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Proper grow room ventilation is important for a number of reasons. Mainly, it will help control temperature and humidity. But good extraction is also important for odour control, and air exchange.

Deciding on the right extraction can be made easier by using the calculations found below. The purpose of this article is to simplify these calculations while helping you to better understand grow room ventilation.

If you need advice on deciding which fans is best for your grow room, then feel free to ask for help in our cannabis growers forum

How to size your exhaust fan

When calculating what extractor fan you need, you have to take, friction loss on carbon filters, increased temperatures and CO2, etc into consideration. The following calculation can be used as a guide for sizing an extractor fan.

Step 1 – Your Grow Room Volume

First the volume of the grow room needs to be calculated. To calculate the volume of your grow space, multiply:

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Length x Width x Height of growing area.

For example, a room that is:

8′ x 8′ x 8′ will have a volume of 512 cubic feet.

Step 2 – Cubic Feet per Minute Required

The fan should be able to exchange the air in a grow room once every three minutes, or at least 20 times an hour. Therefore:

512 cubic feet/ 3 minutes = 171 Cubic Feet per Minute.

This will be the minimum CFM needed for the air exchange in your grow room.

The maximum amount of extraction you should use will exchange the air in the grow room every two minutes, or 30 times an hour. Using the calculation above, ( 512 CF/ 2 minutes = 256 CFM) you can find what extraction is best for your grow space.

extraction kit for grow room,

This same principle works on any grow space, and when you’re buying your extractor fan, you can see how many CFM it will extract. There are some other factors that should be taken into consideration.

Step 3 – Additional factors

Once you have calculated the minimum CFM required for your grow room, the following additional factors need to be considered:

  • Number of HID lights: Add 5% per air cooled light or 10-15% per non-air cooled light
  • CO2: Add 5% for rooms with CO2 enrichment
  • Filters: A carbon filter must be used with the exhaust system, for this add 20%
  • Ambient temperature: For hot climates add 25%, for hot and humid climates, add up to 40%

Active and Passive Intake

Along with an exhaust fan, you must have some kind of opening in the room to allow fresh air in, to replace the air you are exhausting. Without intake, any extractor fan in your grow room will need more energy, make more noise, run hotter, and suck very little air out of the room. This will also reduce the life span of your extractor fan.

Passive intake

Adding an intake port will relieve pressure on your fans, and keep air moving around easily. Intake can be drawn from anywhere, such as, from a gap under the door or an open window. Maybe even a hole in the wall.

Place your intake port diagonally opposite from your exhaust fan. This will force the air to pass across the entire room before being extracted.

Bugs can occasionally enter though your intake. It is recommended you use a piece of screen, or netting, over the opening to keep them out. You can also use an old carbon filter or piece of A/C filter to keep dust, and bugs out of your grow room.

Be careful of light leaks coming through your intake ventilation too, you should check to make sure no light interrupts your plants night cycle.

Active Intake

You will see much better results from your exhaust system if you use a second extractor fan to create an active intake system. Your exhaust fan sucks air out of your room, as a result, air is passively going to get sucked back into the room.

By installing a second fan on the intake side, blowing into the tent, you will reduce the amount of negative pressure created in the grow room. Doing this will cut down on the amount of work the exhaust fan has to do and allow much more air to pass through the grow room.

Ideally, the intake fan should be the same size as the exhaust fan, but it can be a little smaller if necessary. Passive intake will work fine, but active intake will make controlling temperature and humidity much easier.

Fan and Carbon Filter Placement

It is best to place the extractor fan at the highest possible point in the room, outside of the grow tent if possible. The reason for this is that hot air rises and it is the hot air that needs to be exhausted.

Also, place the extractor on the opposite side of your intake. The negative pressure that the fan creates in the room will pull air from the intake opening, across the plants and out of the exhaust. This will bring the fresh air over the plants while displacing the old stale air too.

There are two options that a grower can use with the carbon filter:

  • 1) Suck through the filter or
  • 2) Blow through the filter.
Grow room ventilation, how to set up a carbon filter,

Sucking the air out

Sucking air through the filter is the preferred method. Carbon filters are best hung horizontally, from the ceiling of the grow room, or tent.

From the extractor, attach ducting to the carbon filter, so it exhausts the scrubbed air out the top of the grow room. Keep the ducting as short as possible and avoid any bends. Doing this will reduce the friction loss and noise.

Blowing the air out

Blowing through the filter is not a commonly used method of exhausting. If you’re using the fan/filter combination as part of your exhaust system, the filter will need to be placed outside of the growing area.

Before using a carbon filter for the first time, you should take the filter outside. Use your extractor to first suck, then blow, through the filter before installing it in your growing area. This will blow out any carbon dust which is created in the filter during transport.

During normal operation always keep the dust sock on the filter. This will stop large dust particles from blocking the filter and will help prolong the life of the filter.

