Proper Intake and Exhaust for a Grow Tent


(dpontius)

Can I run the timer every 3 hours for 10 minutes, or do I have to run it 24 hours?




Answer There are three things that running an exhaust fan accomplishes when it exchanges the air inside your grow room with fresh air. First, it lowers the temperature. Second, it lowers the humidity. Third, the fresh air coming into the grow box carries with it a fresh supply of CO2, which the plants need constantly to keep growing. By taking a careful look at these three factors, both during your "lights on" cycle and again during your "dark period", I should be able to give you a very good idea of what you need to do to make sure your exhaust plan is meeting all of these needs, both day and night.

First, let's consider this with the lights on. HID lights, and even fluorescent lights, produce a lot of heat....especially when you close them up into small spaces. Temperatures of 80 degrees or above- measured below the lights, at the plant level- are bad. If your grow tent is inside your home, the ambient air temperature is around 70 degrees. As soon as the lights kick on in the grow tent, the temperature begins to rise.

In the hot spot where you should be taking your temperature readings at, it will probably reach 80 degrees in less than ten minutes. Even with the exhaust running constantly at that point, the grow room will never go below 70 degrees, because that is the temperature of the air in the rest of the house (unless you want to pump AC directly into your grow tent, which I personally have had to do before to make this kind of setup work).

In reality, it is very UN-likely that the grow room will go below 75 or 78 degrees, because you have a screaming hot light burning inside the grow tent and it is not only producing convection heat (which you can remove much of with your fan), but it is also producing radiant heat, which is warming up the temperature of the light itself, and the walls of the tent, and and the PVC that the hydroponic system is made of, etc..

Just to give you an idea of what you are up against, another visitor mailed me just last night. She only has a 105 watt compact fluorescent in her broom closet. After a half an hour with the light on, the closet was 115 degrees!

Well, with the fan running constantly during the lights on cycle, at least you won't have to worry about the air inside the tent becoming depleted of CO2....and you won't have to worry about the humidity getting too high either. However, you will most likely have a heat problem to figure out- that's why most people quit for the summer. It is much easier to do this kind of thing when you can just crack open a window and suck some cool air from outside into your grow room.

Now let's look at this with the lights off. Well, you won't have to worry about the temperature. However, plants do a LOT of growing during the night...using all of the starches that they manufactured during the day and stored in their roots. That means, if you want good plant growth, you also need to make sure your plants got some CO2 during the night. You see, plants and animals are also known as Carbon based life forms, because several components of animal cells and plant cells cannot be made without atoms of Carbon.

So the plants will not (can not) make any new plant cells (which is exactly what they NEED to do in order to grow) unless they have access to Carbon. And where does that Carbon come from? You guessed it! The CO2 in the air. Unfortunately, the level of CO2 in our atmosphere is only 400 parts per million. When the CO2 levels in the air drop below 250 ppm, plant growth stops.

When plants are confined to a small space, they can drop the CO2 levels from 400 ppm to 250 ppm very quickly. It really depends on how much plant matter and how small the air volume in the grow room is. If you want to keep your plants supplied with CO2 and growing during the dark period, I recommend running the exhaust fan long enough to completely exchange the air volume of the tent with fresh air twice an hour. How long you must leave the fan on will depend on how much air volume is inside your grow tent and also on the CFM (cubic feet per minute) rating of your fan. Step by step instructions for doing this calculation are on my exhaust fan calculator page.

By the time you are exhausting your grow tent twice and hour (or at the very least once an hour), high humidity should no longer be a problem during the dark period (which it often is). I realize this all seems like a big pain in the butt, but if the plants don't get it the way they like it, they will not do for you what you want them to do.....so really, all of this stuff needs to be the way YOU like it too. I hope this helps you out, and Happy Growing!

