Ventilation and Environmental Control

How to Calculate the Size of Your Extraction Fan

Before we get into the standard calculations for working out your extraction fan requirements, first understand some key underlying concepts. Lots of beginner indoor growers underestimate their ventilation requirements. Insufficient ventilation is a problem that creeps up on growers. Here's why...

Five sizes of inline ventilation fans. These fans are used by indoor gardeners to bring air in and out of their gardens.

When plants grow (or bloom) they extract carbon dioxide (CO2) from the air. When they are small little cuttings, seedlings or juvenile plants, the amount of CO2 they use is relatively miniscule-after all, those tiny little things probably only have a handful of baby-sized leaves a piece.

The more your plants grow, the more they grow! That is, the more leaves your plants have, the greater their capactity to convert light into plant energy. This equates to an exponential growth rate. 

One way of hammering this important point home is to think of your plants in terms of total amount of leaf surface area. Total leaf surface area gives you a useful insight into the amount of transpiration and photosynthesis a plant will want to undertake in optimal conditions. A young plant with half a dozen small leaves may have a total leaf surface area of, say, 50cm2 whereas just a month or so down the line it could have several hundred leaves, many of them much larger, adding up to 10,000cm2 or more! That's a huge increase! Now, multiply this factor by the number of plants you have in your room and you should be able to see the following:

  • When plants are young, their environmental requirements are relatively easy to take care of.
  • When they mature, all too suddenly they can hit a "glass ceiling" where the amount of CO2 available is not sufficient for optimal development.

That's why so many novice growers report that "everything was going fine" until they their plants got bigger, then they seemed to get "tired" and just limp towards the finish line.

Indoor Gardeners tackle the depleted CO2 problem in two major ways.

  • Ventilation
    They vent out the old, warm CO2 depleted air from their grow rooms and replace with fresh air.
  • CO2 Supplementation and A/C
    They replenish CO2 levels using tanks of CO2 or a burner and cool the room using an air conditioner.

Here we're going to concentrate on the ventilation method. As the whole idea behind this method is air replacement, it's logical that the first thing we're going to need to do is calculate how much air needs to be replaced.

So, step one is to work out the volume of air contained in your active growing space. This is the floor space that's actually occupied by the plants (length x width) multiplied by the ceiling height.

North Americans measure extractor fan size in CFM (Cubic Feet per Minute)
Europeans measure extractor fan size in M3/H (Cubic Meters per Hour)

  • Required extractor fan size in CFM= Volume of active growing space (ft) x 1.33
  • Required extractor fan size in M3/H= (Volume of active growing space (m) x 60) x 1.33
  • In our example with 4 x 600W lights this is 8ft (2.4m) x 8ft (2.4m) x 8.2ft (2.5m), which gives the volume of the active growing area of 524.8 cubic ft (14.4m3)

Why is the volume of your growing area important? Because you need to change all the air contained in it once every minute!

This is a fairly standard rule of thumb for the majority of high intensity indoor gardens (without AC or supplementary CO2)

The CFM calculation is pretty easy. Your room's total volume in cubic ft. is the basis of the figure as we need to move this every minute. For the CMH equation we need to multiply the volume (in cubic meters) by 60 to step it up to the amount of air changes needed per hour.

You may have noticed the x 1.33 in the equations above. What's all that about?

Well, this allows for the use of a carbon filter attached to the extractor fan. Typically we can expect a drop in fan efficiency of approximately 25% when attached to a carbon filter. This figure is not universally definitive; it depends on the make and age of the filter and the length and course of ducting between the fan and filter and some other factors too!

The long and short of it is that we need to allow for this efficiency drop of 25% by simply multiplying our CFM or M3/H figure by 1.33.

Example Calculation:

If we run this equation through our example indoor garden it gives us;

Required Fan size (CFM) = (Volume of Active Growing Space) x 1.33

(8 x 8 x 8.2) x 1.33 =698 CFM

These final figures represent the minimum size extractor you'll need for the growing area in this example. A well-insulated location such as a basement should be fine using an extractor of this specification. Note that less-insulated and more sun-exposed location such as an upstairs, south-facing bedroom or attic will require an extractor around 25% more powerful.

Note that you will need to match your required extractor size to the nearest size available. In this instance the nearest widely available inline fan size is an 8" (200mm) 745CFM extractor.

What Goes Out ...

As we've already touch on, the ventilation method of grow room environmental control is all about replacing the air in your garden. So no disucssion of extraction requirements would really be complete without at least touching on how to get fresh air in!

Growers approach this in two different ways.

Passive air intakes are simply holes that allow fresh air in. The simple act of extracting air will cause the air pressure in your garden to drop so this fresh air will be pulled in through any gaps present. The larger the gaps, the more air. Like water, air will always find the easiest path of least resistance. Passive intakes can take the form of vents or even, at its most crude, an open window or door. We don't recommend this as you want to keep as much control over what comes into your indoor garden as possible-particularly with regard to bugs which can wreak havoc! So by far the best method is to cover all intakes with a bug mesh.

Active air intakes are typically smaller inline extraction fans that blow fresh air into the garden, rather than relying on air pressure. This is preferred by many growers because it puts less strain on the extraction fan and actually helps it to work more efficiently. The intake fan should be connected to an inline air filter to guard against bugs getting a free ride into your garden.