Air Circulation in a Grow Room

Always provide your plants with adequate air movement and circulation in the grow room. Air circulation within the room eliminates trapped air and also reduces chances of rot and mould forming.

If air circulation in the grow room is poor, the air in between the plants will increase in humidity and temperature. This can result in mildew and insect problems, as well as deficiencies.

For adequate air circulation use a wall mounted fan; 2-3 fans will provide sufficient air movement. Never switch these fans off unless you are spraying or fogging your room, whilst lights are on. The fans can blow droplets onto the bulb, and they will break.

Final notes on Grow Room Ventilation

Air flow, and good grow room ventilation is an underestimated part of growing cannabis indoors. So, do not cut corners here, especially with your carbon filter! Covering the smell is rule number three, do not break it.

Ensure the filter, and extraction, is the best you can afford, knowing the smell is not going to escape, will give you peace of mind. And supplying a regular amount of fresh air to your plants will keep them happy, and healthy.

Thanks for reading! You can find more information on anything related to growing in our cannabis grow guides. All of our guides are written by experienced growers, and members of our cannabis growers forum. If you have any questions, sign up. It is free, and we are always happy to help!

Stay safe out there Percy Growers. Happy growing!

10 Comments
  1. blank

    Going to be re writing a lot of guides over the next few…. months i suppose lol. This is one of them mate. Ill be happy to update it with any new info you have 🙂

  2. blank

    @macky I think there is conflicting information.  A couple of websites I’ve noticed tell you the incorrect way of dealing with air flow. 
    There is already fans that automate but controlling carbon levels and everything is another level man lol. 
    Good luck with that mate. 
    Now looking at my set up as I think I’ve been running at a positive pressure for too long. 

  3. blank

    @templegrower that’s a good video. I like what Shane does. He makes things easy to understand.  
    I’m just looking at air flow in my tent. But I only have a .6M square tent and one out fan with a filter. 
    Think I’ve been running on too much positive pressure which may be why I’ve been getting low harvests. 
    Thanks TG 

  4. This is an excellent video regarding a DIY relative flowmeter to measure your exhaust efficiency. It won’t give you quantitative data (i.e., actual CFM data) but it will tell you how efficient (or not) your fan is blowing. Very easy to make and very useful.

  5. blank
    Posted by: Zombie Nation
    Posted by: GuerrillaGrow69
    Posted by: Fukentokin

    I apologize ahead of time if I offended anyone……. but I wanna fuck in that tent!   So airflow is extremely important, good read.  Just a note too if your using the blowing method described, your exhaust vent has to cleaned for mould. 

    I’m gonna go a step further and say……. @Fukentokin ……… Time and Place?

    In all seriousness tho the above is a really good article that’s easy to understand for beginners and cuts out a lot of unnecessary bullshit lies and legitimate but advanced setups and techniques that simply make the tasks seem daunting and unachievable for newcomers.

    Further again I would like to bring up some what of a SUB-TOPIC in home automation. There are lots of ways now that homes can be automated now days both for effectiveness and power savings reasons and one of the big ways apart from lighting and security is air conditioning. In addition to me wondering if any of you happen to employ such a system in your home already, I would wonder if anyone has thought to implement such a system in a grow room / environment. Yes you can control heating and cooling to come on and off at different temperature toggles but what about ventilation? I saw online there is a company that specialize in manufacturing these smart windows which of course you could black out if u wanted to actually have one in a grow room with external wall and they can detect heat and Carbon dioxide/monoxide levels (could it also monitor for nitrogen levels and other gases that make up the air in our atmosphere, idk but it’s a great question) and they can intake and output air in both directions as it needs to maintain room temperature using outside air instead of how a traditional refrigerated aircon unit works. I would assume you would run it in tandem with an actual unit capable of artificial heating and cooling but that’s just me. the thing is these appliances all link up to so many different open source fiirmare and linking software that have app based GUIs so you can literally make it do anything you want as long as you know how to code (I assume there will be a public repository of community made firmwares at some point soon in the future for those who either can’t code or would benefit from using someone elses’ project as a template ) and you can do it all from your phone and monitor iit at any time. I know this thread is about ventilation but it goes without saying you could use the same type of products to control exact time scheduling for lights or various water pumps etc (if you posses powered irrigation) and even security of your space.

    I have a super geeky design pending in my head for an intruder detection system complete with audible warnings and airsoft ammo “bubbles” (lel)  traveling at high velocity but like I said it’s just a super fucked up hypothetical, maybe I’ll make a thread about it if anyones interested in exploring it more.

    Yes…do the thread…bit of a geek myself…would be interested to see what’s in your head 😉

    For sure man! Itll be a good read, if you do a good job of it, maybe you would like to be made and author of the site, so you can write your own articles on the front end if you like 🙂

  6. Posted by: GuerrillaGrow69

    Posted by: FukentokinI apologize ahead of time if I offended anyone……. but I wanna fuck in that tent!   So airflow is extremely important, good read.  Just a note too if your using the blowing method described, your exhaust vent has to cleaned for mould. 

