HOW TO CALCULATE YOUR GROW ROOM FAN SIZE

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.

Factors to Consider in Calculating Grow Room InlineFan Size

First things first, let's clarify what these factors mean, how they affect the grow room fan size calculation, and what exact percentage they've to contribute in the calculation. 

Factor 1: The Volume of The Room/Tent

At first, work out how much space your fans have to deal with, a.k.a, the active grow space. In other words, that's the space covered by your grow lights. And to keep the calculation simple,

Factor 2: Time of Complete Air Replacement

While calculating the inline fan size requirement for your grow room or tent, you've to find the amount of air you need to ventilate in each minute. Because you know, the size unit(CFM) sports the capacity of how much air the fan can ventilate in a minute.

Factor 4: Insulation of the Room

If you grow in a good insulation environment that doesn't have that much of heat exchange with the surroundings, you don't need to consider this factor. But if it's in a less-insulated/more sun-exposed areas like the attic, the basement, upstairs, south-facing bedroom, etc, then there are some important considerations. In any of these cases, the temperature will be either more or less than what you are supposed to maintain in your grow room.
Factor 5: The Ductwork Curves and Length

It's needless to say that, ducting bends and length will hinder the air extraction capacity of the fans. Based on your grow room size and plant numbers, there can be three ducting scenarios-

1. Minimal or no ducting with no bends.
2. Medium ducting runs(5-10 feet) with 2-3 bends.
3. Long ducting runs(10-20 feet) with 3-6 bends.

Factor 6: Lights and Other Heat Sources

As you know, lights and pumps are the two obvious heat sources in a lamp. If you use air-cooled lights in your grow room, that's a different case. But if they're not air-cooled, you've to take each of them in calculating your estimated fan size. And same goes to other heat generative devices like motors etc. 

HOW TO CALCULATE YOUR GROW TENT FAN SIZE

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 grow 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" inline 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!