Controlling humidity during lights off

Interesting situation I have here...I run a window AC unit that is specifically for my 5x5x7 tent in that room, but when lights go out, the AC eventually just recirculates air because the temperature outside at night is low enough that it won't kick on. I know a dehumidifer would solve this easy but I can't afford one right now, so I gotta troubleshoot this with what I have. I'm on day 9 of flower and I've seen 88% humidity.

I'm assuming my options are limited to a space heater in that room to keep the AC engaged, or turning on the central heat, which would also keep the AC engaged or changing the light schedule so they are on at night.
I am running my lights during the day so I can worry less about light leaks, but I could seal the room up with a little effort. My concern is switching up the light schedule and stressing the plants.

I'd like your thoughts though. Even though this is kind of a pain in the ass situation, it's what makes it so fun. Once a week or so you get to be MacGyver.


Here's a rough idea of what's what inside my tent. Outside of the tent is a room about 12x14 that has a dedicated AC. Currently only the light is on a timer and all the fans stay on 24/7

Thanks fellow enthusiasts

gjs4786 said:

Interesting situation I have here...I run a window AC unit that is specifically for my 5x5x7 tent in that room, but when lights go out, the AC eventually just recirculates air because the temperature outside at night is low enough that it won't kick on. I know a dehumidifer would solve this easy but I can't afford one right now, so I gotta troubleshoot this with what I have. I'm on day 9 of flower and I've seen 88% humidity.

I'm assuming my options are limited to a space heater in that room to keep the AC engaged, or turning on the central heat, which would also keep the AC engaged or changing the light schedule so they are on at night.
I am running my lights during the day so I can worry less about light leaks, but I could seal the room up with a little effort. My concern is switching up the light schedule and stressing the plants.

I'd like your thoughts though. Even though this is kind of a pain in the ass situation, it's what makes it so fun. Once a week or so you get to be MacGyver.


Here's a rough idea of what's what inside my tent. Outside of the tent is a room about 12x14 that has a dedicated AC. Currently only the light is on a timer and all the fans stay on 24/7

Thanks fellow enthusiasts
View attachment 4133741

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Sounds like you need more tent air exchanged faster. What is the humidity of the room that you are keeping the tent in?
Shouldnt you have the inlet (4" from AC) just be an open (passive) inlet and let the exhaust fan pull in the fresh air through the tent as it exhausts since the room that the tent is in is air conditioned? Also, where are you exhausting the hot air from the tent to?

HydroRed said:

Sounds like you need more tent air exchanged faster. What is the humidity of the room that you are keeping the tent in?
Shouldnt you have the inlet (4" from AC) just be an open (passive) inlet and let the exhaust fan pull in the fresh air through the tent as it exhausts since the room that the tent is in is air conditioned? Also, where are you exhausting the hot air from the tent to?

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The humidity of that room at night, I haven't measured with a hydrometer, but I can say that when I walk in it feels "dank" and moist. So I can feel the humidity.
The reason I have an active air intake is because I have a 6 inch fan attached to a carbon filter pulling air in, in the tent. If I wasn't blowing air in and relying on passive intake, it wouldn't be enough. The 4inch fan attached to the active intake still gives my tent negative pressure with the 6inch at 40-50%.

While the room is air conditioned, it doesn't kick on during the night cause it doesn't get warm enough. It works great during the day, because that room is always trying to heat up from the sun. At night, humidity throughout the whole house is high, because before now, there was no need to run central air. So I have no source of low humidity air at night. Currently, I'm running the central heat (as of an hour ago) to dry out the house a little, but I feel like lights off at night is no good for this area. High humidity, lower temps, there's no fresh air exchange. If I ran the lights during the night, the light would take care of the humidity and keep the room AC engaged. That's my thinking. Regarding more tent air exchange, I don't think it would help because the room the tent is in becomes humid from the AC fan blowing with no compressor on.

I am exhausting the air straight outside through the floor 8 feet away from the tent. House is elevated with perforated skirting.

Thanks for taking time to help me brainstorm this.

If u walk in and the room feels stale and humid you probably would benefit from more ventilation as red stated, but I guess it depends on outdoor conditions. And yes if you’ve got lots of ventilation then your only hope is s dehum. Try turning the fan speed down on Ac as well, it will remove more latent heat (moisture) if it does come on. Good luck!

Budley Doright said:

If u walk in and the room feels stale and humid you probably would benefit from more ventilation as red stated, but I guess it depends on outdoor conditions. And yes if you’ve got lots of ventilation then your only hope is s dehum. Try turning the fan speed down on Ac as well, it will remove more latent heat (moisture) if it does come on. Good luck!

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Thanks. I have that AC set to Cool, 65, High fan. Lights on, perfect, lights off, temp just goes down to 65 but the humidity skyrockets. I've thought about setting it to Econ mode and auto fan at lights off, but come lights on, if I'm not there to switch it, puts my grow in peril. I will most certainly turn the fan speed down. I didn't know it would remove more latent heat. I guess that makes sense, that means moving air has a longer time to be conditioned, right? Thanks for your input!

I'm surprised no one has referred to switching the light cycle.

gjs4786 said:

Thanks. I have that AC set to Cool, 65, High fan. Lights on, perfect, lights off, temp just goes down to 65 but the humidity skyrockets. I've thought about setting it to Econ mode and auto fan at lights off, but come lights on, if I'm not there to switch it, puts my grow in peril. I will most certainly turn the fan speed down. I didn't know it would remove more latent heat. I guess that makes sense, that means moving air has a longer time to be conditioned, right? Thanks for your input!

