What will it Cost? How to Calculate HVAC Needs in Cultivation Environments

Surna has evolved into a full-scale MEP design firm, manufacturer and provider of HVAC equipment, controls company, and implementer of all components in cannabis cultivation. We provide MEP drawings, support permitting, and coordinate with other design disciplines as needed.
August 11, 2020

When cultivators call us to discuss climate control, the first question they’re typically trying to get to is “What will it cost.” Budget development is obviously of huge importance in cultivation facility planning. However, this is a pretty loaded question and the answer will depend on a number of factors such as precision, energy efficiency, design approach, cultivator operational preferences, and desired climate parameters, to name a few. To begin to answer the budget question, we have to start with load calculations and establishing the size, or tonnage, of the overall system load. From there we can dive into the other details and get to a strong working budget to help the client decide on a design approach that fits both their HVAC needs and their budget.

Rules of Thumb Thinking Can be Fatal

The industry has historically relied on “back of the napkin” or “rules of thumb” for sizing air conditioning systems. While this could provide a reasonable guideline for very small or personal facilities, on a larger commercial scale relying on methods, instead of a well thought out and fully engineered design, can be exceptionally costly. In a small grow, needing a couple of extra dehumidifiers, or using a little more energy than you really need to, isn’t such a big deal. But at scale, mistakes like this can be devastating. Utilizing rules of thumb is acceptable for identifying very rough, order of magnitude, style budgets, but relying on them to actually design the climate control system can be a fatal mistake. The same can be said for relying on past experience to identify the climate system loading in new facilities. Everything from altitude to fertigation schedules will impact how a climate control system functions, and the output it’s capable of.

Considerations for designing climate control systems and accurate load conditions fall into two categories:

  • The direct load on the system (i.e., how many tons of mechanical cooling and dehumidification are required at any given time).
  • The performance derating of the selected equipment (i.e., equipment capacities for heat exchange are based on a very specific set of parameters, and once those parameters change, so does the output of that equipment).

There are Two Primary Considerations for Direct Load

The primary considerations for direct load in cultivation facilities are the lighting loads and the watering rates. These two categories represent the vast majority of the direct load considerations on the climate control system. The direct load created by lighting is a simple calculation (1 watt = 3.41 BTU’s of heat, regardless of the type of lighting selected). Latent heat calculations (dehumidification requirements) are far more difficult to do accurately and require the consideration of a number of variables.

Other variables include the waste heat created by standalone dehumidifiers (this does not need to be accounted for in modulating systems), geography and solar gains, and the impact of any ventilation requested by the cultivator or required by the municipality. In many cases, ventilation can be limited to “on demand” to limit this impact. Each of these will influence the total load requirements of the system and will create variance in the system size requirements from facility to facility.

Consider the Actual Output

Once system output requirements are established, the next consideration is how much output you can expect to get out of a given piece of equipment. HVAC equipment is generally rated for output based on AHRI standards, and most dehumidification equipment is rated based on the AHAM standard of 80 degrees and 60% RH. Geography will impact the output of a given system in a number of ways.
For instance, the nominal performance of all condensing units is based on specific outdoor temperatures. In hotter climates, the performance will decrease, and in cooler climates, the system’s outdoor unit might gain efficiency. This means that if the load requirements are established to be 20 tons, the system that’s installed will need to be larger than 20 tons nominal to ensure that it will provide the correct output.

Altitude and air pressure will affect the output of outdoor condensing units and possibly indoor air handling systems as well, with higher altitudes generally resulting in diminished performance vs. sea level applications.

Desire the Best, but Plan for it

Desired climate parameters will also impact the output of a system. Generally speaking, the colder and dryer the desired parameters, the less sensible and latent heat exchange the air handling units are capable of providing. The same is true of desiccant dehumidification systems, although they are generally capable of lower temperature operation than refrigerant based systems (they will still de-rate, but they may be capable of reaching lower temperatures/humidities in extreme circumstances). This means that cultivators who desire colder, dryer conditions will have to upsize their systems to compensate for the diminished heat exchange capacity, even if their direct load is the same as another cultivator working in warmer conditions. Refer to our recent blog post about the relations between VPD and HVAC for a deeper dive on this subject.

Our years of experience have taught us that accurately calculating HVAC loading for cultivation facilities looks simple on the surface, but the reality is it’s anything but. The old “1/2 ton per kW” rule of thumb may be close enough for order of magnitude budget development, but precise load calculations and budget development really need the input of an engineer. Relying on back of the napkin calculations can be costly. Luckily, we know a few engineers who specialize in this sort of thing. If you need help developing budgets or estimating electrical requirements of your mechanical system give us a call.


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