Correctly Sizing a Cooling System for your Indoor Garden

Designing an indoor garden can be complicated. Cannabis is a new industry and though growers can pull best practices from similar industries, the fact remains that setting up a commercial cultivation facility is complicated and hard to navigate for new entrants. From navigating regulatory requirements to finding an appropriately zoned facility to choosing the right equipment, nothing about this process is simple. As soon as one task is completed, another task needs attention.

Written By Celia Daly
March 9th, 2017

Designing an indoor garden can be complicated. Cannabis is a new industry and though growers can pull best practices from similar industries, the fact remains that setting up a commercial cultivation facility is complicated and hard to navigate for new entrants. From navigating regulatory requirements to finding an appropriately zoned facility to choosing the right equipment, nothing about this process is simple. As soon as one task is completed, another task needs attention.

Here at Surna, we’ve been designing controlled cannabis environments for almost a decade and are well-versed in the cultivation set up process. This involves balancing numerous variables and is as much of an art as a science; but what we’ve seen to be one of the most crucial aspects for cultivators to get right (and what so many get wrong) is properly sizing a cooling system.

The first thing anyone trying to determine the correct cooling system size needs to know is how to measure cooling. Cooling is measured in tons. Sometimes chillers will be sold based on horsepower, but this is really just an indication of the size of compressor the chiller uses to circulate refrigerant, giving you an idea of how it cools itself but not necessarily speaking to its ability to cool your space. So, it’s safer to always think about cooling in terms of tons.

The next thing to understand is how cooling works. It is not the introduction of cool air that counts, but rather, the removal of heat. Therefore, it is important to have a solid understanding of how much heat (measured in ”British Thermal Units” or BTUs) is being introduced to the space and where it is coming from so a cooling system can be sized accordingly. Most heat will be introduced to the space via electrical equipment. Generally, each watt of electricity converts to approximately 3.41 BTUs. Usually, cultivators primarily look at the number of lights in the space, as this usually accounts for the largest electrical load.

From there, it is a relatively simple calculation into tons. However, this is where we see cultivators make the biggest mistake. If we just stopped the article right here, any grower using this as a guide would go out and get a cooling system that would be simply too small. Why? Because they would be overlooking other sources of heat that have not been accounted for. As was previously mentioned, anything electrical will produce heat. Things like fans, pumps, stand alone dehumidifiers and CO2 burners have to be included in the calculation. Physical location, ambient conditions, occupancy and even heat created by plants themselves during photosynthesis are other sources of heat to consider and require precise calculations.  

Another source of heat that is often overlooked is heat created by the process of dehumidification. Any standard cooling system will dehumidify as a byproduct of cooling but when liquid condenses, it releases BTUs, adding an additional heat load to the cooling system. Adding this heat into the calculation starts to make things complicated, as it will depend on many different factors like growing mediums, target temperature and humidity levels. As we all know, indoor cultivation uses a substantial amount of water, calling for more dehumidification, and thus more heat than you would encounter in other applications like server farms, offices or homes.

As you can see, if you size your system solely based on lighting, without taking into consideration other heat loads from additional equipment, occupancy, ambient conditions and dehumidification, you will most certainly undersize your system, disrupting the indoor garden environment and potentially risking yields.

Sizing a system can be tricky for those unfamiliar with all of its different aspects. But some systems, like a Surna chilled water system, are easier to size than other HVAC options like split and mini-split ACs. Due to the nature of chillers and hydronic cooling, Surna can utilize a bank of chillers to service all rooms in a grow facility. This means that instead of trying to size each room individually, we only have to consider the maximum total heat load for the whole facility at any one time, making it more accurate.

So, how do you size your system correctly? Understanding the delicate balance between cooling and dehumidification means that sizing a cooling system on your own can be tricky. To truly be sure that you have it right, you should ask an expert with both the mechanical and cannabis-related knowledge, like Surna.

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