Here are some basics to consider when setting up a kick ass ventilation system for your grow.

Reasons to Have Good Ventilation

  • Humidity control (40-80%, depending on stage)
  • Prevent mold and mildew
  • Temperature control (75 degrees)
  • Keep all your equipment cool to avoid fire hazards
  • Stabilize harmful gases
  • Control air-flow
  • Control odor (using a carbon filter)

Ventilation System

Exhaust Fan

When purchasing an exhaust fan to vent your grow room aim to clear the air out at least 1x every minute, average 3x a minute, and up to 5x times a minute if your room is hot. How often you want to clear the room depends on:

  • How hot your grow room is
  • The temperature outside and how air-tight your room is
  • If you are using a carbon dioxide system
  • How big of a fan you can afford (lets be real)

Length x Width x Height / (Times per Minute, 1-5)

So for my room the calculation is my room size, 14 feet long, 7 feet wide and 7 feet high, with a goal of clearing the air every 3 minutes:

10 x 7 x 7 / 3 = 163

I personally use the Hydrofarm Active Air Inline 165 CFM fan with 4″ and 6″ ducting and oscillating fans. This size fan is perfect for my room according to the math above.

Exhaust Fan Timer

You may want to set your fan on a timer for a few different reasons:

  • Turn off your fan during and after use of your carbon dioxide system
  • Turn your fan on and off to regulate temperature (and minimize cost and on time)
I personally feel that having the fan on 24 hours a day is overkill for a smaller grow room. 15 minutes on and 15 minutes off is what I am trying for this grow. Besides the 24 hour recommendation I have also seen using the temperature of your room as a guide. Here is how that works:
  1. Turn your exhaust fan on until your room has reached the ideal temperature, about 75 degrees
  2. Turn it off and see how long it takes the room to get to the highest temperature you are comfortable with, for me that is 80 degrees. Record how long this takes – it will be your interval
  3. Set your fan interval timer for your measured amount to get the minimum amount of time between turning your fan on and off
  4. Leave your fan on for at least 5 minutes, up to 15 minutes

Intake System

A passive intake system (one without a fan) is probably fine for almost all personal grow rooms that are medium sized or smaller. This is just a duct from outside to the bottom of the room to allow in fresh air from outside.

If you are fancy you can have an active intake system using a small computer fan to regulate how much air comes in.

Temperature & Humidity

  • Ideal temperature is 72 – 76 degrees, ideal range is 60 to 85 degrees
  • Try to not let your day and night temperatures vary by more than 5 – 10 degrees, no more than 15 for sure
  • Ideal humidity is 50% to 65%, ideal range is between 40% and 80%. Depends on stage:
    • During the vegetative stage the ideal humidity is 60-65%
    • During flowering the ideal humidity is 50%
  • Over 120 degrees and below 30 degrees can kill your plants
  • A carbon dioxide system will increase both the heat and humidity in your environment
  • If your plants develop a “lettuce leaf edge” they probably were exposed to excessive heat and/or humidity

You can increase the humidity in your room by spraying water into the air, leaving a bucket of water out to evaporate, or by using a humidifier. To decrease the humidity use a dehumidifier.

Cost of Running the Fan

In case you are curious, here is a quick way to calculate the cost of running your fan:

Wattage (per hour) x Hours On Daily = Watts per Day

Watts per Day / 1000 = Kilowatts per Day

Kilowatts Per Day x Electricity Rate (kWh) = $Cost Per Day

So my fan is rated at 113 watts per hour:
113 x 24 = 2,712 Watts per Day
2,712 / 1,000 = 2.712 Kilowatts per Day
2.712 x $0.07722 (current electricity rate) = $0.20 Per Day
So monthly this particular fan costs $6.28 a month to run.

Fan Muffler/Silencer

Once you get your awesome fan you may notice that it is much louder than you expected and that it is annoying the crap out of you. Getting a fan muffler can fix this.

When researching them I found that they cost about $70.00 to $100.00, but you can also DoItYourself. I choose the DIY route and it ended up costing about $50.00 anyway:

  • 2 foot piece of ducting – $10.00
  • 2 Reducers – $15.00
  • Hardware cloth – already had, $6.00
  • Poly-fill batting – $15.00
  • Metal foil duct tape – $5.00
  • Scissors, Wire cutters, Pliers – already had
  • Duct crimpers – went without and mimicked the crimping from the intact side using needle nose pliers

Tip for doing it yourself, make sure the ducting from the fan to the muffler is as short as possible, this is where the sound escapes. You can see in the picture that I actually duct taped mine to get it as close to the fan and muffler as possible to plug up this noise leak.

Looking back I should have just purchased one, but it does work rather well. The DIY muffler lowered the decibals level from 78 to 60, a significant decrease. Before I installed the muffler you could hear the fan when you walked by outside, not loudly but you might wonder what it was. Now when you walk by you cannot hear anything. Even if you try really hard to listen for it, the sound is just gone.


Buy the largest size ducting and fan you can afford BUT that is also necessary for your sized grow room. A medium to small size grow room probably does not need ducting larger than 6″. A few tips and things to consider regarding ducting:

  • Minimize the length, amount of curves, connections/parts and other seams in your system by using careful planning. These things can significantly reduce airflow
  • Ensure your ducting does not cause light leaks and try not to puncture it
  • Use screens on ducting that goes outside to keep your ducting clean
  • Use wire cutters to cut the wire in the ducting when trimming pieces
  • Careful with the wire wrapped in this ducting, it WILL try to hurt you when you cut it or there is an exposed end

Oscillating Fans

Use at least one oscillating fan to circulate the air around your plants and simulate the wind it would get outside. I recommend using 2 to 3 fans for a 10 x 10 room. These have many benefits:

  • Simulates the natural environment
  • Encourages stem and branch development
  • Evens out the airflow from your ventilation system

I use one on either side of my tray, keeping them just above the canopy. At first you might start with a low setting and increase the speed as the plant develops a stronger root system. You shouldn’t turn the fan on so high that you are causing visible damage or causing the plant to uproot, but it can be very vigorous.

From my research this causes micro tears on the stems and branches that are then forced to heal themselves, causing the plant to be stronger and thicker so that it can support more bud. Plants naturally adapt to wind.


  • Hang your fans using cabling or rubber straps to minimize vibration (as seen in the pictures above).
  • The intake is a good place to install a temperature gauge to monitor for temperatures below 60 degrees.
  • The passive intake should be low in the room and equipped with a screen to keep your room clean.
  • The exhaust vent fan should be installed high in the room since hot air rises. I also installed a screen going out just in case so nothing can get in.
  • Clean the screens on your ducting at the start of each grow or at least twice a year. You can do this by rinsing them and letting them dry completely before re-using. Replace at least once every year.
  • Screens can be cut from the stove hood screens you can purchase from home improvement stores. They are even carbon filtered usually. They cost about $15.00 for a pack of two large ones.
  • Your exhaust fan should ideally create a vacuum when your intake is closed up. Try to plug up any air leaks if possible to ensure you have full control over air flow and to avoid wasting carbon dioxide.
  • Make sure that all ducting to the outside is light proof. The easiest way to do this is to ensure proper, tight installation and use black duct tape around the seams.
  • Install a carbon filter into your ducting system to eradicate the odor from your grow.
  • You can remove the front shields on your oscillating fans to increase the airflow even more (then you don’t have to clean them either, just be careful).