Grow Tent Ventilation Setup & Exhaust Guide

Ventilating your grow room is way too important. To such an extent that the worst cannabis problems and leaf symptoms come from bad ventilation.

CO2 levels, temperature, humidity, and more. Cannabis plants can only thrive in certain climate conditions. That’s where grow tent ventilation and exhaust come into play. 

Why Your Grow Tent Ventilation Is So Important?

Your grow tent should try to replicate nature’s growing conditions. And the more control you can exert over those conditions the better.

Ventilation and exhaust are the core of it all. It helps ensure that the air in your grow tent is conducive to healthy growth, strong plants and great yields. 

Keep The Perfect CO2 Level For Cannabis 

Like all plants, cannabis needs carbon dioxide to survive. But when plants are in a contained space, the CO2 in the air will quickly be depleted and replaced by oxygen without proper airflow.

This means we need to replenish it in some way. Your setup should have 2-way air circulation, so the cool fresh air filled with CO2 goes in, while the used air goes out.

Control Humidity And Excess Heat

Bud rot because of bad ventilation

Cannabis plants thrive in what we would consider “comfortable” temperatures. That is to say a range between 70-85 °F (or 20-30 °C) during the vegetative stage. When your plants are ready to flower, you’ll find that dropping the temperature to around 65-80°F (18-26°C) results in better yields and quality buds.

As well as controlling your grow tent’s temperature, you also need to consider the humidity. Apart from ambient humidity, plants transpire and release water into the atmosphere. 

Without a proper exhaust you may find yourself with a damp grow space where mold and fungi will start to develop. Believe me, you want nothing to do white powdery mildew or bud rot.

Avoids Pests And Plant Diseases In Your Tent

Mold and fungi aren’t the only life forms against which growers need to be vigilant. You also need to protect your plants from insect pests which can thrive in stagnant air and moist topsoil. 

Spider mites and fungus gnats find it difficult to fly in well ventilated spaces. Meaning that your plants are much safer from them when you have the right fan and exhaust setup.

Strengthen Stems

Finally, it’s important to remember that cannabis plants benefit from a gentle breeze, and even needs it for optimal growth. 

This gives the stems a nice workout and prevents them from becoming too weak to support proper bud growth. 

Without a replicated breeze in your grow tent, your stems can become anemic which may lead to bowing. This in turn can prevent the leaves from absorbing enough light to properly photosynthesize, which impedes proper growth.

Ways To Setup A Ventilation System In Your Grow Area

Getting the ventilation right for your setup may require some investment and experimentation. Nonetheless, it’s an aspect of your setup that will definitely pay out.

For starters, there’s two ways you can set up your ventilation, passive or active intake.

Exhaust Fan With Passive Intake

passive-intake-ventilation-setupPassive intake relies on negative pressure to create passive airflow. 

Put simply, you have a fan blowing air out of your tent, but there is no fan blowing air from outside in. Instead you use an intake hole that passively allows new air to enter the tent. 

This will usually be around 3 or 4 times the size of the exhaust hole, or you can have multiple passive intake holes. 

The differences in pressure between the inside and outside of the tent will keep the air moving. This system will reduce energy use and work best for most growers.

Active Air Intake And Exhaust System

020-active-intake-and-exhaust-systemAlternatively, an active intake system uses a fan to actively draw air into your grow tent. 

So there are two fans working together. One to keep air from inside the tent blowing out, and another to keep air blowing air in

The size of the intake hole is less important when using this system. So if your grow tent has an intake hole that’s about the same size as your exhaust hole, your tent will lend itself well to an active intake system. 

Since most fans allow you to control their power, you can find your own balance that helps you to get the right temperature and humidity in your tent.

The Ultimate Ventilation Setup

The principle for exhaust systems is always the same. It needs to keep clean, fresh air flowing in while venting out used and humid air.

But there are many different accessories and equipment that you can include in your setup. The ultimate ventilation setup will be one that not only keeps your crop healthy, but also discreet, minimizing noise and odors. 

I’m going to assume your setup consists of some or all of the following:

  • Rope ratchets to hang your equipment.
  • An exhaust fan (we’re going to be using a passive intake).
  • A carbon filter.
  • Ducting and clamps. 
  • A silencer.
  • An oscillating fan.
  • Air cooled grow lights (most type of lights, such as LED, create almost no heat so don’t need to be included in your ventilation setup). 

Ducting Your Fans And Accessories

In the following steps you’ll be connecting the rest of your equipment to your exhaust fan. We will do this with aluminum ducting. 

I find that flexible aluminum ducting provides the perfect combination of affordability, versatility and ease of installation. But, if you want heavier duty ducting and have the budget for it, insulated aluminum will also work well. 

To ensure peak performance, make sure that all kinks and wrinkles are smoothed out.

