Active Member
Please excuse me if this has been posted all ready. I found it to be a good find and figured that someone else could benefit from the info.
Ventilation Explained
Part 1 - Ventilating your growing area

Correctly ventilating your growing area is a vital part of greenhouse/grow room design. Adequate ventilation in the grow room is important for a number of reasons – temperature, humidity, disease and odor control, and fresh/stale air exchange. The calculations that are used to correctly design a ventilation system can be quite complex and require numerous known variables. The purpose of this article is to simplify these calculations while helping you to better understand grow room ventilation.

How to size your exhaust fan

There are many calculations on the web for sizing a fan for ventilating grow rooms; however, what these calculations fail to take into consideration is the friction loss on carbon filters, increased temperatures from HID lights and CO2, etc. The following calculation can be used 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 required to ventilate the grow room):

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

Step 2 – CFM Required
The fan should be able to adequately exchange the air in a grow room once every three minutes. Therefore, 512 cubic feet/3 minutes = 171 cfm. This will be the absolute minimum cfm for exchanging the air in a grow room.

Step 3 – Additional factors

Unfortunately, the minimum cfm needed to ventilate a grow room is never quite that simple. Once the grower has calculated the minimum cfm required for their 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 – 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%.

Example 1:

In our 8’ x 8’ 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°F, however, we will be using air from another room that is air-conditioned.

Minimum Required CFM to ventilate room:

(CFM required for room – step 2) + (CFM required for room – step 2 x 10% (2 air cooled lights)) + (CFM required for room – step 2 x 5% (CO2)) + (CFM required for room – step 2 x 20% (Carbon Filter)) + (Ambient Temperature 0 (Air coming from air-conditioned room)).

= (171cfm) + (171cfm x 10%) +
(171cfm x 5%) + (171cfm x 20%) + ( 0 )

= 231cfm – this is the absolute minimum cfm required to ventilate your room.

The next step would be to match the closest fan to this CFM. 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. The filter that we will use is a Dutch Breeze Carbon Filter - DFS4. The DFS4 has a 6" flange. Our options for 6" fans are as follows:

- Eco Plus 6” – 440cfm
- Fantech 6” – 252cfm
- Fantech 6” XL – 392cfm
- Elicient 150B – 309cfm

The DFS4 filter has a cfm rating of 255-309, so the fan that we will choose for our room will be the Elicient 6” fan. This will be an ideal fan for this room and carbon filter, since the extra cfm will help compensate for the small amount of ducting we may have to run.

If only step one and two was used (the calculation that is most commonly displayed on garden websites) then the grower would have chosen a 4” fan and a 4” carbon filter; the grower would have soon found out that this fan and carbon filter would have been inadequate for their grow room.

Example 2:

If all the variables are kept the same and we changed the room size from 8’ x 8’ to a 12’ x 12’ then the minimum required CFM would be 519 cfm, thus the fan would be Fantech 8” XL, Elicient 8” or EcoPlus 8” and the DFS5 or DFS6 Dutch Breeze carbon filter.

The Really Simple Method:
If you want to keep things really simple, just remember that you want to replace the air in your grow room every one to three minutes. If you're in a hot area, exchange it every minute; if you're in a cooler area, you can take up to three minutes.

The Very Important and Often Overlooked Intake Port
It is very important to understand that you can't simply put an exhaust fan in a sealed room and expect to suck air out of it. You absolutely must have some kind of opening in the room to allow fresh air to replace the air you are exhausting. If you don't understand this concept, just put your finger over one end of a straw and try sucking out of the other end - you'll find that the walls of the straw collapse inward and that you can't suck any air out. If you were to poke a small hole somewhere in the straw, you will find that you can now suck air through it; the bigger the hole, the more air you can suck. Obviously, any fan you install in your grow room isn't going to be powerful enough to create enough negative pressure to suck the walls inward, but what will happen is that your fan will draw more energy, make more noise, run hotter, break down much sooner than it should, and finally, suck very little (if any) air out of the room. 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, odor, etc. from passing. You can get creative with these devices and use one fan to control two rooms, etc.

One last 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 grow room, thereby cutting down greatly on the amount of work the exhaust fan has to do and allowing much more air to pass. Ideally, the intake fan should be the same size as the exhaust fan, but it can be a little smaller if necessary. If you're not sure or you don't want to spend the money, start out with just an exhaust fan. If it's not performing as well as you thought it would, try adding an intake fan - you'll smile when you see the difference!

Fan and Carbon Filter Placement

If no carbon filter is being used then place the fan at the highest possible point in the room; the reason for this that hot air rises and it is the hot air that needs to be exhausted. Also, place the fan 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 garden and towards the fan. This will bring the fresh air over the plants while displacing the old stale air.

If a carbon filter is being used then place the fan on top of the filter. There are two options that a grower can use with the carbon filter: 1) Suck through the filter or 2) Blow through the filter.

Sucking air through filter is the preferred method. Carbon filters should be placed on the floor standing upright or hung horizontally from the ceiling. Place the fan and filter in the room opposite the intake port. From the fan, attach ducting so as to exhaust the scrubbed air out the room. Keep the ducting as short as possible and avoid any bends in the ducting. Keeping the ducting as short and as straight as possible will reduce the friction loss and noise.

Blowing through the filter is a less commonly used method of exhausting. The benefit is that the fan can be placed at the highest point in the room and the hot air is sucked out of the room and forced through the filter. However, if you're using the fan/filter combination as part of your exhaust system (as opposed to just an odor scrubber), it will need to be placed outside of the growing area.

Before using a carbon filter for the first time always take the filter outside and then first suck, then blow through it 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 stops large dust particles from blocking the filter and will help prolong the life of the filter.

Air Circulation

Always provide your plants with adequate air movement and circulation in the grow room. Air circulation within the room eliminates stale spots and also reduces the formation of micro-climates within the leaf canopy. If air circulation in the grow room is poor then the micro-climate in between the plants increases in humidity and temperature. This can result in mildew and insect problems, as well as less 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.


Well-Known Member
WTH is wrong with people?
This is a great read. Tons of useful information here. Take three minutes and learn something...


Well-Known Member
NICE!!!! this is deff. going to help me out!! I knew a bit about the venting but you right, there not alot talkin about the add-ons and such.Thank You!


Active Member
This thread helped me out a lot! I was thinking I was going to go with bigger fans but with my 24"x24"x70 ill go with 2 65cfm for intake, exhaust and a larger one to pull heat from my 400wt hps with cool tube.


Well-Known Member
This thread helped me out a lot! I was thinking I was going to go with bigger fans but with my 24"x24"x70 ill go with 2 65cfm for intake, exhaust and a larger one to pull heat from my 400wt hps with cool tube.

This is a good find for me, thanks. I was actually just getting ready to draw up some ideas.


Well-Known Member
Thanks for posting. I wish I would have read this before I bought my filter. I was venting just fine before I added a filter, now the heat is getting out of control. Too many people just give CFM for the fan without taking filter and ambient temp into account...