    I’m gonna go a step further and say……. @Fukentokin ……… Time and Place?In all seriousness tho the above is a really good article that’s easy to understand for beginners and cuts out a lot of unnecessary bullshit lies and legitimate but advanced setups and techniques that simply make the tasks seem daunting and unachievable for newcomers.Further again I would like to bring up some what of a SUB-TOPIC in home automation. There are lots of ways now that homes can be automated now days both for effectiveness and power savings reasons and one of the big ways apart from lighting and security is air conditioning. In addition to me wondering if any of you happen to employ such a system in your home already, I would wonder if anyone has thought to implement such a system in a grow room / environment. Yes you can control heating and cooling to come on and off at different temperature toggles but what about ventilation? I saw online there is a company that specialize in manufacturing these smart windows which of course you could black out if u wanted to actually have one in a grow room with external wall and they can detect heat and Carbon dioxide/monoxide levels (could it also monitor for nitrogen levels and other gases that make up the air in our atmosphere, idk but it’s a great question) and they can intake and output air in both directions as it needs to maintain room temperature using outside air instead of how a traditional refrigerated aircon unit works. I would assume you would run it in tandem with an actual unit capable of artificial heating and cooling but that’s just me. the thing is these appliances all link up to so many different open source fiirmare and linking software that have app based GUIs so you can literally make it do anything you want as long as you know how to code (I assume there will be a public repository of community made firmwares at some point soon in the future for those who either can’t code or would benefit from using someone elses’ project as a template ) and you can do it all from your phone and monitor iit at any time. I know this thread is about ventilation but it goes without saying you could use the same type of products to control exact time scheduling for lights or various water pumps etc (if you posses powered irrigation) and even security of your space.I have a super geeky design pending in my head for an intruder detection system complete with audible warnings and airsoft ammo “bubbles” (lel)  traveling at high velocity but like I said it’s just a super fucked up hypothetical, maybe I’ll make a thread about it if anyones interested in exploring it more.

    Yes…do the thread…bit of a geek myself…would be interested to see what’s in your head 😉

  7. Posted by: FukentokinI apologize ahead of time if I offended anyone……. but I wanna fuck in that tent!   So airflow is extremely important, good read.  Just a note too if your using the blowing method described, your exhaust vent has to cleaned for mould. 

    I’m gonna go a step further and say……. @Fukentokin ……… Time and Place?In all seriousness tho the above is a really good article that’s easy to understand for beginners and cuts out a lot of unnecessary bullshit lies and legitimate but advanced setups and techniques that simply make the tasks seem daunting and unachievable for newcomers.Further again I would like to bring up some what of a SUB-TOPIC in home automation. There are lots of ways now that homes can be automated now days both for effectiveness and power savings reasons and one of the big ways apart from lighting and security is air conditioning. In addition to me wondering if any of you happen to employ such a system in your home already, I would wonder if anyone has thought to implement such a system in a grow room / environment. Yes you can control heating and cooling to come on and off at different temperature toggles but what about ventilation? I saw online there is a company that specialize in manufacturing these smart windows which of course you could black out if u wanted to actually have one in a grow room with external wall and they can detect heat and Carbon dioxide/monoxide levels (could it also monitor for nitrogen levels and other gases that make up the air in our atmosphere, idk but it’s a great question) and they can intake and output air in both directions as it needs to maintain room temperature using outside air instead of how a traditional refrigerated aircon unit works. I would assume you would run it in tandem with an actual unit capable of artificial heating and cooling but that’s just me. the thing is these appliances all link up to so many different open source fiirmare and linking software that have app based GUIs so you can literally make it do anything you want as long as you know how to code (I assume there will be a public repository of community made firmwares at some point soon in the future for those who either can’t code or would benefit from using someone elses’ project as a template ) and you can do it all from your phone and monitor iit at any time. I know this thread is about ventilation but it goes without saying you could use the same type of products to control exact time scheduling for lights or various water pumps etc (if you posses powered irrigation) and even security of your space.I have a super geeky design pending in my head for an intruder detection system complete with audible warnings and airsoft ammo “bubbles” (lel)  traveling at high velocity but like I said it’s just a super fucked up hypothetical, maybe I’ll make a thread about it if anyones interested in exploring it more.

  8. I apologize ahead of time if I offended anyone……. but I wanna fuck in that tent!   So airflow is extremely important, good read.  Just a note too if your using the blowing method described, your exhaust vent has to cleaned for mould. 

  9. blank

    Nice Rocco, I’ve always wanted good aircon, but always forget about it when summer is over lol. 

  10. I agree with all the points that proper ventilation is required in order to maintain a comfortable temperature in the home. On the other hand, it also reduces the air pollutants in the home, which is very beneficial for the family. The smart HVAC system is the best option to have a proper temperature in the interior environment. I also have a complete HVAC system in my home, which I generally used for the heating, ventilation, and an air conditioning purpose. 

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