Several key measurements can tell a lot about a facility’s environmental control requirements
By Tom Dillon
Growers understand the humidity requirements of cannabis plants, but achieving and maintaining that ideal level can be a challenge, particularly during and after the flowering stage.
When humidity becomes a problem, a few simple measurements will help determine where attention is needed. Sampling temperature and humidity in a few key places will help facility managers adjust their HVAC and dehumidifier equipment, both of which affect humidity. These measurements can be used to analyze how well equipment is working. If all equipment is performing as designed and humidity levels remain high, then moisture is being released faster than dehumidification equipment can process.
Small, hand-held measuring instruments are excellent tools for checking air conditioner, heater or dehumidifier performance. First, record the temperature and humidity in both the air of the room and the equipment supply air.
When using better measuring instruments, the dew point temperature of the air will also be displayed. Dew point temperature correlates to the amount of water in the air. If humidity control equipment is working properly, the dew point temperature of supply air will be lower than room dew point temperature.
Next, compare the room dew point to the dew points of the air conditioner and dehumidifier supply air. Since the air conditioner moves large air volumes, even a small difference in dew point temperature results in significant moisture removal. Dehumidifiers move smaller quantities of air and depress dew point more than air conditioners to deliver their moisture removal capacity.
Just knowing that dehumidifier air is hot and has a low relative humidity is not enough information for an accurate assessment. For example, supply air of 100 degrees Fahrenheit at 25% relative humidity has a dew point temperature of 58 degrees. If that is the same dew point as the room, that means zero moisture removal. This is an indication that the dehumidifier requires servicing. A larger than normal dew point depression could indicate an airflow restriction through the dehumidifier (dirty coils or filter). When restrictions become excessive, damage to the unit can occur. Routine filter cleaning or changing will improve the performance and life of dehumidifiers.
Measurements can be used in the calculation of water removed by the equipment. The best way to determine removal rate is by allowing the dehumidifier to drain into a bucket for two hours. Measuring the water recovered and dividing by two will provide a pints-per-hour rate of removal under current room temperature and humidity conditions.
Daylight humidity control can be accomplished by using standard air conditioners if desired room conditions are higher than 55% relative humidity. Some larger package air conditioners allow leaving air temperature to be lowered (leaving air is that which is supplied to the room from the equipment). This lowers dew point temperature of supply air (while also lowering actual temperature of the air). Some facilities run air conditioners at night for humidity control and use some form of electric or refrigerant hot-gas reheat to raise the supply air temperature. This procedure is not an energy-efficient solution and does not provide low relative humidity in cool rooms — however, it does allow growers to avoid purchasing dehumidifiers.
Achieving 50% relative humidity in a sealed, commercial grow facility generally requires dehumidifiers. Refrigerant dehumidifiers remove more water in warm, humid rooms than they do under cooler, drier conditions. At night, when sized properly, dehumidifiers can control room relative humidity. Short-term relative humidity spikes are normal when lights are turned off if the room is allowed to cool at night. A properly sized system will regain humidity control in less than an hour.
Self-contained plug-in dehumidifiers generate heat along with removing moisture. This heat can be a nuisance for cool, night room conditions, so some growers will need to run the air conditioner at night to counter this heat. A split-system dehumidifier provides cool to warm leaving air, providing some temperature control to the room.
Dehumidifier water removal is typically rated based on 80 degrees at 50% relative humidity inlet (room) air. If cool night conditions are desired (for example, 70 degrees at 50% relative humidity) water removal rates fall by up to 70%. Don’t blame the dehumidifier. Cooler air carries less water vapor (lower dew point temperature) meaning there’s less moisture passing through the evaporator coil, so less water is condensed and removed. Desired night room temperature plays a critical role in sizing dehumidifiers. The bottom line is that existing climate control systems must be enhanced if additional humidity removal is required.
Tom Dillon is the general manager of Subcooled Air. For the past 15 years, he has engineered humidity control solutions for industrial customers, including food manufacturing, shipyards, horticulture, power plants and more. Subcooled Air has developed a dehumidifier line designed for commercial indoor cannabis growers that require removal of at least 600 pints per day.