earlier flowering
• Wrap well-supported plants with polyethylene to shut out wind and preserve heat.
• Use patio-type propane heaters. The units spread heat in a large circle. They can be lined up to provide heat to rows of plants.
• Agricultural gas heaters use natural gas or propane. The heaters are used to prevent frost damage on fruiting plants such as grapes and stone fruit.
• Heat the roots. When they are kept warm, the canopy withstands cool temperatures with less damage. Both electric heat mats and tubing-based hot-water heaters keep the roots warm. Keep roots warm by using black containers or by wrapping with dark plastic to absorb the sun’s rays and warmth.
• Place a half-inch (12 mm) or thicker Styrofoam sheet as a barrier between the ground and the containers so that heat isn’t drawn down.
• Cover the ground or raised bed with black plastic to keep the soil warm during cooler months.

Greenhouses

Greenhouses provide a defined airspace with opportunities for environmental control and all the benefits of natural light.

There are many types of greenhouses to choose from. Popular greenhouse options include hoop, Venlo, commercial gable, cold frame, gothic arch, and others. There are a number of factors that one should consider when pondering the type and model greenhouse to acquire. Among these factors are:

• Goals: what is the greenhouse to be used for? Is it just to be used seasonally to start plants early in the season? To extend the season for additional crops? Or is it to be year-round?
• Location: the structure and infrastructure of any construction is highly dependent on the environmental conditions (light, temperature and wind) it will be exposed to.
• Financial Investment: greenhouses range in price depending on sophistication.

Environmental Considerations

Along with selecting a greenhouse appropriate to the environment, equipment must be chosen that is capable of regulating temperature, humidity, and CO₂. The selection process will depend on whether the system will be run as an open or closed environment. Both have advantages and disadvantages.

For instance, using an open-loop system, a greenhouse in South Florida may exceed the cooling ability of a pad and fan system, also known as a “wet wall.” Another option might be a chiller system.

Forever Flowering greenhouses have been designed around the specific needs of cannabis growers for over 15 years. The G-series are designed for high wind and snow loads and use passive cooling technology through its use of side and ridge vents (single or double).

Closed-loop greenhouses offer many advantages because there is less chance of infection, and it is often cheaper in the long run to maintain the temperature of the same air rather than running an air-exchange system.

Indoor facilities have insulative values or what is commonly referred to as an R-value. R-values represent the capacity of any building material to insulate or resist heat flow. The higher the R-value, the greater the insulating power. Engineers use R-Values and other factors such as location, record highs, record lows, depth of the slab, and internal gains such as lighting to determine just how much HVAC and humidity control is needed for an indoor facility.

Traditional greenhouses have low R-values because they have thin walls. External gains are determined by the outside environment, such as the sun’s efficacy through rain, cloud cover, and shadowing from other structures. Expenses and consistency of cannabis will change from day to day and season to season. Newer coverings, sometimes composed of several layers, have higher R-values.

Hybrid greenhouses were developed to increase G-efficiency. They have solid walls on one to four sides. Some only have a northern wall, which never gets direct light. Some have half-height walls. The most extreme are buildings with the only light being let in through the translucent roof. Hybrid greenhouses work well, especially in areas with extreme weather. They use insulation to lessen the external environment’s impact, allowing the internal heating or cooling systems to work efficiently.

Standard features of modern-day greenhouses are automated lighting systems that adjust the electric lighting based on the intensity of solar radiation. They also include blackout curtains and chilling and heating systems to accommodate year-round cultivation.