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Jan 02, 2014
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Can't win NEW
by: Morgan

Info 1st: I'm in the uk in jan cold01 to 04 at night into09 to 07 in the day,could snow 2morrow could be sunny that's England on a allotment I have a 6 x 8 stone floor shed( I built it brand new not long ago insulated roof 3 walls one left waiting for sum bubble wrap.i went into 12/12 2days ago I have an exhaust fan straight line to wall with many 15mil holes totally camaflaged.my Mylar tent is got 3 layers of blankets ALL over for the 12 h when my girls are asleep (daytime obviously temps!)they have a reptile pad underneath 2x 10 gal square pots 2x ak47 x Lowrider in one n aBBXWW in the other also if real cold a cold frame paraffin heater with clay plant pot on top just offset so oxygen gets in but no light the pot heats up lovely and my gals love the co2 ! Right I've explained my set up my problem is I veged at 250w MH I'm flowering at 400 dual spec HPS (digi ballast 250w400w600w n 600+)so this400 has my extraction fan on all night I have a thermostat but it couldn't get cool enough I have it half way up the tent on pole(advice pls) I opened one flap still no goodso 2nite I've loosened the blankets a made plenty of air space outside the flaps I only have one low hole in the bottom but it's full off wires ideas pls boys n gals? Ps on the coldest day that 10w reptile mat never let's the roots go under17c but usually 19c-21c best buy so far!

May 13, 2013
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keep good adjusting trends NEW
by: Anonymous

the Its polar environment company notice. Anybody today wishes to keep good adjusting trends along with this distinct brand name folks may be current with all the manner structure Knockoff Utah Leather.

Feb 04, 2013
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Cooling ? NEW
by: Smitty

I have 4- 1000 watt xxxl hoods powered by 4- Solis-tek digital LCD display digital ballast 3- bassass blades osolating fans, c02 tank in a sealed 12x10x8 room in the basement during the winter it about 38 degrees in the basement. i'm getting ready to set my equipment up in it and was wondering if i have the 4 hoods daisy-chained together with 8'' duct and a 1200+ cfm inline fan sucking in and blowing into house for heat''during winter''outside during summer and also a 12,000 b.t.u a/c will with what i have here take care of my heating problem and at night will i have to have a heater too and if so what size. also have an autopilot master controller to run everything together...have never used one before so that's why i'm asking about this......thanks

Feb 03, 2013
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Help needed NEW
by: Anonymous

Hi I have a 9m2 tent 3x3x2 and I am looking to use nine lights inside it 600 watt per square metre. I have two 8" td silent fans hanging neatly and they meet into a y joint before leaving the tent with 8" duct I have a regulator on my fans too I believe I have 1600m3 of air flow an hour 800 on each I need to know if this will work ok as I know it will be very hot with 9 x 600 watt hps burning. I have no air in jus plenty of flaps on my tent I have four lights on now covering my 19 plants in 18ltr pots I am going to repot these to 30 litre pots an make them trees before switching. Do you think my temp will be ok without an intake ?



Feb 03, 2013
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starstarstarstarstar
Help needed NEW
by: Anonymous

Hi I have a 9m2 tent 3x3x2 and I am looking to use nine lights inside it 600 watt per square metre. I have two 8" td silent fans hanging neatly and they meet into a y joint before leaving the tent with 8" duct I have a regulator on my fans too I believe I have 1600m3 of air flow an hour 800 on each I need to know if this will work ok as I know it will be very hot with 9 x 600 watt hps burning. I have no air in jus plenty of flaps on my tent I have four lights on now covering my 19 plants in 18ltr pots I am going to repot these to 30 litre pots an make them trees before switching. Do you think my temp will be ok without an intake ?



Dec 25, 2012
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question for you??? NEW
by: jay lows

Hello my name is jayson.. I'm going to grow in a 10x10x7.. I am going to us 4 six hundred watt lights. Each in double sets hanging in the room and bothe will have exhaust fans,and room will have 3 oscilating fans. Does this sound like it will work..?

Dec 15, 2012
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temp and rh problems NEW
by: jules

ive got a 1m by 1m by 2m grow tent,temp getting up to 85 and rh only 25 when lights and fans,and extracter on its tucked away under the steps in my cellar,when everything is off temp dropping to 55 and rh about same,is this ok for my plants,there in veg at the moment,gonna be putting an intake fan in today any comments would be appreciated.

