How to Design a Complete Exhaust System for a Grow Tent - Happy Hydro

Proper ventilation can make or break a small grow space. Cannabis plants, like all plants, need fresh air and good air circulation to thrive. Installing an exhaust system to expel old, stale air and excess heat, in exchange for new cool air, can dramatically improve the health of your cannabis plants, yielding bigger, better harvests. 

The wrong sized fan and exhaust system can significantly limit the potential and lead to system inefficiencies. So whether you are new to growing cannabis indoors, or well versed, how do you match the fan size to your grow space?

As we will see, fan size isn't just about cubic feet per minute (CFM). There are many other grow tent factors to take into consideration. The type of light you use, the rate of CO2 supplementation, and even where you live can change the system requirements.

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Why Fans and Exhaust Are So Important In Your Grow Room

Fresh air can make-or-break your yield. So much so, that it's a substantial expense for commercial growers in terms of initial equipment investment and ongoing energy costs. While the requirements within smaller grow tent are not nearly as costly, it doesn't mean the small-time cultivator should ignore or under ventilate their crop.

Air circulation is crucial for mitigating issues with mold, fungi, and common indoor pests. Grow tents and grow closets get hot and humid because cannabis transpires. This is the process by which the leaves excess moisture from cells located on the leaves.

The evaporated moisture is why greenhouses and grow tents feel extra hot and humid compared with the outside environment. Moving this damp, stale air out helps improve plant growth, and reduce the risk of disease and pests.

Plus, combined with ventilation, fans also improve overall plant robustness. In nature, wind provides this service free of charge, naturally strengthening the plants through gentle resistance training.

An exhaust system combined with carefully placed circulation fans helps improve airflow and replicate the outdoor environment. Happy plants need fresh air, and consistently moderate temperatures to thrive.

Finally, a properly sized fan and exhaust system can help fix common mistakes in the grow room, like overwatering. Should you have accidentally overwatered your plants, a fan with humidity and temperature controls will kick into action based on pre-set programming.

Once per minute (Happy Hydro's recommended rate of air exchange ), the system pulls humid air and pump in fresh cool air, thereby helping to regulate the moisture content. In only a short matter of time, your previous overwatering issue is returned back to normal.

Putting Together a Complete Ventilation and Exhaust System

There are a hundred and one ways to set up ventilation within a grow room, but largely they all require similar components in the same order. Minimal operations can get by with fewer parts, but adding carbon filters and grow room monitors into the mix becomes increasingly necessary the more plants are involved. 

Helpfully, most grow tents these days come set up with a complete exhaust system in mind. Following the order of intake to exhaust, here are the essential components you can expect to need in most grow tents.

  • Intake Filters

Perhaps the least common component (at least for small grow tents) are the intake filters. These are placed on the intake line to remove possible contaminants before entry to the sealed grow space. Larger facilities install intake filters to remove potential pollen, spores, or dust mites.

There are likely passive air intake holes (filtered or nonfiltered) along the bottom of the space. Exhaust systems should always get installed at the top of the tent, thereby pulling fresh air from the bottom up for a natural line of airflow.

  • Ducting

Ducting is a basic part of an exhaust system, but one worth mentioning. Depending on the order of operations, components, and the specific grow room ducting will be necessary to connect the individual parts. The longer the ducting, the more work the fans have to do to move air.

Additionally, the more twists and turns (and the more severe the angles), the more power a fan will need to pull air through the ducting. For example, a 90-degree turn needs more power than a 45-degree one. Aim to create a system with the least number of angles, and the shortest ducting possible.

  • Circulation Fans

One of the fundamental pieces in a ventilation system is the interior fans meant to circulate air through the grow tent gently. Growers typically place these above and below the canopy to strengthen the plants and avoid stagnant pockets of hot, humid air. Circulation fans are nothing fancy, just the basic fans you'd use to cool down your kitchen or bedroom.

  • Inline Exhaust Fans

An inline exhaust fan is essentially the system engine. It is responsible for the majority of air movement through the space. When placed on the exhaust end of a ventilation system, an inline exhaust fan pulls air out of the grow tent, which will pull fresh air through the intake holes. Inline fans are usually better for pulling air, than for pushing, and therefore placed at the end (before a carbon filter if you are including one).

As mentioned above, you should always place the exhaust fan at the top of the grow tent, because heat rises. Most light fixtures (even LED options) put off a significant amount of heat for the small space they occupy, and this needs to be ventilated.

If you are supplementing with CO2 you will have to improvise so you're not wasting it all. A good solution for this is having a lung room and the most simple example of this would be a grow tent inside a bedroom. Your AC unit goes in the window, the exhaust fan and carbon filter in the tent, and you exchange air between the grow tent and the bedroom without losing all of your CO2 enriched air.

Throw a towel under the door and seal off the window/room the best you can and you're good!

