LoveGrowingIt said:

About air exchange, especially for smaller tents... There's that never-ending dilemma about heating and humidifying the air in a tent, only to pump that air out. I can't think of a good reason to do that. Devices working against each other aren't good for the electricity bill. So, I either run the exhaust fan on low or cycle it on and off, depending on the size of the tent, and the size and life-stage of the plant(s) in the tent. I have one 28"x28"x74" tent that always runs hot, so that tent's exhaust has more work to do. I've never figured out why that tent is so hot all the time, even with different lights. I even moved the driver out of the tent, and it didn't make a difference.

As for VPD, I check it often, but I don't use it to control my tent's environment. The VPD is often the first thing I look at because it's a quick way to check the tent's internal environment, and I target specific VPD ranges. If it's out of range, I tweak the temperature or the humidity.

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PooToe said:

Moving moisture, heat and expelled gasses............

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LoveGrowingIt said:

Removing excessive heat is probably the best reason to exhaust air. I doubt there's much impact regarding gases. The main gases are oxygen and carbon dioxide that are exchanged through transpiration and respiration. Both are expelled by and used by the plants. As for moisture, if the humidity is in the desired range (and there are circulation fans in the tent), I don't foresee a moisture problem.

I just think exhaust, which is better thought of as air exchange, tends to be overemphasized. If the air in the tent is replaced several times a minute, why even bother having a heater or a humidifier in the tent? At that point, we should be talking about the lung room environment, which tent controllers don't directly control. So, aside from controlling excessive heat, my thinking is that the key factor for a tent environment is maintaining a relatively low rate of air exchange. I don't know what that rate should be, but I suspect it depends on the size and life stage of the plants in the tent. I doubt replacing the air several times a minute is necessary.

Every situation is different, though, which makes it impossible for me to say precisely what's best for someone else. My largest tent only has 48 cubic feet of air without adjusting for the space taken up by the things in the tent. Even a 4" fan can pump a lot more than 48 CFM.

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