How to calculate the exhaust fan cfm with carbon filter and take in consideration HID lights, temperature and humility, a beginner guide ๐Ÿ‘

Hereโ€™s how it can be done:

Calculating By Room Volume
You will find many calculations on the web for sizing a fan for ventilating indoor gardens; however, what many of these calculations fail to take into consideration is the friction loss on carbon filters and increased temperatures from HID lights. So hereโ€™s my calculation method which you can use as a guide for sizing an exhaust fan for a growing area (keep in mind that this calculation will give you the lowest required CFM (Cubic feet of air per minute) required to ventilate the indoor garden.)

Step 1: Room Volume
First the volume of the room needs to be calculated. To calculate multiply length x width x height of growing area e.g. A room that is 8โ€™ x 8โ€™ x 8โ€™ will have a volume of 512 cubic feet.

Step 2: CFM Required
Your extraction fan should be able to adequately exchange the air in an indoor garden once every three minutes. Therefore, 512 cubic feet / 3 minutes = 171 CFM. This will be the absolute minimum CFM for exchanging the air in an indoor garden.

Step 3: Additional factors
Unfortunately, the minimum CFM needed to ventilate a indoor garden is never quite that simple. Once the grower has calculated the minimum CFM required for their indoor garden the following additional factors need to be considered:

Number of HID lights โ€” add 5% per air cooled light or 10-15% per non-air cooled light.

CO2: add 5% for rooms with CO2 enrichment

Filters: if a carbon filter is to be used with the exhaust system then add 20%

Ambient temperature: for hot climates (such as Southern California) add 25%, for hot and humid climates (such as Florida) add up to 40%.

An Example
In our 8โ€™ x 8โ€™ room we have 2 x 1000w air cooled lights, and we plan to use a carbon filter. We also plan to use CO2 in this room. The ambient temperature is 90 ยฐF (32ยฐC), however, we will be using air from another room that is air-conditioned. Hereโ€™s the minimum required CFM to ventilate room:

  1. Calculate the CFM required for room (see above.)

  2. Add 10% (for 2 air cooled lights.)

  3. Add 5% of original CFM calculation (For CO2.)

  4. Add 20% of original CFM calculation for Carbon Filter.

  5. Air is coming from air-conditioned room so no need to add any other percentages.

  6. CFM = (171 CFM) + (171CFM x 10%) + (171 CFM x 5%) + (171CFM x 20%) + ( 0 )= 231 CFM.

This is the absolute minimum CFM required to ventilate your room.

The next step might seem to match the closest fan to this CFM. However, for this example Iโ€™d choose a six inch fan with a CFM of around 400 or more, and a 6 inch carbon filter to match. The extra CFMs may seem a bit excessive (calculations on most indoor gardening websites would recommend a 4" fan and a 4" carbon filter) but itโ€™s always better to over-spec since we need to compensate for air resistance in ducting too.

Also, as we are using a carbon filter we will need to match the fan with the filter so that the fan that will neatly fit onto the filter.

If all the variables are kept the same and we changed the room size from 8โ€™ x 8โ€™ to a 12โ€™ x 12โ€™ then the minimum required CFM would be 519 CFM.

The All-Important Inflow!

An intake port can be anything from a gap under the door to an open window - even a hole in the wall. The best place for an intake port is diagonally opposite from your exhaust fan; that way, air has to pass across the entire room - very efficient. You can put a piece of screen over the opening to keep insects and animals out, a piece of A/C filter to keep dust out, or a louvered shutter or backdraft damper that opens when the fan turns on and closes when it turns off. You can also use a motorized damper. This gets installed in-line with your ducting and is plugged into whatever device controls your exhaust fan. When your fan turns on, it allows air to pass. When your fan shuts off, it seals completely, preventing CO2, air, etc. from passing. You can get creative with these devices and use one fan to control two rooms, etc.

One additional note about intake ports - you will see much better results from your exhaust system if you install a second fan to create an active (as opposed to passive) intake system. Normally, when your exhaust fan sucks air out of your room, air is passively going to get sucked back into the room. By installing a second fan on the intake side, you will reduce the amount of negative pressure created in the indoor garden, thereby cutting down greatly on the amount of work the exhaust fan has to do and allowing much more air to pass through. If youโ€™re not sure or you donโ€™t want to spend the money, start out with just an exhaust fan. If itโ€™s not performing as well as you thought it would, try adding an intake fan - youโ€™ll smile when you see the difference!

Hoping that this will help as many grower as possible :wink::+1::ok_hand::v::innocent:


Very nice great info bro :+1::wink::v:๏ธ๏ธ:sunglasses:

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Thanks bro :innocent: Coming from you , a specialist in Hvac/R, mean greatly :wink::innocent::+1::ok_hand::v:

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My pleasure
I think this will be helpful info for a lot of the new growers that show up on a daily basis looking for answers
Iโ€™m sure in time this will become a favorite thread or resource :+1::wink:
Iโ€™m willing to help if needed

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exceptional info!! i have this book marked for future reference. thanks @Niala

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Great explanation. Iโ€™m not sure I completely comprehend the whole piece but I bookmarked as well and well reference it as needed. Thanks @Niala

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@bob31 come on bro you know my alter ego and I have always got your back bro :+1::sunglasses::grin: lmao :stuck_out_tongue_closed_eyes:


@Countryboyjvd1971 @Niala Absolutely I filed that away for future reference as well. LOL. My brother was an HVAC guru as well and I have a lot of respect for you guys.


Thanks @BIGE just sharing information when I find something :wink::v::innocent:


My pleasure @bob31, you know I will always try my best when I can help :slight_smile: :grinning::wink::innocent:


lol on my new setup will have complete max exchange rate roughly 3 times a minute


Very helpful, thanks for making it easy for me :+1: you answered my questions so I didnโ€™t have to ask :v::green_heart:

great post, glad someone @GreenJewels saw it and bumped it up the list. thanks a bunch @Niala if your still around

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