Using exhaust to control temperature

I am curious what problems I could encounter if I use my exhaust fan to control the temperature inside my tent.

- grow space is my basement which includes a 5x5 tent for vegging and a 10x10 "room" for flowering.
- Ambient temp in the basement right now (northern IL) is 71-72F/62%RH. I have a large dehumidifier ready for the flower cycle. That temp drops down to 60-65 during the winter.
- Most all of my seeds are regular, so I am trying to follow Jorge Cervantes' advice in "Marijuana Horticulture: The Indoor/Outdoor Medical Grower's Bible" for getting more females from seed: lower temps, higher humidity (75%) and longer night cycles right off the bat. My light cycle is 14/10.
- Lights are one Mars TS1000 and one HLG 135. I've got a blurple Viparspectra "V600" if needed but I'll be picking up some more QB's from Alibaba for flower.
- I have plenty of fans inside the tent. I have a large exhaust fan that is on a speed controller
- I have an inkbird temp/humidity controller: Link
- Medium is sohum and purple cow indicanja ultimately going in 7 gallon fabric pots


So here is my problem as I see it: I've only had my TS1000 in the tent up until yesterday. However, now that I am transplanting into larger pots, I decided to hang my other light due to the larger area needing light. Doing so raised my temps from ~75F to over 80F. Also prior to doing this, I was using the exhaust fan as the RH control where the humidifier would run 24/7 and the exhaust would kick on when RH got over 75%. The other kicker is that I work from home in my basement and I don't have an office, so opening all the vents on the tent to get more ambient air inside won't work without also having something to block the light on those vents.

- If I leave everything as is, temperature skyrockets during lights on.
- If I use the exhaust to control temperature (and the humidifier on the RH side, shutting off at 75%), not only does it suck ALL the moist air out and causes huge fluctuations in RH, it also doesn't do anything at lights off.

My question, I think, is: what are the ramifications of not doing ANY air exchange during lights off? As I said, I am pretty confident I have sufficient airflow inside the tent so I am not really worried about mold, but this is only my second grow and my first grow of 2 cannabis plants is the ONLY other plants I've ever grown.

Hopefully this longwinded post contained enough detail.....

So you have exhaust fans but no intake fans bringing in fresh outside air? Without an intake your exhaust is just circulating back into your room and grow. Intake fan was the biggest temp problem fix I have ever used.

Oh also your temps are much more important than managing humidity. With proper air exchange humidity would not be able to get too intense with constant change of air and air flow throughout.

Depends on how big your basement is and if it gets AC and heat down there.

I have a similar set up. A tsw2000 (305W) in a 3x3 increases the tent temperature by 6F-8F. 400cfm extractor fan and filter. No intake fan. Passive intakes only. Running the system only raises the ambient temp of the room by 1-2F. I have air ducts in the basement.

Sade said:

So you have exhaust fans but no intake fans bringing in fresh outside air? Without an intake your exhaust is just circulating back into your room and grow. Intake fan was the biggest temp problem fix I have ever used.

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I guess I did leave something out: I have one vent open that has a dark cloth over it. It is on the opposite side of the exhaust.

Sade said:

Oh also your temps are much more important than managing humidity. With proper air exchange humidity would not be able to get too intense with constant change of air and air flow throughout.

Click to expand...

Right which is why I asked because if I leave the exhaust controlling the temp, then there is little to no air exchange during lights off. Is air curculating sufficient even if the only control I have for high RH during lights off is simply having the humidifier shut off at 75% which means nothing to stop it from going much higher than that?

Shape Shifter said:

Depends on how big your basement is and if it gets AC and heat down there. .

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It's something like 1200sf and is not "finished" (exposed foundation and concrete floors). Oddly enough, the HVAC does not vent down here. I own the house and can build on as I see fit, so a proper grow room is in the works but I am finding that better ideas of how to use the space and what I want in a grow room are coming to me as I grow more so I am holding off on implementing a permanent structure. My 10x10 flower area is really just some panda film hung from the floor joists.

LawnXweeds said:

My question, I think, is: what are the ramifications of not doing ANY air exchange during lights off? As I said, I am pretty confident I have sufficient airflow inside the tent so I am not really worried about mold,

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Stink will creep out at night. And mold is still a concern even if you think it isn't. I let the fan shut off for 6 hours during veg. Always on during flower.

