Stealthy Exhaust Systems for Indoor Plants

How To Set Up Your Exhaust

Optimally, you want the exhaust to pull air from your hot lights and directly out a window, in a short, straight line if possible.

Exhausts become less and less efficient at cooling the further your air has to travel, and the less straight the path. A short, straight, direct line to a window is the best case scenario when it comes to setting up your exhaust.

Use ducting and a strong fan to pull air directly from your air-cooled hood and out your exhaust. 

HID grow light exhaust setup diagram - See how to set up your exhaust system for MH/HPS grow lights

When worried about heat, it's important to create enough suction with your fans to completely replace all the air in your grow tent as rapidly as possible, at least once every few minutes. Even when you're not worried about heat, it's still important to regularly replace the air and make sure there's plenty of airflow and circulation above and below your plants.

Here are a few guidelines that will help you achieve the best exhaust system possible:

  • It's important to have as strong-enough exhaust fan for the grow space to make sure that you are completely changing out all the air in the tent often.
  • Designing your exhaust will be much simpler if you take the time to understand airflow in a room – the whole idea of setting up an exhaust system is to create negative airflow in the grow space or grow tent so that any hot air is regularly replaced with new, fresh cool air.
  • All the air in the tent should be replaced by new air every 1-3 minutes for the best results. You can calculate the strength of exhaust fan you need based on the dimensions of your grow space.Learn how to calculate the type of fan you need below…
  • When your grow tent is bending in from the exhaust fan, it means you've got good suction going in there and have created negative airflow
  • Make sure your intake hole is bigger than your exhaust hole to achieve proper suction – in other words the holes/spaces for new air to come in (vents, intake fans, doors, windows) should be bigger than the hole/space where air is being pulled out the tent by your exhaust fan
  • Even with a good exhaust system, it is recommended that growers still install small fans in their grow space to blow air above and below to the plant canopy to improve air circulation and help prevent any hot spots or humid pockets of air

How to Calculate The Exhaust Fan Strength You Need

Most important – remember you want to completely replace all the air in your grow area at least once every few minutes. If you're worried about heat, exchange all the air every 1 minute. If it's cooler where you live, then exchange all the air about once every 3 minutes. 

Here's how to determine what fan you need for your space

  1. Determine the cubic area of your space by multiplying Length x Width x Height.
    Example: for a 4'x4'x7' grow tent, 4x4x7 = 161 cubic feet.
  2. Choose how often you want to exchange your air. Once every 1 minute for a hot areas, once every 2 minutes for areas with moderate climates, and once every 3 minutes for a cooler locales.
    Example: For areas with moderate (medium) temperatures, I want to exchange all the air in the tent once every 2 minutes, so my answer is "2".
  3. Determine how much cubic feet/minute of air you need to move by dividing your number from step 1 by your number from step 2
    Example: 161 / 2 = 80.5 (that's how many cubic feet/minute I need the exhaust fan to move)
  4. Pick your fan. Fans are measured by CFM (cubic feet/minute), so you need a fan that has a CFMhigher than the number you figured out in Step 3.
    Example: I would need a fan rated at least 80.5 CFM or more if I wanted it to exchange all the air every 2 minutes. 

When in doubt, it's always better to get a bigger fan than you need. You can always use a speed controller or put the fan on a timer if the fan is cooling your space too fast, or if you go through a cold spell.

Cannabis plants like having a lot of fresh air all the time, and they will reward you for your diligence in setting up a proper exhaust system.

Choose the Placement of Your Grow Tent & Exhaust Carefully

Consider that you want a direct line to the exhaust, with as short a distance as possible, so that hot air is quickly and easily released outside of your home.

This diagram illustrates visually what you need to understand about setting up a grow room with an exhaust system that works

Learn how to make a stealthy "secret window" exhaust system like this below…

Having trouble finding a cheap & easy way to stealthily exhaust out hot air from your grow tent? I used a combination of the following items to create an awesome exhaust system for about $45 (just add your fan). 

What You Need

  • wicker blinds
  • foam
  • corrugated plastic
  • duct tape
  • ducting
  • super glue/hot glue
  • staple gun

*All these items can be found at a home improvement store, as they are common items for upgrading around the house

First, hang the wicker blinds in your window like normal. Air can escape through these blinds, but you can't see through them to the other side. They are commonly used in bedrooms and other windows that lead into private areas.

Hang the wicker blinds in the window as the first step of your stealthy exhaust system

I bought a huge sheet of foam and glued (super glue and hot glue for best results) it to a sheet of corrugated plastic. You can buy both these items for a few bucks at a home improvement store like Home Depot.

Next, use some ducting to trace a round, duct-sized hole on the foam/plastic. Note: Cut out the duct-sized hole in the foam/corrugated plastic near the top of where it will be mounted. Remember, heat rises, so having the exhuast exit near the top is much more efficient.

Run the ducting through the hole you cut, and right up to the wicker blinds. The ducting should be butted up against the wicker blinds so that heat escapes directly outside (since it will be able to pass through the blinds easily). Use duct tape to hold the foam/plastic in place, then use more duct tape to hold the ducting in place. At the end after everything is placed and you're happy with how everything looks, you will use a staple gun to staple all your duct tape in place, but don't worry about that yet.

I also used duct tape to secure the foam to the window because it's cheap, easy to use, and is very durable when combined with staples.

Prepare the foam for use by gluing to a sheet of corrugated plastic and then cutting a hole for your ducting to go through

Use a staple gun to reinforce anywhere you use duct tape, including where the ducting goes through the hole you made in the foam.

Reinforce the duct tape with staples or it will fall off after a day or two

I like using foam + corrugated plastic because it's inexpensive and easy to cut the hole for the exhaust. Having the foam/plastic combo in the window increases the suction in the room so more hot air is vented out and blocks light (no light gets in or out). As an added bonus, the foam dramatically reduces the noise of the fan (so you can just barely hear it even when you're standing right outside the window).

Why should you use foam as part of your exhaust system?

You now know everything I wish I'd known before I started growing with HID lights. If you're serious about growing cannabis, HID grow lights are going to give you the best yields, most ease of use, and (in my opinion) the most enjoyable growing experience.

Now that you know exactly how to get started…

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