Also, try and keep your path as short and straight as possible until the air is outside the tent. The longer and more circuitous your path, the less efficient your system.   

Inline Exhaust Fan

exhaust-fan-hanging-in-grow-tentAfter setting up your grow tent the first thing to do is to set up your exhaust inline fan. But make sure not to plug anything into your power supply until you’re ready to start growing.

Step inside your tent and connect your exhaust fan with your tent’s exhaust hole. 

You can hang your fan from the ceiling of your tent. And either connect the fan’s exit port directly to the exhaust hole, or by ducting it if you find it more comfortable.  

Carbon Filter To Deal With Odor

If you want to grow with discretion you can use your exhaust system to your advantage. Using a carbon filter you can prevent your living space from smelling of cannabis. 

Your carbon filter, this needs to be factored into the way you set up your exhaust. The air from your grow tent needs to pass through your filter before being exhausted outside.

The simplest setup that many growers use is to hang the carbon filter inside the tent. Making an inline connection with the exhaust fan, by ducting them together. 

This way, the air will go through the carbon filter first, then the exhaust fan vents the filtered air to a space outside (first picture).  

Alternatively you can exhaust the air out of your tent first, and then duct it into your CO2 filter before letting the air out (second picture).

Get this right and the only place where you’ll be able to smell cannabis is when you’re actually inside your grow tent.

Silencer For Extra Stealth

Silencers are designed to go after the fan, better if directly after but if needed you could place it further along the duct. 

stealth-ventilation-setupIt works by muffling the air from the exhaust, making a lot less noise.   

And they work really well. I have a silencer installed in one of my tents which sits inside the house, and the reduction in noise is amazing. It also changes the sound type, it goes from being a terrible whining  to a gentle swooshing, more so like wind.

Keep in mind that it will restrict airflow and you’ll need to factor it in your CFM calculations (more on that below). 

Connecting An Air-Cooled Reflector (Mostly for HID grow lights)

If you’re using HID grow lights it’s very likely that your hooded fixture allows for ducting.

You’re going to need to have direct airflow going through the light’s reflector. This will help you keep temperatures down, as well as keeping the light bulbs cool, since HID lights output lots of heat.

Depending on your reflector and how many other accessories you’ve got, this can complicate your setup quite a bit.

You need to connect everything inline. In case you’ve got a CO2 filter, start ducting together to your reflector.

Then, connect to your reflector to your exhaust fan, and finally to your tent’s exhaust port.

Sealing Your Grow Tent To Create Suction

A sealed grow tent ensures air only passes through the intake hole and leaves through the exhaust fan. Without proper sealing, your exhaust system will be prone to air leakage and be terribly inefficient

Sealing creates an independent ambient, so you have control over temperature, humidity, and CO2. And ensures that your exhaust system is operating at peak efficiency. 

By preventing air leakage, your exhaust fan will create negative air pressure, letting new air in through the intake hole. You will even see that the sides of your grow tent bow inwards. 

Combined with a carbon filter, this will further prevent cannabis odors from permeating your home.  (On top of that, a sealed tent helps keeping ambient light out during plants’ dark hours). 

Most commercial tents take measures to ensure a good enough seal. But if you have tears or pinholes, you can use anything from duct tape to Black RTV Silicone.

If your grow room is the DIY type (like a closet), you may want to look into weatherstripping the door, and shutting off any type of opening it may have. 

Check For Air Leaks

To check for air leaks, first power up your system and close your tent from the inside. 

Then, with a lit incense stick, move it around the inside of your tent and see how the smoke behaves. If the smoke drifts straight upwards, the air is where it needs to be. 

Finally, take the incense stick to the outside of the tent, and place it close to the intake hole. If the smoke snakes towards the intake, is an excellent sign that you have achieved good sealing and negative pressure.  

Oscillating Fans

oscillating-fan-distributed-airflowOscillating fans are affordable and really effective. Blowing and distributing air around your grow tent evenly, while also ensuring the breeze is strong enough to keep your plants’ stems strong. 

You have floor fans that sit in a corner of your room, or specialized grow tent fans that are much smaller and clip-on to your tent’s poles. 

If you’re a bit of a DIYer and a fan lying around, you can try hanging it or have it sit in your room. Just make sure the breeze it’s not too strong on your plants, as this can cause wind burn (and damage your leaves).

Depending on the size of your grow tent and the size of the fans, you may do well with one or you may need several. There’s not a simple way of knowing how many you need.

But, try to set the fans in a way so your whole crop gets some action.

How To Calculate CFM For Exhaust Fans

When ventilating your grow tent, you need to know how much fresh air must be provided to ensure healthy growth for your plants.

Fortunately, there is a formula that allows you to calculate the proper Cubic Feet per Minute (CFM) necessary to ventilate your tent.

