Ventilation Setup: The Short Answer
Size the exhaust fan to move the tent's full air volume once per minute. Add 25 percent for the carbon filter and 20 percent for duct bends. A 4x4x6.5-foot tent holds 104 cubic feet, so a fan rated near 160 CFM works. Run the exhaust fan 24 hours a day. Add one oscillating fan inside the tent. Cut a passive intake opening at least twice the area of the exhaust duct.
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Step 1: Measure the tent volume
Multiply length by width by height in feet. A 2x4x5 tent holds 40 cubic feet. A 5x5x6.5 tent holds 162.5 cubic feet. Write the number down. Every later step uses it.
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Step 2: Pick fan CFM
Inline fan ratings vary by size. A 4-inch fan moves 100 to 200 CFM. A 6-inch fan moves 300 to 450 CFM. An 8-inch fan moves 600 to 750 CFM. Ratings come from tests at zero static pressure, so real airflow in a tent is lower.
Step 3: Match the carbon filter
The filter CFM rating must meet or beat the fan CFM rating. An undersized filter restricts airflow and pushes odor out of the tent seams. Replace carbon when odor returns, which is 12 to 18 months in most grows. Higher humidity shortens that life.
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Step 4: Route the duct
Keep duct runs under 10 feet. Each 90-degree bend costs about 10 to 20 percent of airflow. Two bends turn a 200 CFM fan into roughly a 150 CFM fan. Insulated duct lowers noise and heat gain from the room.
Step 5: Build the intake
Passive intake with a light trap is the common setup. Intake area should be twice the exhaust duct area. Active intake fans work only if their CFM stays below the exhaust CFM. Otherwise the tent runs at positive pressure and leaks smell.
Step 6: Move air inside the tent
One clip fan or oscillating fan per 2x2 feet of canopy. Aim it across the top of the canopy, not straight at buds. This breaks up humid pockets and strengthens stems.
Environment targets
- Temperature: 70 to 85 F with lights on.
- Lights-off drop: keep it under 10 F.
- Relative humidity: 60 to 70 percent in veg, 40 to 50 percent in flower.
- Air exchange: once per minute, more in hot rooms.
Common problems
- Odor outside the tent: filter undersized, or duct joints not sealed.
- Tent walls pulled in hard: intake opening too small.
- Heat buildup: fan too small, or duct run too long.
- Noise: use an EC fan on low, insulated duct, and a silencer.
What Is Not Known
No standard test measures grow tent fan airflow under a filter load. Published CFM numbers are start-of-life ratings. Airflow drops as the filter loads with dust and carbon fines. Clean pre-filters and check for a partial clog every 30 days.