Ventilation and nighttime temps

I’ve got something going on with my ventilation I’m not sure how to resolve.

Preface: I moved House over the pandemic and have my new grow space all set up in the basement.

Here’s the issue - my fan controller is set to kick on if RH is above 60% or temp is above 79F. During the day temps are consistently 78-79 which I’m fine with and humidity is 45-50. Overnight the light is off and the room cools down as you’d expect and the exhaust fan shuts off. Then the RH starts to creep up. Then the exhaust fan kicks on bring the RH down and in doing so pulls in more cool air. Then start the vicious cycle of getting cooler and cooler and as the air cools the RH doesn’t drop.

By the morning it’s like 63° in my tent. Obviously not a big deal for the temp to be dropping into the 60s at night but it’s like does my fan really need to be chugging along all night long??

I realize how relative humidity works and the fact that the air is getting cooler just means that it holds less moisture and so for the same amount of water in the air at a given temperature my relative humidity is either going to go up or at least remain constant as the temperature continues to drop.

The most obvious solution to me is a fancy your fan controller that I can program for day and night modes but makes me wonder if I am missing a simpler solution

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You can try running your lights at night and off daylight hours see if that might help you a little. Also a framed out area in basement an control temperature and humidity in the room you’re pulling the fresh air from. Just my thought good luck !

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Both good ideas.

I can certainly try to shift the light schedule for the next go around. Presently I am 3 1/2 weeks into flowering. If you know of any good way to make that sort of change in the middle of flowering I am all ears. I suppose I could try to shift the light timing by half an hour every few days and eventually end up with the opposite schedule online now but I sort of feel like by the time I get there it’ll be harvest time :sweat_smile:

The slightly more detailed description of my basement is that it is a finished basement with a dedicated 5 x 10‘ grow room. In the grow room I have a 5 x 3 tent. I don’t exhaust the entire room, just the tent. That tent is ventilated with a AC Infiniti 6 inch with their smart controller which is vented to outside. Just Passive intake into the tent

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To switch light cycles just prolong youre dark period and kick lights back on at what ever time you wish to switch to the longer dark period wont hurt the girls and also maybe buy a dehumidifierand set it in the tent or lung room that you can set to say 55 or 60 rh that’s can get a bit expensive depending on the size you buy


If I was making the swap i would just go 24 hours dark and swap it over. I also have framed out room with tent inside space. I run my exhaust fan 24/7 and exhaust outside. I control temperature in room when needed.


Plug your fan into a digital timer that is on when lights on and off at lights off so can’t run 10.00 fix from harbor freight or run a handi heater or both.

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