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My name is Walter Stark. I am a mechanical engineer designing Climate Control systems for Cannabis facilities. My company manufacturers Dehumidification and temperature control equipment for Cannabis facilities and must rely on growers for the information need to properly design a mechanical system.

Growers typically give us irrigation rates in gallons/day and we then need to divide this between day and night use in order to properly size our dehumidifier. We have been assuming a day/night split in the rate of transpiration to be 90%/10% but there are indications that night transpiration may be higher. If anyone has data to support the actual water usage, we are all ears.

Also, we offer ourselves as a resource to anyone seeking information on Climate Control for Cannabis facilities.

Walter Stark
Cannabis Climate Solutions
Div of MSP Technology
[email protected]

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I do not know the split percentages, but I can know anecdotally that the night transpiration is fairly high. During late flower I will dial our humidity down to 40%. I use 2 Qwest 150 and one Qwest 155 for 72 plants in a 1,200 sqft room with 8.5’ ceiling. Daytime while a/c is running, there is no problem keeping the humidity down, but when lights go out and a/c isn’t cycling, the humidity will peak at around 60% even with the 3 dehuys running full-tilt.
Hope this helps.
Utopia Gardens


Thank you Greg

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Hey Walter,

So what we generally see is the nightime rate to be at least 20% and if we factor in the temperature drop off it in effect turns to 50/50. The reason for this is as the lights go out for 12 hours, the temperature drops at least 10-15F and the humidity will spike due to the dew point shifting dramatically in minutes. When this happens the humidity will go from approximately 50-55% during the day cycle to 75-95% in the night. This is a major issue as molding will occur leading to powdery mildew, leading to massive crop casualties. Dehumidification during the night cycle is far more important than during the day as the A/C system will handle all the daytime duties if sized correctly. Cheers!

Brandon Kion
Excel Air Systems



Relative humidity naturally increases as temperature drops. If you have sufficient heat available when the lights go out, the humidity will not increase and slowly drop as transpiration decreases. You can then drop the temperature slowly to the desired night value.

I’d be curious to know if a sunrise/sunset lighting strategy reduces any radical behavior during switch over periods.

Do you also use VPD as a guide?



The sunrise/sunset would absolutely work for temperature/humidity control we’re looking to achieve. I’d say even doing a sunset would be the most beneficial as it takes around 15-30 mins for any given room to raise up to temperature when lights flip on but minutes to lose the heat when lights go out. Unfortunately, unless you’re adding an incredible amount of heat when lights go off or stage off your lighting the temperatures drop fast. We have a handful of clients that either operate on VPD or are looking to use the strategy but still early days… Cheers!

Brandon Kion
Excel Air Systems

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almost 65% of it comes from the first 2ish hours after lights out. of the dark cycle and your looking at a 68%-32% roughly IIRC but not sure on day to night transp. rate of total given or transpired per night/dark cycle

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