Ventilation/Fan During Lights Out Question

Hello,
I have a new setup I am trying to dial in and had a question around the exhaust fan while the lights off. I have browsed here for years and tried searching but could not find anything specific enough. If anyone knows of anything I missed, a point in the right direction would be greatly appreciated.

Flowering tent:
3x3x6
600w Dimmable
400 CFM fan

Airflow is your basic setup: carbon filter inside the tent, air being pulled through the filter, through air cooled reflector and out the tent. Passive intake with a fan inside for air movement.

My question is if I should have the exhaust fan running (the whole time) while the lights are off. During lights on, I maintain about 79-80 degrees. With lights off and exhaust still running, that can drop down to 62-66. Is it worth setting up a separate timer for the fan to have it shut down periodically during lights off - while having it turn on every now and then to exhaust air? I do not currently have the ability to automatically lower the fan speed during lights out. Would alternating exhaust fan on/off during lights off have any positive or negative effects, CO2, etc (not running CO2)? I plan to keep the fan inside the tent on always just moving around, but it would not be exhausting air.

Thanks in advance.

I personally never turn my exhaust off. My lights out temps vary greatly depending on the season. I keep them in check by adjusting the temps in the adjacent room which it takes its air from.

Maybe you can try putting a dimmer on the fan if necessary? Air exchange is pretty important, so I wouldn't recommend turning it off (some growers leave fans off for lights out).

Another option is to cycle a small heater on/off during lights out in the winter. This is what I do. Usually 5 mins on/ 10 mins off. FYI, i used to ignore lights off temps in the past, and they would get as low as 50 degrees. I now keep night time temps btwn 65-70, and there was definitely an improvement in my winter grows.

Good luck

Hi Marlo,
Thanks for the response. I have not adjusted the exhaust fan myself either in previous grows. I would agree that constant air exchange is important. Your confirmation on how the temperature swings impacted your runs when that was involved is something I expected and is beneficial to hear from your true experiences. I have had troubles with high temps in general in the past, but this is my first time with this wide of temp swings.

I will focus on normalizing my lights out temps and explore what options I have. I am not present for most lights out periods so automation will be key.

Appreciate the insight!

I'd run it 24/7 wouldn't want humidity to climb very high with lights out.

Keep the fan on, and run a dehumidifier in the room the tent is in. That will help to get rid of the moisture the plants transpire during lights out, and helps to keep lights out temps higher. 10 degree swings between lights on/off delivers best results.

wakislander said:

Hello,
I have a new setup I am trying to dial in and had a question around the exhaust fan while the lights off. I have browsed here for years and tried searching but could not find anything specific enough. If anyone knows of anything I missed, a point in the right direction would be greatly appreciated.

Flowering tent:
3x3x6
600w Dimmable
400 CFM fan

Airflow is your basic setup: carbon filter inside the tent, air being pulled through the filter, through air cooled reflector and out the tent. Passive intake with a fan inside for air movement.

My question is if I should have the exhaust fan running (the whole time) while the lights are off. During lights on, I maintain about 79-80 degrees. With lights off and exhaust still running, that can drop down to 62-66. Is it worth setting up a separate timer for the fan to have it shut down periodically during lights off - while having it turn on every now and then to exhaust air? I do not currently have the ability to automatically lower the fan speed during lights out. Would alternating exhaust fan on/off during lights off have any positive or negative effects, CO2, etc (not running CO2)? I plan to keep the fan inside the tent on always just moving around, but it would not be exhausting air.

Thanks in advance.

Click to expand...

Lights off temps are a function of the environment around the tent, basically the same w/ the fan on or off. If you don't want to add a dehuey then you want some air moving thru at all times. That doesn't need to be a lot but some for sure.

I use two stage ventilation for that reason- see the ventilation 101 thread. Alternatively, you can make the fan run slower during lights out w/ a fan speed controller & a flipflop relay to extend filter life.

DoubleTripleOG said:

Keep the fan on, and run a dehumidifier in the room the tent is in. That will help to get rid of the moisture the plants transpire during lights out, and helps to keep lights out temps higher. 10 degree swings between lights on/off delivers best results.

Click to expand...

I never understood this shit.. because lol outdoors.. day temps could be 100.. and nights at 65-70 lol. and plants seem to love it..

lol iuno why it matters but ya I heard 10 degree swing is best..

Your best bet would be a thermostat. I bought a cheap one on amazon for $5 and wired it up with a $20 VARAC speed controller. It runs on the speed controller when it is cool enough (lights on or off) and when it gets too hot the fan runs full speed.

Grozone makes a controller called the TV2,
it's a fan speed controller but with a thermostat control and it has day and night options. You can set it to turn your fan on full power at a set temp, below that point it will run on a lower speed (25-60 percent, depending on where you dial the knob).
On one setting it will turn your fan completely off 10 degrees below the setpoint.

There's a couple companies that make similar controllers, and there is also a guide on here for a "DIY thermostatic fan speed controller" in the DIY link o rama.

I just speak on this particular one because I use it and depend on it to turn my exhaust of to not freeze my plants. It's like 90 bucks but very worth the price if it's what you are looking for and it sounds like you are.

if your room is small, an oil filled radiator type heater works great on a timer set to turn on during the dark. you have to tweak the dial on it to put out the right heat you want, but they work great and are like 40 bucks at big box stores, even cheaper at thrift stores

Stay away from space heaters IMO, they eat electric juice and dont actually do much.


If you have a larger space, I can recommend a propane space heater rated for indoor use, thanks to DoubleTripleOG for recommending that one, it saved my space with the combo of that and the TV2 controller. there are a lot of options, so shop around if you are going the propane heater route.

I have my exhaust and intake fans set up on a zephyer 1 along with a heater on a separate sephyer 1 made by titan controls. I'm able to control my humidity at night (30-50% depending on outside weather) because my exhaust fan turns on about every 15-20 minutes and the heater keeps my room between 64-66°F. I plan on getting a Saturn 6 by titan controls to help my ocd (it's digital and a little more tunable)and because it will allow me to add co2 during light on

I use a temp controller from ebay

that keeps it all in check

also use a small fan on a 15 min timer

and avoid the worst of winter by not growing lol

cheers/

I setup a lights out exhaust.. that only slowly takes are out of the room so it doesn't need to be heated..

got on a speed controller and a timer.. once my lights kick off.. the light exhausts and the other 2 carbon filter exhausts turn off.. and the single turns off.. keeps my humidity perfect.. keeps temps great.. and keeps fan noise down during the day time lol .