Dec 15, 2012
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temp and rh problems NEW
by: jules

ive got a 1m by 1m by 2m grow tent,temp getting up to 85 and rh only 25 when lights and fans,and extracter on its tucked away under the steps in my cellar,when everything is off temp dropping to 55 and rh about same,is this ok for my plants,there in veg at the moment,gonna be putting an intake fan in today any comments would be appreciated.

Dec 04, 2012
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intake/exhaust NEW
by: Anonymous

to clear that up for you,basically the plants go through "photosynthesis" during the dark cycle,so it's important to regulate your temperatures AND humidity levels along with your air intake/exhaust during this time.during the summer months this may be tricky due to higher climates.basically how i have mine set.this time of year(early winter) i set my intake to constantly run for fresh air.i exhaust the air once every 4 hours just to maintain my temp/rh(relative humidity) levels.during the light cycle my intake/inline is constantly running to disperse of any heat coming from the equipment ex:light hood(s),ac unit,etc... while at the same time,i have 2 clip on ocsillating fans in 2 opposing corners of my tent.my tent is approx. 4'wX8'LX7'H.so about 225 sq. feet.you have to keep in mind,the air movement of your fans.if you have a fan that runs at 200 cfm(cubic feet per minute,you shouldn't have to worry too much about fresh air intake.only issue here,is how often to exhaust the air.this also depends on your temps and rh levels as well.pay close attention and monitor with detail.in the cooler months,you can exhaust once every few hours and run the fan for about 30 minutes or so.every setup is different so it's hard to say for sure.if you have objects in the tent as well as your lights,fans,plants,these all put off heat.the light will also heat up the temperature of the water if running hydro/aeroponics,but will also cook the top layer of soil which is why its important to keep that puppy moist...not wet,but moist.i mist during the dark,as well as feeding schedule.reason being is that light radiates off of the water molecules.thus creating excess heat/moisture which is NO GOOD and at the same time will put excess heat to the plant causing damage to the leaves which is what you want to avoid.water/feed at night,or whenever you have the lights kick off.early mornings are ok,or at least 2-3 hours AFTER the lights turn off is suffice.with lower humidity,you might notice you have to water more frequent,this is due to the dry air.you want to keep your plants at approx 35-45% RH during the flower stage.temps should range anywhere in between 70-80F anything above 80 will cause issues with the growth.hope this clears things up a little bit...i tried lol.it's much easier doing this in person rather than trying to explain.sorry if this seems a bit scrambled or off topic in any way.

Nov 14, 2012
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Hmm? NEW
by: Anonymous

Plants use O2 at night and co2 during the day. There is much more O2 in the environment I believe it is about 20% so you should be able to extend the time between air exchanges during the dark cycle compared to less time during the day. I'm continuously exchanging air. I'd to would like to cut this down at night but I'm not sure how often to do it. I do not want to do the bare minimum. Why ride the line if you don't NEED to. It's been my understanding that without co2 enrichment you should be fully exchanging all your air in your tent/area no more than every five minutes. Twice an hour might work during the dark cycle but as I understand it that won't cut it during the light cycle. Please clear this up for me.

Nov 14, 2012
Rating
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Hmm? NEW
by: Anonymous

Plants use O2 at night and co2 during the day. There is much more O2 in the environment I believe it is about 20% so you should be able to extend the time between air exchanges during the dark cycle compared to less time during the day. I'm continuously exchanging air. I'd to would like to cut this down at night but I'm not sure how often to do it. I do not want to do the bare minimum. Why ride the line if you don't NEED to. It's been my understanding that without co2 enrichment you should be fully exchanging all your air in your tent/area no more than every five minutes. Twice an hour might work during the dark cycle but as I understand it that won't cut it during the light cycle. Please clear this up for me.

Aug 07, 2011
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at night cycle exhaust fan sucks cool air out making ac work all nite
by: Anonymous

hi can i put exhaust on a timer say 15 min on and off all nite so my ac doesnt work so hard and exhaust is on lowest setting

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