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The HVAC system is critical to the success of an indoor grow. Cooling, Dehumidification, and Airflow effect crop quality, quantity, and safety.
In the Cultivation and Dry rooms of indoor grow facilities, temperature and humidity control can be used to help optimize plant growth, improve quality metrics, reduce microbial risk, and dry cannabis flower into a shelf-stable consumer product.
This page includes the following sections:
Temperature and Humidity Targets by Room Type in Indoor Grows
HVAC System Options for Grow Rooms
Technical Notes on Grow Room Loads and Control
Airflow and Air Mixing and T/RH Distribution in Cultivation Rooms
The types of rooms covered here are flower, veg, mom, clone, dry and cure/storage.
Flower: These are the rooms where "flower" is grown. Plants are typically in this room around 8-10 weeks (though can vary) corresponding to the length of the flowering cycle of the plant. Plants are brought in from the veg room and usually start at relatively high temperature and humidity setpoints. This might be 80-85F temperature and 65-75% target RH. Temperature and humidity may decrease a bit at night, to maybe 78-80F, and 60-70% RH. Humidifiers are usually needed to meet these high humidity setpoints, though some growers opt to forgo humidifiers and let RH sit below setpoint. Toward the end of the flower cycle, lower temperature and humidity is desirable, for various reasons including to reduce microbial growth. The plants are transpiring a lot more moisture into the air at this point, requiring a lot more dehumidification. Rooms are often lowered to 72-78F with the lights on, and maybe 68-75F with the lights off, and around 50-55% RH. Without sufficient dehumidification (with reheat and/or standalone dehumidifiers), microbial issues will develop and the room will be a constant operational struggle.
Veg/Mom/Clone: From an HVAC perspective Veg, Mom, and Clone rooms like to be fairly warm and humid. Clones like it very humid, but they are often grown under domes or in small tents to have a very high localized relative humidity. The greater veg and mom rooms may operate around 80F and 50-70% RH. Some growers will use humidifiers in the veg rooms to maintain high RH.
Dry and Cure/Storage rooms: Typically target conditions for Dry Rooms are 60F/60% RH. The lower temperature is desired to reduce the loss of volatile organic compounds like terpenes which make up the flavor and aroma profile. Dry rooms need sufficient dehumidification to be able to reduce the humidity to 60% within a day or so of the recently harvested flower getting loaded in. Cure and dry flower storage rooms should also be kept cool and dry to continue to maintain product quality. Special care should be taken to choose a cooling unit that will be able to operate with a low room temperature. Some equipment will have issues with the evaporator freezing or will be limited by their internal settings to not run this cool.
HVAC or "HVACD" systems in indoor grows can be categorized in various ways. Sometimes the "D" is added to the acronym "HVAC" to highlight the importance of dedicated dehumidification in these facilities. All Indoor grows require cooling and dehumidification. A small amount of heating is generally needed too. If reheat is used for dehumidification, then a lot of heat is needed. Although rooms are typically operated without outside air to avoid blowing away CO2, it is good practice and often required by code to have ventilation (meaning bringing outside air into a room) available on demand and triggered to operate on high CO2.
One way of categorizing types of systems is by how they dehumidify as "integrated" or "decoupled" systems. Decoupled systems use different devices for cooling vs dehumidifying.
In a decoupled system any cooling air conditioning unit can work, splits, VRF, packaged RTUs, chilled water fan coils to name a few. Then separate standalone dehumidifiers are used to handle the moisture load.
In an integrated unit, a single cooling coil provides cooling and dehumidification, and then heat is added back to the supply air before it is delivered to the room. Integrated units can be packaged or split systems or 4-pipe chilled water air handlers. Integrated systems can use any form of heat for reheat, including electric, gas, hot water, or hot refrigerant gas instead of sending that to the unit's condenser ("hot gas reheat").
Using heat recovered from the cooling process from either via hot gas reheat or via heat recovery chillers in a 4-pipe chilled water system saves energy. Reheat without heat recovery will use a significant amount of energy. Dehumidifiers also have their condenser in the room and when they run are effectively doing cooling and hot gas reheat dehumidification within a single box, discharging hot, dry air into the room. Dehumidifiers, like a refrigerator, overall add heat to a room.
A couple points. Technically, 1 kW = 3.412 MBH heat load. With heat from minor heat loads, like circulation fans, this might increase to 4 MBH heat load in a room of actual heat load per kW of light (before considering dehumidifiers). However, typically more nominal capacity is required to provide setpoint flexibility across the grow cycle, especially in integrated systems.
Rule of Thumb Sizing for Decoupled and Integrated Systems
In decoupled systems, a half ton cooling per lighting kW is a good rule of thumb. Though more can be needed to reach lower setpoints. Similarly, 2 PPD nominal dehumidifier capacity per square foot of canopy is a good rule of thumb for dehumidifier capacity, though more can often be desirable.
In integrated systems with reheat and no dehumidifiers, sizing can be in excess of 1 Ton per kW of light when sizing to hit lower temperature and humidity targets. In these systems I prefer to have integrated unit sizing closer ~6-10 MBH capacity per kW lighting, with dehumidifiers, in addition.
To go beyond rules of thumb and calculate system sizing based on calculated loads, setpoint targets, and psychrometric calculations see our HVAC system sizing tool.
The dehumidification capacity of integrated units and dehumidifiers are additive. If the equipment has a very high degree of control such as with chilled water control by a modulating valve, oversizing has lower potential downside. DX systems should consider if the units selected have enough turndown capacity to operate smoothly in early flower with the lights on and off, as well as in late flower. Of course, every installation has different considerations.
Consistent airflow at the plant canopy and uniform Temp/RH distribution should be goals of any grow room HVAC system. Easier said than done.
Canopy airflow and cooling/dehumidification supply airflow should be seen as two separate, yet interconnected systems. Sometimes engineers will try to design the supply ducts such that they provide enough velocity to the canopy while avoiding "redundant" fans. However, 1) conditioned air is not always an appropriate temperature and humidity to blow directly onto that plants and 2) it is very hard to design airflow velocity for one off designs and in practice it is better to design a system that will provide good supply air distribution and allow for adjustability of actual canopy airflow using fans later.
For room temperature and humidity distribution, single level and multi-level are separate topics.
Single-level airflow:
There are numerous configurations for single level airflow that often include some combination of wall and ceiling mounted fans. Fans can be classified as "HAF" which stands for horizontal airflow fan or "VAF" which stands for vertical airflow fan (also called V-Flo, tradename). Ceiling fans and ducted inline fans are also sometimes used. Wall Fans are good for narrow rooms, wider rooms will likely benefit from HAF fans if airflow from the wall cannot reach to the center of the room. For narrow rooms with wall fans, supply air can be dropped along the wall and wall mount units are good. For wider rooms, supply air above the canopy with fans helping to push the air across and down into the canopy will often make sense. The airflow fans should be oriented to reduce short circuiting of the HVAC and dehumidifier supply air.
Multi-level airflow:
Consistent airflow at the plants as well as temperature and humidity uniformity throughout the room are challenges in multi-level grow facilities. Integrated cooling and dehumidification units can provide better uniformity by having a single temperature supply air for the room. Narrow rooms can have air movement provided by wall mounted fans and are most compatible with decoupled systems. Air tube or plenum systems can pull air from the front and/or back of the room and distribute to the plants.