Grow Room VentilationApril 1, 2019
Deciding on the right extraction can be made easier by using the calculations found below. 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
When calculating what extractor fan you need, you have to take, friction loss on carbon filters, increased temperatures and CO2, etc into consideration. The following calculation can be used as a guide for sizing an extractor fan.
Step 1 – Your Grow Room Volume
First the volume of the grow room needs to be calculated. To calculate the volume of your grow space, multiply:
Length x Width x Height of growing area.
For example, a room that is:
8′ x 8′ x 8′ will have a volume of 512 cubic feet.
Step 2 – Cubic Feet per Minute Required
The fan should be able to exchange the air in a grow room once every three minutes, or at least 20 times an hour. Therefore:
512 cubic feet/ 3 minutes = 171 Cubic Feet per Minute.
This will be the minimum CFM needed for the air exchange in your grow room.
The maximum amount of extraction you should use will exchange the air in the grow room every two minutes, or 30 times an hour. Using the calculation above, ( 512 CF/ 2 minutes = 256 CFM) you can find what extraction is best for your grow space.
This same principle works on any grow space, and when you’re buying your extractor fan, you can see how many CFM it will extract. There are some other factors that should be taken into consideration.
Step 3 – Additional factors
Once you have calculated the minimum CFM required for your 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: A carbon filter must be used with the exhaust system, for this add 20%
- Ambient temperature: For hot climates add 25%, for hot and humid climates, add up to 40%
Active and Passive Intake
Along with an exhaust fan, you must have some kind of opening in the room to allow fresh air in, to replace the air you are exhausting. Without intake, any extractor fan in your grow room will need more energy, make more noise, run hotter, and suck very little air out of the room. This will also reduce the life span of your extractor fan.
Adding an intake port will relieve pressure on your fans, and keep air moving around easily. Intake can be drawn from anywhere, such as, from a gap under the door or an open window. Maybe even a hole in the wall.
Place your intake port diagonally opposite from your exhaust fan. This will force the air to pass across the entire room before being extracted.
Bugs can occasionally enter though your intake. It is recommended you use a piece of screen, or netting, over the opening to keep them out. You can also use an old carbon filter or piece of A/C filter to keep dust, and bugs out of your grow room.
Be careful of light leaks coming through your intake ventilation too, you should check to make sure no light interrupts your plants night cycle.
You will see much better results from your exhaust system if you use a second extractor fan to create an active intake system. Your exhaust fan sucks air out of your room, as a result, air is passively going to get sucked back into the room.
By installing a second fan on the intake side, blowing into the tent, you will reduce the amount of negative pressure created in the grow room. Doing this will cut down on the amount of work the exhaust fan has to do and allow much more air to pass through the grow room.
Ideally, the intake fan should be the same size as the exhaust fan, but it can be a little smaller if necessary. Passive intake will work fine, but active intake will make controlling temperature and humidity much easier.
Fan and Carbon Filter Placement
It is best to place the extractor fan at the highest possible point in the room, outside of the grow tent if possible. The reason for this is that hot air rises and it is the hot air that needs to be exhausted.
Also, place the extractor 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 plants and out of the exhaust. This will bring the fresh air over the plants while displacing the old stale air too.
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 the air out
Sucking air through the filter is the preferred method. Carbon filters are best hung horizontally, from the ceiling of the grow room, or tent.
From the extractor, attach ducting to the carbon filter, so it exhausts the scrubbed air out the top of the grow room. Keep the ducting as short as possible and avoid any bends. Doing this will reduce the friction loss and noise.
Blowing the air out
Blowing through the filter is not a commonly used method of exhausting. If you’re using the fan/filter combination as part of your exhaust system, the filter will need to be placed outside of the growing area.
Before using a carbon filter for the first time, you should take the filter outside. Use your extractor to first suck, then blow, through the filter 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 will stop large dust particles from blocking the filter and will help prolong the life of the filter.
Air Circulation in a Grow Room
Always provide your plants with adequate air movement and circulation in the grow room. Air circulation within the room eliminates trapped air and also reduces chances of rot and mould forming.
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, whilst lights are on. The fans can blow droplets onto the bulb, and they will break.
Final notes on Grow Room Ventilation
Air flow, and good grow room ventilation is an underestimated part of growing cannabis indoors. So, do not cut corners here, especially with your carbon filter! Covering the smell is rule number three, do not break it.
Ensure the filter, and extraction, is the best you can afford, knowing the smell is not going to escape, will give you peace of mind. And supplying a regular amount of fresh air to your plants will keep them happy, and healthy.
Thanks for reading! You can find more information on anything related to growing in our cannabis grow guides. All of our guides are written by experienced growers, and members of our cannabis growers forum. If you have any questions, sign up. It is free, and we are always happy to help!
Stay safe out there Percy Growers. Happy growing!