Exhaust and Ventilation
Exhaust and Ventilation - What's The Difference?
Exhaust and Ventilation go hand in hand when discussing air control in a grow room but it important to understand that they are two different functions with two different goals.
Exhaust is the removal of excess heat and humidity to help maintain your environment.
Ventilation is the addition of fresh air into the grow room so the plants can thrive.
While both if these functions can be achieved with a single exhaust fan, more often it is easier to control the environment in your room when using two separate fans - one for removing excess heat for your lights and another one just for air exchange in the grow area.
Single Fan Setup
The most simple setup is a single in line exhaust fan connected to a carbon filter for odor control. If using HID lights and air cooled hoods, the exhaust ducting runs through the air cooled hood and then to the carbon filter and is set to run whenever the light is on. If using LED only the fan and filter are necessary.
This setup can work well when the ambient outside air is similar to the desired air temp and humidity inside your room. Any fresh air brought into the room will already be the same or close to the ideal conditions you are trying to achieve.
Dual Fan Setup
When the ambient outside temperature and humidity is very different to the conditions you are trying to achieve, (think Summer and Winter), it is often better to use a dual fan setup. The reason for this is that if you are constantly exhausting all of the air out of the grow room, you are also exhausting all of the heat and humidity you are providing. causing your environmental control system to work overtime - and costing you money!
With a dual fan system, one fan is connected to your air cooled light hood and fed fresh air from outside the room and the second smaller fan connected to a carbon filter is used to exchange the air in the room. The first fan is connected to the same timer as the light and will turn on whenever the light is on.
This second fan should be connected to an intelligent controller that turns the fan on and off periodically or when the heat and humidity exceed the desired levels in the room. Programming the controller just at the upper temp and humidity ranges causes the fan to come on often to ensure enough air exchange in your room.
Air Intake
In order to provide fresh air into your room an intake air vent needs to be provided. Intake vents can be passive or active.
Passive intake relies on natural airflow through holes or vents in the walls of your room. The flow is created from the exhaust fan pulling air out of the room and creating negative pressure, causing new air to be pulled in. An air filter should be used on the intake vent to scrub any impurities from the air before it enters the room.
Active intake, on the other hand, works by pulling air into your grow room using an intake fan This setup actively pulls fresh air into the room while the exhaust fan is also pulling air out of the room. This can be helpful if the air vent is too small to provide enough fresh air to replace the exhausted air. Most small grow rooms will only need a passive intake vent.
Exhaust fans and CFM (Cubic Feet per Minute)
Exhaust fans are rated in CFM or Cubic Feet per Minute. This measures how much volume of air can be exchanged per minute. It is important to size your exhaust fans correctly to ensure the fan will be able to meet the demands needed. You should always pick an exhaust fan that is just slightly larger than the one you need and get an exhaust fan with adjustable controller so you can adjust the fan down to your specific setup.
Calculating CFM for Single Fan System
When calculating the CFM for a Single Fan System, calculate the total volume of your grow room by multiplying the L x W x H to get the total cubic feet of the room and divide by number of minutes you want the air to be exchanged. Air should be exchanged every 3-5 minutes in a healthy grow room environment so you would divide the total by 3.
An 8 x 8 x 8 grow tent equals 512 cubic feet divided by 3 is 170 CFM to exchange the air every 3 minutes.
You also need to account for the grow lights, ducting and carbon filter. For HID lights ad an additional .25 per watt of light. For LED lights add an additional .125 per watt.
For a 600 Watt HID light multiply 600 x ..25 and you get 150 additional CFM. for a total of 320 CFM.
For a 600 Watt LED light multiply 600 x ..125 and you get 75 additional CFM. for a total of 245 CFM
.
Add another 20% for each carbon filter and 10% for every 10 feet of ducting. For a simple system with one 600 Watt HID light, one filter and 10 feet of ducting in the above room would be"
320 + 64 + 32 = 416 total CFM needed.
Calculating CFM for Dual Fan System
When calculating the CFM for a Dual Fan System, you need to calculate the CFM for your light fan (Fan 1) and your ventilation fan (Fan 2) separately. Using the the example above we would have:
Fan 1 - 600W x .25 = 150 CFM add 10% for the ducting and you get 165 CFM.
Fan 2 - 8 x 8 x 8 = 512 divided by 3 = 170 CFM. Add 20% for the carbon filter and you get 207 CFM
You will notice that the two examples don't add up to the same CFM. This is because the carbon filter in a single fan system restricts the air flow of the entire system, while in a dual fan system the filter only restricts the air flow of Fan 2 lessening the overall CFM needed between the two fans.