1. First let’s find out your grow tent’s volume

While your grow tent may vary, we’re going to use a 4×4 grow tent with a fairly typical 8’ height as our example.

4 (width) * 4 (depth) * 8 (height) = 128 ft3 

2. Calculating Base CFM

Your inline fans should, ideally, replenish the volume of air in your grow tent once every two to three minutes.

128 ft3 / 2 minutes = 64 CFM 

But ambient temperature and humidity need to be taken seriously. If the room you’re getting the air from it’s too hot, factor in 20% more. And if you’re fighting humidity, add in another 40%. 

In a hot an humid climate like my own, it’d end up like this.:

64 (cfm) * 1.4 (humidity) * 1.2 (warm climate) = 108 Base CFM 

That means you need to exchange 108 cubic feet of air per minute in order to properly ventilate your 128ft3 grow tent.

We will call this your base CFM.

3. Factoring In Accessories

Now that you’ve got your base CFM, you also need to factor in your accessories such as your ducting, grow lights, and your carbon filter or silencer (where necessary).

These can all affect how hard your fan needs to work to achieve the perfect growing conditions.

For instance, any sharp bends in your ducting will significantly reduce airflow efficiency. A 90 degree bend can reduce airflow by up to 60%! Even a bend of 30 degrees will result in a 20% loss of efficiency.

Throw in another 60% if you’re using a carbon filter to neutralize odors coming from your tent. 

You can see how these will impact your base CFM:

AccessoriesEffect on CFM
LED grow lights20%
HID grow lights50%
CO2 filter60%
Silencer20%
Ducting 30º Bend20%
Ducting 90º Bend60%

If we only had LED grow lights, a CO2 filter, and a few bends. Our final CFM formula would be:

108 (base) * 1.2 (lights) * 1.6 (filter) * 1.9 (ducting) = 394 Total CFM Needed

Now you need to get an exhaust fan that can output this much Cubic Feet of air per Minute, as per your own grow tent and accessories needs. Check the best 6″ inline fans that can do the job quite well.

Other Air Flow And Ventilation Tips

How To Manage Fan Noise

Because you need a high-powered fan to ventilate your grow tent, you can expect some noise. But, there are ways to mitigate this so that your growing is more discrete and unobtrusive.

Choose a fan with speed controls and a CFM rating 25% higher than your needs, this way you can run it at a lower speed making less noise.

Also, most of the inline fan models come with noise reducing features. But, as we saw above, you can also attach a silencer which will dampen your fan’s sound.

CO2 Injection To Increase Yields

co2-injection
CO2 injection in a grow tent

As most growers know, plants absorb Carbon Dioxide (CO2) and “breathe out” oxygen. While plants don’t have lungs, they have hundreds of tiny pores called stomata on their surfaces, which allows them to absorb ambient CO2.

Unlike us, they need more carbon dioxide than oxygen in the air to remain in good health. This invisible gas assists in the process of photosynthesis, and without it your plants will “suffocate” and die.

So… you may want to inject a little extra CO2 into the atmosphere of your grow tent to improve your plant’s health and increase yields.

Doing so can even make your plants more resistant to heat and light. Meaning the temperature in your grow tent can be higher, and your lights more powerful. Maximizing everything in the process.

I recommend this approach only to technical people that want to go the extra mile with their yields. Novice or hobbyists can definitely get amazing yields without this.

Supplementing CO2 is a complex process and there’s a few ways of doing it. Check out this expert guide from CO2Meter on how to go about it.

Grow Room Air Conditioner

While this may be surplus to requirements an air conditioner may be a worthy investment if you’re dealing with heat. Specially for larger spaces.

It can be a great way of avoiding excess heat and have the most control over your temperature.

Whether you’re growing on a scorching summer’s, your plants will enjoy the perfect cool breeze and offer you strong and consistent yields.

You can place your air conditioning unit on the ground. Although some growers prepare to mount them on the ceiling or walls. It’s also a good idea to place it near an exit to make it easy to get rid of residual water.

Make sure that your unit’s BTU output is well matched to the size of your grow tent. According to this calculator, 5,000 BTU is enough for grow tents up to 4x4x8. You can calculate your specific metrics (make sure you select (feet not meters).

Final Thoughts

As you can see, ventilation is incredibly important to the health and productivity of your plants. The more you leave airflow to chance, the more you can expect to be disappointed by lackluster yields.

There are many ways in which you can exert control over your grow tent ventilation to ensure optimal yields. Hopefully you found the one that bests suits you. And make sure to return to this article whenever you have doubts.

Happy growing!

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Santiago Pardo
Santiago Pardo
I live to help out cannabis growers have a greater experience with their crop, and not loose their mind over the many aspects of growing marijuana.

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