LawnXweeds said:

It's something like 1200sf and is not "finished" (exposed foundation and concrete floors). Oddly enough, the HVAC does not vent down here.

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In that case, the heat will just keep creeping upward in the summer. You will be fine in the fall and spring. Might get a little too cold in the winter. It's best if you can run the fan 24/7. Your humidity shouldn't get below 40% with the lights and fan on. It will rise at night.

Shape Shifter said:

Stink will creep out at night. And mold is still a concern even if you think it isn't. I let the fan shut off for 6 hours during veg. Always on during flower.

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I'm not concerned at all about smell. We grow legally and the people that come around know we grow AND we all enjoy the smell. That said, i am going to be venting outside because venting humid air back into the basement is less than ideal.

If you shut the fan off, the humid air from the outside ends up in the tent. Another reason to leave it on 24/7.

Shape Shifter said:

In that case, the heat will just keep creeping upward in the summer. You will be fine in the fall and spring. Might get a little too cold in the winter. It's best if you can run the fan 24/7. Your humidity shouldn't get below 40% with the lights and fan on. It will rise at night.

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Yea, the worst I've seen the RH drop is still above the ambient RH of the basement but as mentioned, i am trying to keep it mid 70's in an attempt to promote for females plants. At any rate, this was really all the convincing I needed to ensure the exhaust is running more. I will find other ways to control the other factors.

LawnXweeds said:

I am curious what problems I could encounter if I use my exhaust fan to control the temperature inside my tent.

- grow space is my basement which includes a 5x5 tent for vegging and a 10x10 "room" for flowering.
- Ambient temp in the basement right now (northern IL) is 71-72F/62%RH. I have a large dehumidifier ready for the flower cycle. That temp drops down to 60-65 during the winter.
- Most all of my seeds are regular, so I am trying to follow Jorge Cervantes' advice in "Marijuana Horticulture: The Indoor/Outdoor Medical Grower's Bible" for getting more females from seed: lower temps, higher humidity (75%) and longer night cycles right off the bat. My light cycle is 14/10.
- Lights are one Mars TS1000 and one HLG 135. I've got a blurple Viparspectra "V600" if needed but I'll be picking up some more QB's from Alibaba for flower.
- I have plenty of fans inside the tent. I have a large exhaust fan that is on a speed controller
- I have an inkbird temp/humidity controller: Link
- Medium is sohum and purple cow indicanja ultimately going in 7 gallon fabric pots


So here is my problem as I see it: I've only had my TS1000 in the tent up until yesterday. However, now that I am transplanting into larger pots, I decided to hang my other light due to the larger area needing light. Doing so raised my temps from ~75F to over 80F. Also prior to doing this, I was using the exhaust fan as the RH control where the humidifier would run 24/7 and the exhaust would kick on when RH got over 75%. The other kicker is that I work from home in my basement and I don't have an office, so opening all the vents on the tent to get more ambient air inside won't work without also having something to block the light on those vents.

- If I leave everything as is, temperature skyrockets during lights on.
- If I use the exhaust to control temperature (and the humidifier on the RH side, shutting off at 75%), not only does it suck ALL the moist air out and causes huge fluctuations in RH, it also doesn't do anything at lights off.

My question, I think, is: what are the ramifications of not doing ANY air exchange during lights off? As I said, I am pretty confident I have sufficient airflow inside the tent so I am not really worried about mold, but this is only my second grow and my first grow of 2 cannabis plants is the ONLY other plants I've ever grown.

Hopefully this longwinded post contained enough detail.....

Click to expand...

My turn to apologize for the long winded post

I have a relatively similar situation, but with wildly fluctuating humidity at different times of year, and winter temps that get to 50F in the winter (my basement is unheated unless I need to burn my woodstove).

I'll show a couple of pics to explain, but I need to lead into some background first. I actually blow the warm air from my veg tent into its accompanying flower tent when the temp in veg reaches 79F. When this happens, the exhaust in the flower tent ramps up to handle the excess intake air. When veg fan turns off, the flower exhaust will wind down to re-stabilize at 78F.