I'm surprised no one has referred to switching the light cycle.

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Switching the light cycle will likely only be relevant if you are trying to drop temps. If the room you are pulling air from into the tent is humid, then it wont be any help to the tent either and if you are exhausting directly outside from the tent, then its seeming like the only option at that point is a dehu for the room that the tent is in.

HydroRed said:

Switching the light cycle will likely only be relevant if you are trying to drop temps. If the room you are pulling air from into the tent is humid, then it wont be any help to the tent either and if you are exhausting directly outside from the tent, then its seeming like the only option at that point is a dehu for the room that the tent is in.

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The heat from the sun kicks the AC on which dehumidifies the room. At night, no AC's are running cause its cooled down, but the humidity is in the 80's. I am thinking the culprit is that there isn't anything triggering the AC at night, so I end up with stale, humid air. If I put lights on at night, that 1000 watt HPS would take care of a lot of the humidity, and would kick the AC on too. This is just my current thinking. I'm not sure you guys understand 100% my dilemma or understand what's going on. But i sure appreciate you trying! At night: Cooler, humid as hell like 88%. All over the house. Day: Temps high 70's, 40-45% RH, even though its hotter outside, cause the sun has the AC working.
So, my tips, I can control either day or night...My current thinking, is with nothing on at night, with the cooler outdoor temperatures, it just fogs right up. If i turn lights on for that, the AC will kick i, and during the day, lights off, the sun will kick in the AC. Just m current frame of thought but I welcome alernatives

Dehumidifier
Set it to 45% and,,,,forget it...other then to dump the tank, if you don't use a drain feature..
Boom, Done!

Tent's must exhaust 24/7 to reduce RH build up's at night..

Hes trying to do it without buying a dehu, which I believe is his only option at this point.

HydroRed said:

Hes trying to do it without buying a dehu, which I believe is his only option at this point.

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Yeah, I dont expect dehu results but Im confident I can get it down 15-20 points which would do for no until I can get a dehu

some window ac unit has dehumidifier function and at day 9 it's no biggie to switch your light schedule

Xs121 said:

some window ac unit has dehumidifier function and at day 9 it's no biggie to switch your light schedule

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The dehum function is typically lower blower speed and a bit of over cool. If the room fogs up then the air coming in must have lots of moisture, you are bring it from outdoors correct? What are the lights off outdoor temps and humidity? Also be aware that humidity is based on temp and the amount of moisture that air is able to hold. At 65 your humidity would be higher but the actually moisture content could be the same . You have some time to fix it as stated, not a huge deal while still young.
Edit: most of this post is directed at OP Xs just to clarify .

Budley Doright said:

The dehum function is typically lower blower speed and a bit of over cool.

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Honestly, window AC unit dehumidifier sucks...took a long time to control humidity in a small room. When I used to live in a high humidity State, I would actually run my AC colder instead of using the dehumidifier function.

run a dehu or use lights during night hours.

If not, you would need to build some kind of split intake between the a/c & outside and this can be a bitch to setup efficiently.

SPLFreak808 said:

run a dehu or use lights during night hours.

If not, you would need to build some kind of split intake between the a/c & outside and this can be a bitch to setup efficiently.

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It really is, and kind of expensive lol. I have it set up with an enthalpy control to open intake damper and shut power to my fan coil (water cooled). My damper was $150 whole sale .

Xs121 said:

Honestly, window AC unit dehumidifier sucks...took a long time to control humidity in a small room. When I used to live in a high humidity State, I would actually run my AC colder instead of using the dehumidifier function.

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Your right they do suck unless you get a VRF unit that actually does lower capacity in dry mode. There like 10x the price though lol.

Whats the difference just cause the bulb went off, shouldnt affect humidity one single iota.

Where are you getting all this extra humidity from lights out from, maybe im just confused and your kicking in a load of moisture from an external source at lights out or maybe idk but lights on off humidity should be the same pretty much unless your fucking up badly reading somthing ...

Kingrow1 said:

Whats the difference just cause the bulb went off, shouldnt affect humidity one single iota.

Where are you getting all this extra humidity from lights out from, maybe im just confused and your kicking in a load of moisture from an external source at lights out or maybe idk but lights on off humidity should be the same pretty much unless your fucking up badly reading somthing ...

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The AC is running when lights are on so it’s dehumidifying and when lights are off the AC quits so no more dehum. But also as temps drop relative humidity goes up if it has the same moisture content. You are only reading what that air can hold at that temp with RH. It’s not a great measure really, if running using VPD chart 80-85% rh at 85f is optimal but most would freak at the 85%.

Block windows with thick black plastic to avoid light leaks. Run lights at night for best result ?
-good luck!

Budley Doright said:

The AC is running when lights are on so it’s dehumidifying and when lights are off the AC quits so no more dehum. But also as temps drop relative humidity goes up if it has the same moisture content. You are only reading what that air can hold at that temp with RH. It’s not a great measure really, if running using VPD chart 80-85% rh at 85f is optimal but most would freak at the 85%.

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My bad but for lack of trying i couldnt see why.

Just vent the air at lights out, cheap fan and hole in door or wall.

Yup that’s what was suggested but I guess it depends on what he’s bringing in. It does seem like a strange issue if he is actually ventilating enough.