In this chapter, we deal with air circulation and exchange within the grow tent while taking special care of dust and smell filtering.
When we talk about ventilation, we need to mention:
Internal fan: Moving air within the tent to help plants evaporate water and grow stronger stalks
Extractor fan: Removing hot, humid air and filtering smell
Air Intakes: Allowing fresh, cool, and dust-free air to flow into the tent
Internal air circulation is usually achieved by mounting one or two small 15-20cm / 6-8" oscillating fan called monkey fans. The name comes from their ability to cling to the tent frame using a rubberized clip. The air movement generated by these fans helps the plants to evaporate water and improve their water and nutrient uptake. Constant swaying motion also makes the stalks grow stronger. Do not use a too powerful fan (above 30W) as strong winds can cause windburn.
My choice: Instead of using a monkey fan I went with a 30cm desk fan that I disassembled and kept only the head unit. I mounted it from the top of the grow tent using a shoestring and 51mm binder clip, directing the airflow on the LED light below the fan. Running the fan at its lowest setting is enough to cool the lights while also provide sufficient air movement to the plants below the lights.
Hot and humid air is pulled out from the top of the tent through an active carbon filter to remove the smell. Having passive intakes and an active extractor fan creates negative pressure inside the tent, forcing air to get sucked into the tent through any openings. This ensures that air can only leave the grow area through the carbon filter. When sizing the exhaust fan try to get one that can exchange the air volume of the grow area in ~1 minute. When sizing the extractor fan calculate with extra airflow requirements:
Carbon filter(+30%)
Ducting (each feet +1%, each meter +3.3%)
90-degree bend in tubing (+10% per bend)
External temperature (Compared to the ideal 68-78°F / 20-26°C range: hot environments +20%, cold environments -15%)
The wattage of the lights (+1% every 100W)
You can use the below Google Sheets spreadsheet to calculate a rule of thumb fan rating. Copy the spreadsheet using the File - Make a copy option and fill the cells marked green. Try to find an extractor fan that matches the results close enough, preferably by overachieving the calculated rating.
As a practical example, we are going to check out how to control a 2-speed inline fan using Arduino. Out of the factory, the fan has a power cable with three wires: ACL, ACN, and Ground. The fan has a 3 state switch (Off/Low/High) directing the power between the 2 input wires of the motor. We are going to remove the switch and the power cable and replace it with a 4 wire power cable. This way we can power the two motor ACL inputs straight from the relay module controlled by the Arduino. We`ll need to take special care not to accidentally power both ACL lines at the same time. The same method shown below was used to automate the internal fan too.
The industrial carbon filter used for this build is rated up to 500 m³/hour - 300 CFM. The 5 cm / 2" thick filter layer is made from crushed carbon and comes with a dust cover. The filter is connected to the in-line extractor fan with a PVC coupling, this piece can be replaced using thick duct tape. I chose the PVC coupling as it is easier to disassemble. The filter and the in-line fan assembly is mounted on the grow tent rails using a couple of carabiner ratchets.
Note: During vegetative growth, the filter is not needed and can be disconnected from the exhaust fan. This will prolong the filter’s lifetime (typically 1 year).
The coupling is only needed if the filter does not fit directly in the extractor fan
Grow tents come with air intakes located at the bottom of the tent with a Velcro flap. Select a grow tent that has at least two intakes with dust filtering mesh screens on them.
