To Calculate the fan size that you’ll need for your grow room or grow tent is a relatively straight-forward process. However, before we get into those calculations – let’s take a brief look at the concepts that make up the need for having a correctly-sized extraction, and, intake fan in the first place.
First of all, having the correct fan size is something that first-time growers tend to overlook. Incorrect fan size can lead to insufficient ventilation – which is a recipe for problems during the entire growing and flowering cycle.
As you may already know, plants survive on CO2 (carbon dioxide) which they extract from the air around them. When they’re first beginning to develop, the smaller-sized plants don’t need all that much CO2 because they’re not supporting a large number of leaves. But as the plants grow, the oxygen requirement increases drastically.
Consider, for example, the total surface area of your plant’s leaf. This is where your plant’s photosynthesis and transpiration takes place. The process is vital for energy provision for the plant. Young plants may only have total leaf surface area of, let’s say, 50cm2. But in a short period – maybe 30 days, the plants’ leaf total surface area may get up to 1000cm2 or more! (This is just a conservative estimate). So, in essence, when your plants are in the early stages of their growing cycle, taking good care of them is a relatively easy task in regards to providing enough CO2.
In a matter of time, the plants hit a specific size where optimal plant growth and development is hindered by not having enough CO2.
When plants lack enough CO2, their production drops. Indoor growers with little or no experience are often walloped when their product is affected. There are two proposed methods to fix the problem:
Better ventilation – Venting the old CO2 - heavy air out of the growing area and replacing it with fresh air.
Cooling and Supplementing- Room cooling with A.C. and adding more CO2 (with a burner or tanks of CO2).
Performing the calculations
First, a quick note regarding measurement standards. There are two types, European and American. Below are the workings for both:
Europeans use the standards Cubic Meters per Hour (M3/H) when measuring their extractor fan size.
In North America, extractor fan size is measured in Cubic Feet per Minute (CFM).
Let’s take things a little further for you to understand the difference between the above two standards:
For Cubic Meters per Hour (M3/H) …
You need extractor fan size in meters per hour. That is, the volume of active growing area in meters multiplied by 60 then multiplied by 1.33.
A quick, simple example;
Suppose your growing tent measures 10ft long, 8ft wide, and 8.5ft tall. The first step is to convert your measurements into meters. 1ft = 0.3M. Our new measurements will be 3M by 2.4M by 2.55M, which gives 18.36M3 as the total volume in cubic meters of the growing area.
As mentioned above, the total volume is multiplied by 60, then by 1.33. Why? We multiply by 60 because our equation is in meters per hour and we need the number of air changes per hour. We then multiply by 1.33 to allow for the use of a carbon filter attached to the extractor fan. The 1.33 takes care of the drop of efficiency of approximately 0.25 or 25% once we assign a carbon filter. Note that 0.25 is subject to change depending on several factors such as the age, make, and length of the screen, among others.
For the Cubic Feet per Minute (CFM) …
The workings here are quite straight forward. Using our previous tent measurements of 10ft by 8ft by 8.5ft, we can get the required fan size by performing the following calculations:
Note that there’s no converting the units into meters as we’ve already got the feet measurements as the base dimensions.
The volume of the active growing area = 10ft by 8ft by 8.5ft by 1.33 = 904.4 CFM.
(Please continue reading as there are further factors that need to be taken into consideration).
Please ensure that …
You work out the correct calculations whether you’re working with European or American measurements.
Look at the fan's specs before buying, and, ensure that the CFM is enough for your grow room or grow tent.
How much air exchange is necessary?
The rule of thumb here is that you need an exhaust fan that can exchange the air in your grow tent/room at least once in every three minutes. Since most exhaust fans have their rates in CFM, we shall carry out our calculations in CFM.
To determine how much air exchange is necessary or our CFM requirements, we’ll use our above example. Note that we’re assuming there’s no carbon filter attached, and therefore are not multiplying with 1.33.
Our tent measures 10ft by 8ft by 8.5ft. So our volume will be 680 cubic feet. We divide our volume by 3 minutes to get the minimum expected exhaust fan for our room. That is, 680/3 gives us 226.67 CFM. Please understand that this is to provide you with a quick estimate and under no circumstance should you settle for it as your final exhaust fan requirements. You’ll often need more ventilation as we’re going to discuss below.
Before you can get excited about anything that we’ve just covered above, we have other considerations – that are equally important. Let’s have a look …
Filters – although we had already captured this in our very first example, it’s imperative you understand that whenever you add a carbon air filter to your exhaust system you should add on 25% of your CFM.
High-Intensity Discharge Lights (HID Lights) – If you use grow lights that emit heat, such as Metal Halide or HPS lights, you’ll need more ventilation. To take care of this effect, add 5% of your CFM for each HID light.
Temperatures – if you realize that your grow tent gets hot, you may want to add 25% of the CFM to manage the temperatures. If your room temperatures don’t get that hot, there’s no need for this addition. Note that areas with hot and humid climates may need an addition of up to 40%.
Let’s now have an example that captures all of the above considerations. We shall stick to our original measurements.
Suppose our grow room is 10ft long, 8ft wide, and 8.5ft tall.
Calculate the volume of our grow room. 10ft by 8ft by 8.5ft = 680 cubic feet.
want fresh air exchange every 2 minutes. So we divide our CFM in step one above
by 2. That is, 680/2 = 340 CFM.
We now want to factor in all other variables in our calculations. We use the above-recommended percentages, as shown below:
CFM in step 2 + (CFM x 25% for our carbon filter) + (CFM x 5% for air-cooled HID lights) + (CFM x 25% for temperatures)
340 + (340x0.25) + (340x0.05) + (340x0.25)
340 + 85 + 17 + 85 = 527 CFM
NOTE:If you’d like to exchange the air in every three minutes instead of two, then divide your CFM in step one above by three. If you’d like air to be exchanged in every minute, then leave your CFM in step one as it is.
Please do not get confused. In our very first example, we only factored in the carbon air filter. We instead used 1.33. But remember we said that it’s because the carbon air filter affects the efficiency by 25%. In our last example, instead of using 1.33, we’ve used 25%, which is equally the same concept.
Once you've worked out your CFM as shown above, it will represent the minimum size of the fan extractor that you’ll need to obtain for this particular growing area. For areas that are not exposed to high temperatures you can ignore the last part for temperatures.
Of course, you will want to match the size of your calculations to the nearest available size fan. It’s usually a better idea to measure slightly up, rather than down.
Air that comes in
Since we’ve explained the need to have air circulated in your growing area, we should also touch upon getting new, fresh air into your growing area. There are two common ways to look at this task.
An active air intake consists of a smaller fan that will draw in more fresh air into your garden area. This is preferred by many growers because it puts less strain on the extraction fan and helps it to work more efficiently. These are some ways to tackle the necessary task of ensuring that your growing area remains at an optimal level regarding CO2 content. When you accomplish the process, your garden will produce far better, and your plants will remain healthier.
A passive airintake will cause a growing area room to have a drop in air pressure. The result is that fresh air is drawn in from any crack and crevice. The higher the number and size of these gaps, the greater the amount of new air that is brought in. The benefit of a passive air intake solution is that it merely consists of holes in your growing room that lead to the outside; therefore, there’s no cost involved. However, you’ll want to have a bug mesh that covers each intake hole.
Calculating Carbon Filter Size
Once you've worked out the size of your fans you can then simply find the same sized carbon filter. For example if your grow room needs a 6 inch fan you just need to buy a 6 inch carbon filter. Then simply connect the carbon filter to the extraction fan within the grow room.