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Reasons you aren’t meeting setpoints (HVAC/D)

Cultivators often face challenges in maintaining HVAC/D setpoints, with issues ranging from humidity to equipment failures. Both commercial and home growers recognize the importance of a reliable system for optimal cannabis and greens production. Identifying common failures and implementing best practices ensures consistent climate control in grow spaces.
October 12, 2023

We are often approached by cultivators who are struggling to meet their HVAC/D setpoints for one reason or another–humidity issues, problems with temperature control, and alarms or failures of air conditioners used in the grow spaces are all common issues that cultivators call us an ask for help with.

Reliability of HVAC/D systems is especially vital for commercial cultivation facilities, but even home growers struggle sometimes and can benefit from the lessons learned by commercial cultivators.

Maintaining the optimal climate is critical to quality and yields for cannabis and greens producers. So what are the most common causes of failure to meet setpoints in cultivation spaces? And what are the best actions cultivators can take to ensure consistent temperature and humidity in their grow space?

Understanding HVAC/D Setpoints

The sensible (temperature) and latent (humidity) heat removal capacity of your HVAC/D system is dependent upon a number of variables. All HVAC/D systems have a certain sensible and latent capacity, but that capacity is constant at at specific condition, usually 80 degrees Fahrenheit and 60% relative humidity. When the temperature and/or humidity are lower than those setpoints, the system’s capacity to remove heat and moisture declines(known as derating).

In indoor grows, dehumidification capacity is usually the area that is most affected by lowering temperature setpoints, as latent loads are typically higher than sensible loads in most indoor grows. Mechanical engineers or HVAC/D system designers should take into account the performance of the systems specified at the desired conditions, so if your setpoints deviate from the design condition, that could be the cause of fluctuations and you may be forced to return to the setpoints specified during the engineering design.

If excess humidity is the primary concern, you might also be able to find a sweet spot that works by adjusting setpoints to mirror the dewpoint temperature the system was designed to, while still reaching your target VPD. 

Read more about reaching your target VPD using the right dehumidifier

Undersized systems

The most common engineering or design mistake we seen in specifying air conditioners, dehumidifiers, and integrated cooling/dehumidification systems is a failure to take into account all of the loads and under-sizing systems as a result.

Indoor cultivation produces a lot of heat that must be managed through the climate control system. The sensible (cooling) loads are generally straightforward, as they can be fairly easily calculated by identifying the wattage of all of the electrical equipment in the facility (lighting primarily, but also pumps and other equipment that produces heat) and adding that to the building envelope loads (the heat load expected based on the outdoor temperatures, the building’s solar exposure, and insulation values).

While sensible loads are sometimes underestimated, the most common cause of under-sizing systems is usually related to latent load, or dehumidification.

While plant transpiration rates vary, the design of the system should focus on the watering rate over time and assume that 100% of the irrigation water that the plant uptakes will eventually make its way back out into the space through transpiration and become water vapor, which is the primary source of humidity in the room. (Evaporation from soil and other media, spills, and ambient humidity if there is significant ventilation will also affect relative humidity).

Humidity in grow rooms is the most complicated part of designing indoor plant environments, and if the design assumptions did not account for the amount of water vapor that will be present in the air, ideal humidity levels cannot be reached.

Further complicating the issue, if a cultivator simply adds stand alone dehumidifiers to reach a specified humidity, it’s possible that the air conditioners in the space are not large enough to manage the waste sensible heat produced by the additional dehumidifiers.

Maintaining ideal humidity levels is a crucial part of achieving a perfect climate for indoor cultivation. There are so many different types of systems that can be applied, and it’s important that the components of the climate control system work together.

It can become very costly to modify climate control systems after the fact, and may cause higher energy usage than system that was properly designed to begin with. Proper sizing of climate control systems at the very beginning of construction is critical, and it’s extremely important to engage with a qualified, experienced design professional like Surna to identify the loads in the facility and ensure that systems are properly sized at the outset in order to maintain optimal conditions for plant growth.

Before making changes in your facility such as adding plants or canopy square footage, increasing lighting wattage, or increasing irrigation volume, double check with your mechanical system designer first, as this could cause your existing system to fall behind and fail to meet setpoints.

Oversized Systems

Oversized systems in indoor farms can be as problematic as undersized systems, but for different reasons. While HVAC/D systems have to be sized for the maximum watering rate and heat load to ensure optimal conditions, the watering rate (and sometimes lighting intensity) usually increases over time in grow rooms as the plant matures.