To handle the veg to flower ventilation, I have a piece of duct at one end of a cooltube bent and pointing upwards. On top of the open end, I have a filter, then a 120V 6" fan that blows the air from veg to flower, through the ducting on the other side of the tube. This fan only turns on at specific temperature or humidity levels:



From veg to flower. I reduced the 6" duct to 4" simply because I ran out of 6":



To protect light from entering the tent from veg, I put a 5/8" plywood floor, elevated to exactly the top of the passive intakes. The elevated platform also keeps the plants off the ground (even though I have insulation under it), and with warm air from veg underneath, keeps the pots warmer. The vent as seen in the pic above goes underneath of that floor. At the front and back of said floor, there's a 3" gap for the veg and passive intake air to enter the growing area of the tent (note how this side's passive intake flap is closed):



The other side of the flower tent, showing the passive intake flap open on top of the board, allowing air intake while blocking light:



Those are banana boxes under the wood platform. Ran out of cinder blocks.

Regarding allowing the exhaust to shut down, I designed the controller to balance temp and humidity, even if it means turning off all exhaust for periods of time. However, because air circulation is still important, the controller will kick on both veg fan and flower exhaust on for one minute out of every 15.

Here's how things look down there right now:



This is high humidity season here, but its declining. For two weeks I had 90%+ humidity in the flower tent at lights off. It's all a balancing act, but so long as you have extremely good airflow, and plants properly lollipopped and spread out, things work out well.

spek9 said:

To protect light from entering the tent from veg, I put a 5/8" plywood floor, elevated to exactly the top of the passive intakes. The elevated platform also keeps the plants off the ground (even though I have insulation under it), and with warm air from veg underneath, keeps the pots warmer. The vent as seen in the pic above goes underneath of that floor. At the front and back of said floor, there's a 3" gap for the veg and passive intake air to enter the growing area of the tent (note how this side's passive intake flap is closed):

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Holy crap thank you for this amazing idea.

Shape Shifter said:

If you shut the fan off, the humid air from the outside ends up in the tent. Another reason to leave it on 24/7.

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Yep best to just leave fans running and get them praying haha.

.

LawnX I do pretty much what you're looking to do, I use a Temperature/Humidity switch to keep temps near 85 F during lights on and 72 F during lights off.

The switch also goes on if the humidity gets above your set limit.

.

HT-2 humidity tem controler


.

Hobbes said:

LawnX I do pretty much what you're looking to do, I use a Temperature/Humidity switch to keep temps near 85 F during lights on and 72 F during lights off.

The switch also goes on if the humidity gets above your set limit.

HT-2 humidity tem controler

HT-2 Humidity Temp Controller

Maintaining proper temperature and humidity in your garden is crucial to maximizing yield. The Grozone Control HT-2 Controller keeps your garden’s temperature and humidity properly maintained through all plant life-cycle stages. Simply plug the ventilation fan in to the HT-2 controlled outlet...

www.blackdogled.com

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That looks great and I've been kinda wanting something that does 2in1 on a single outlet but holy crap, $200 is a bit much. At any rate, I've done a bunch of tweaking with my tent and in doing so, I failed to realize that my humidifier is warm mist so that may be contributing just a tad. The next thing I am going to do is pick up a ultrasonic fogger/mister so I can have cool humid air. I doubt it's going to get my temps where I want, but it will help.

I've even switched out my HLG for my blurple and mounted the driver for the Mars outside my tent. Temps are still >80F during lights on...... I may just have to say fuck it and live with the fact that some of these seeds are going to be male because the ONLY reason I am concerned about any of this is because I am trying to promote more females.

LawnXweeds said:

My question, I think, is: what are the ramifications of not doing ANY air exchange during lights off? As I said, I am pretty confident I have sufficient airflow inside the tent so I am not really worried about mold, but this is only my second grow and my first grow of 2 cannabis plants is the ONLY other plants I've ever grown.

Click to expand...

Airflow won't reduce RH%, it prevents stale spots from having higher RH% than other spots but that is all. If the RH% gets too high in the tent then mold / mildew will happen. So venting during lights off is key when you have sufficient biomass in the tent transpiring humidity. If you MUST stop venting when the lights are off then wait at least 30 minutes after the lights go out because the RH% spikes during this period.

LawnXweeds said:

reason I am concerned about any of this is because I am trying to promote more females.

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Female / male is genetic.

If you want females get some feminized seeds.
Plus try to keep it simple.