Calculating Fan CFM Requirements

Dan's Method - 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 here'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 8' x 8' x 8' 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 — 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 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 (32°C), however, we will be using air from another room that is air-conditioned. Here'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 I'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 it'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 8' x 8' to a 12' x 12' 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 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!

Fred's Method - Calculating By Wattage

Hello there. First off, I'm used to working with Celsius, not Fahrenheit, but I've done my best to provide formulas for both. My method for calculating fan requirements does not cover active cooling with air conditioning systems or cool-tube designs. We're talking about everyday grow chambers here, totally enclosed for airflow control, with no large amounts of radiant heat into or out of the box. Your mileage may vary some for these reasons.

RIGHT THEN, LET'S GET STARTED:

1) Start at the beginning and design this right! Before you even buy or cut anything for your new project, determine the highest temperature that your intake air will ever be when lights run. Call this T (inlet).

2) Use these formulas to determine difference in temperature you can tolerate. 80°F (27°C) is just about the optimal for growing most plants. You can go up to 86°F (30°C) if you have to, but aim for 80°F (27°C).

Tdiff = 27 °C – T (temperature of inlet air)

3) Add up wattage for all power sources in your indoor garden. Lights, pumps, heaters, humidifier, radio, coffee maker, whatever! Add it ALL up and call it Watts. If it is on for more than three minutes and uses more than a watt, add it up. This will make your number worst-case and therefore a conservative value.

4) Compute the absolute minimum fan power you will need using the following formulas. Fan power is measured in the amount of air (cubic feet) shifted per minute. The formula below is the minimum fan rating you must have to achieve your temperature goals. You will have to increase fan power to compensate for duct constriction, small inlets, carbon scrubbers, screens, or other items that block airflow.

CFM = 1.75 x Watts /Tdiff (in Celsius)

If you prefer to work in Fahrenheit, try this formula:

CFM = 3 x Watts / Tdiff (in Fahrenheit)

5) Get at least this fan power or don't come and ask questions! If you are going to have more than one fan, they should be mounted side-by-side rather than inline if you want to add their different CFM ratings. For inline fans, use the lowest airflow rating of all fans in the path. A fan on the inlet and a fan on the exhaust of the box are considered inline fans. Fans just circulating air inside the indoor garden should not be counted for airflow but must be included in your initial wattage calculations.

OK, TO SEE THESE FORMULAS IN ACTION WE'RE GOING TO HAVE TO DO A LITTLE NUMBER CRUNCHING:

An Example 
Ok, let's say you have 2000 watts in a 8 foot by 8 foot room with an 8 foot ceiling height.

So what amount of air do I need to move to keep the room at 82°F (28°C)? My incoming air temperatures are 68°F (20°C) during the lights on period.

Tdiff = 28 – 20 = 8°C

For Celsius the formula comes out at: 
CFM = 1.75 x 2000 / 8 = 438 CFM

For Fahrenheit we get the following: 
Tdiff = 82 – 68 = 14°F CFM=3x2000/14=429 CFM

Remember, Tdiff shows how much your temperatures will rise above your inflow air temperature for a given wattage and air movement.

If you are adding any carbon scrubbers or extensive ductwork, this is where you add to the fan size to account for air pressure losses. You have to move this many CFM, or the numbers don't come out right. Exactly how much these items diminish your airflow depends on your exact configuration and is beyond the scope of this introductory article!

What to do when your outside temperatures are higher than your maximum allowed indoor garden temperatures!

YOU HAVE A FEW CHOICES:

1) Stop growing for a while till things cool off or try running your grow lamps at night when inlet air will be cooler.

2) Reduce your lighting to drop the heat load. Not good if the incoming air is already over critical when it arrives in the box. Might be possible if the inlet air temperature is lower but you are running too many lights to keep up with the cooling.

3) Use active air conditioning.