Additionally, there is a long lights off cycle where the sensible loads are very low, but the latent loads still exist. For that reason, proper sizing of HVAC/D systems must accommodate both the period of lowest loads, and the period of highest loads.

At best, oversized systems will rapidly cycle, resulting in excessive energy usage and inconsistent temperature and humidity. At worst, oversizing systems may cause the systems in use to alarm for low refrigerant pressure, shutting the system down and forcing maintenance staff to respond. In order to allow for successful operation at both low and peak loading times, it is recommended that systems designed for cultivation be equipped with the ability to stage (either with multiple units, multiple compressors, multiple compressor stages, or a combination of the three), or modulated (where the systems ramp up or down based on real time load as opposed to simply coming on and turning off.)

Both approaches can be successful, with the added benefit of energy savings and equipment redundancy.

Outdoor Climate

While the outdoor climate in grow facilities has less of an impact on the sizing of the climate control system than in comfort cooling applications, the types of systems applied still need to consider the outdoor climates to ensure appropriate year round operation.

In hotter climates, the cooling capacity of the outdoor unit can derate considerably, meaning that it is not providing as much cooling or dehumidification as it would otherwise be capable of. If your HVAC/D system generally keeps up during milder times of year but you see an increase in temperatures during the summer, this is the likely cause.

Additionally, in excessively hot climates, high ambient operating kits may be required to avoid component failure or high pressure alarms causing shut downs.

In colder climates, low ambient operating kits may be required to avoid component failure or low pressure alarms causing shut downs. Air conditioners by their nature are designed to keep the inside of a building cool when it’s hot outside–so asking them to operate for cooling in the dead of winter can be a tall order for a system that isn’t purpose built for the application.

Cold temperature operation requires that the units be equipped to operate in low ambient or extreme low ambient conditions, for which conventional air conditioning is rarely equipped, so if you’re having issues with your system in cold weather, verify that the system is designed for low temperature operation.


Ventilation in cultivation facilities is an often debated subject. Some cultivators like to seal their cultivation rooms to the degree possible, creating a closed loop system where exhaust fans are only utilized in emergencies.

Cultivators who are not actively dosing with CO2 must continuously ventilate to ensure that their plants have access to at least atmospheric normal levels of CO2. And other cultivators prefer to ventilate at a constant level for pressurization purposes, or on a schedule. There is not necessarily a “right” way to do it, but ventilation strategies will have a significant impact on the climate in the room.

Proper ventilation systems should be equipped with effective filtration to ensure that cultivators are not exposing their plants to pests and pathogens such as powdery mildew and spider mites from external sources.

In some cases, energy savings may be achieved through airside economization (or ventilation for the purposes of maintaining temperature and humidity), but ventilation’s effectiveness is variable depending on the outside conditions and mechanical systems must still be sized to account for the total load in the space for the times that the outdoor conditions aren’t at the ideal temperature or humidity.

The most efficient ventilation systems utilize energy recovery, where the air leaving the facility exchanges heat with the air entering the facility (without mixing) to precondition the air coming into the space without using mechanical energy. And no matter what the ventilation strategy, the HVAC/D system must be designed to take the ventilation strategy, and associated loading on the system, into account.

If your ventilation strategy differs from the ventilation strategy devised at the time that your HVAC/D system was designed, this could be contributing to variances in temperature and humidity in your grow space.

Maintenance Fails

HVAC/D systems may be the most under-maintained systems in grow facilities. No one really notices them until they aren’t doing their job, so it can be easy to overlook their maintenance needs.

At a high level, coils on indoor units and outdoor units should be regularly cleaned, refrigerant pressures should be checked, and all other maintenance suggested by the manufacturer should be regularly undertaken. Failure to appropriately maintain your HVAC/D system usually results first in increased energy usage (and associated operational cost), then may result in a slow decline, impacting the system’s ability to maintain setpoints.

When maintenance is long overlooked, system failure and alarms result, usually requiring emergency response, which is often far more costly than simply maintaining the system correctly to begin with. If you find that your climate parameters are starting to slip, it’s a good idea to verify that your system has been cleaned and that all maintenance has been performed as scheduled.

Often, operators of grow facilities overlook the need for regularly scheduled maintenance on major equipment, which can be an expensive mistake.

Commercial facilities in particular are well served by allowing for the salary of a maintenance person in their financial models, as they will typically pay for themselves very quickly in the form of reducing downtime and avoiding the need for costly emergency repair. For more information on this topic, we recommend our blog titled, “4 Reasons to Invest in an HVAC Maintenance Plan for Your Grow Room.”

If you’re building a new cultivation facility, or struggling to maintain the climate parameters in your existing facility, contact us for help today!

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