  • Carbon Filters

Most growers no longer need to worry about the legal concerns about a smelly grow room, but the potent aroma of even a small grow isn't always ideal. Carbon filters exist on the exhaust end of the system to scrub the strong cannabis aroma out of the air before expelling it into the world. They can also rid the atmosphere of any lingering foliar sprays, pesticides, or other unwanted contaminants.

Carbon Filters like the AC Infinity series come in various duct sizes, from 4" to 10", with several silent options.

Keep in mind, most of the components add drag on to the system and will mean you need a more robust fan (higher CFM) to ensure it can efficiently pull air in and continue to pull it out.

How to Calculate CFM For Your Grow Tent

You may have noticed that there are many differing calculations (and opinions) online for determining appropriate exhaust and ventilation within your grow tent. Based on ten years of experience, and our own research, we aim for an air exchange rate (or scrub) of once every minute. Once per minute, simplifies the formula, as you'll see below.

The first step to determining the proper fan size and exhaust requirements is to work out the cubic feet per minute, or CFM. The CFM will predict how much air exchange is required, per minute, for the given size of the space. You can use the same formula for an entire room, or a small grow tent, the calculation will be the same. It also works for determining exhaust or intake components.

Although the CFM calculation is itself quite simple, there are always additional factors in a grow room, which will likely increase the size of the fan required. Hot HID lights, CO2 supplements, carbon filters, and hot climates will require slightly higher powered exhaust systems. 

Never forget to consider the impact of grow lights on your grow room set up! 

Happy Hydro's Simple CFM Calculation

1. Measure the width, length, and height of the space. Multiple together to get the cubic feet.

  • Ex: 4ft x 4ft x 7ft = 112 sq ft

2. Add on additional factors, like HID lighting (5 percent per air-cooled light, 10 percent for non-air-cooled), CO2 supplement (5 percent), hot climates (25 to 40 percent depending on humidity) and/or carbon filter (20 percent). These are rough estimates but will help adjust the final number for a more accurate reading.

  • Ex: 112 base CFM plus two HID lights (10 percent) =  123.2 CFM
  • Ex: 112 base CFM plus two HID lights and carbon filter (30 percent) = 145.6 FM

3. Install an inline exhaust fan and appropriate sized ducting based on this final CFM rating.

Choosing the Right Size Inline Exhaust Fan for a Grow Space

You'll want to consider the following additional specs before choosing an inline exhaust fan:

  • CFM 
  • Duct size
  • Noise 
  • Speed Controller
  • Carbon Filter Rating (if using)

Once you've worked out the estimated CFM, including all influencing factors, from the ventilation system to the outside climate, round up and look for options that fit this number. Inline fans are first based around CFM rating, and it should be easy to weed out those rated above and below your grow tent target. For example, the Cloudline S4 is rated 205 CFM, while the Cloudline S10 goes up to 1011 CFM. 

The CFM plays into duct size. The higher the CFM, the larger the duct required. Each inline exhaust fan uses a specific duct size (from 4" to 10" and beyond). Use the following as a general guideline:

  • 4ft x 4ft = 4” Duct Size
  • 5ft x 5ft (or 4ft x 8ft) = 6” Duct Size
  • Larger than 4ft x 8ft = 8” or larger

While noise won't play into your exhaust system's efficiency or functionality, it may be important depending on where you place the grow tent in your home or garage. Indoor growing is a lot noisier than some people expect, largely thanks to the need for a full ventilation system. If noise considerations are important, look for fans with silencer components, like the AC Infinity Duct Fan Silencer series.

For a bit more grow room control and flexibility, invest in a fan with a controller, like the Cloudline T6 with Temperature and Humidity Controller. This feature creates intelligent control over temperature and humidity programming, fan speed control, timer, and alarm system.

It's always better to invest in a slightly larger fan, with speed controller, than you need for the system. This gives your grow room to expand, and gives you a little leeway in terms of CFM, without needing to buy an entirely new fan system.

Finally, if you are using a carbon filter, you'll want to ensure your fan is compatible. A powerful exhaust fan may push air through the filter too fast, making the filter ineffective. Always choose an inline exhaust fan with a lower CFM rating than the filter. 

Grow Room Fans and Exhaust Should Never Be an Afterthought

We can excuse newbie growers if an inline exhaust fan and ducting comes only as an afterthought to the initial setup. But today commercially sold grow tents, like the Gorilla Tents, all come with the necessary spaces to insert a complete exhaust system. With the structure already in place, installation, and set up are much easier.

Taking the time to calculate CFM and install a properly sized fan, filtration, and ducting will create immediate returns on the investment come harvest. As it turns out, plants are happier with a constant supply of fresh air.

Author | Chris McDonald

With two decades of expertise, Chris leads Happy Hydro in redefining sustainable gardening and delights in backpacking adventures, mind-expanding journeys, and creating memories with his loved ones.

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