CannaGLAP

Blog · Grow tents

Grow tents · 7 minute read · Updated October 15, 2026

Passive vs active intake: which does your tent need?

Plenty of growers buy an intake fan to fix a heat problem and end up with a louder tent, a smell problem and the same heat. The intake fan was never the bottleneck. So before you buy one, what are your tent walls doing right now with the exhaust running?

How passive intake works

An exhaust fan pulls air out of the tent. That leaves the air inside at slightly lower pressure than the room, so room air pushes in through any opening it can find. If you give it big, low openings, that is where it comes in. That is passive intake: no second fan, just holes in the right place.

That slight vacuum is called negative pressure, and it does two jobs:

• Smell control. Every leak at a seam or zipper pulls room air in, so tent air leaves only through the carbon filter. AC Infinity points to slightly concave walls as the sign you have it.
• Simple control. One fan sets the whole exchange. Turn it up, more air flows. Turn it down, less.

How big passive vents need to be

AC Infinity's rule of thumb is that the passive intake opening should be about three to four times the area of the exhaust opening, and several smaller openings can add up to that. Treat that as a target rather than a law, and lean toward the high end if the vents have mesh or socks over them, since those block part of the hole.

Why so much bigger? Think of air speed. A 6 inch fan moving 300 CFM pushes air through its duct at about 1,500 feet per minute. If the intake were only the same size, the air would have to rush in just as fast, and the fan would spend its effort fighting the vacuum instead of moving air. With four times the area, incoming air slows to under 400 feet per minute, and the fan delivers closer to its rating.

Count what you actually have. A 6 inch round port is about 28 sq in. Two of them is only about 57, short of the target for a 6 inch exhaust. Most tents also have rectangular mesh vents near the floor. Measure them: a 6 by 12 inch flap is 72 sq in on its own.

How to tell if your intake is too small

• Walls sucked in hard, not just gently bowed.
• The tent "breathes", walls pulsing in and out.
• Whistling at the zipper or vents.
• Turning the exhaust up barely changes the temperature. The fan is spinning faster but not moving much more air.
• Opening the front door drops the heat fast. That is the clearest test: if unzipping the door cools the tent within minutes, the tent was starving for air, and more low vent area is the fix.

Try the cheap fix first: open every low vent, and, during the lights-on period, partly unzip a lower corner of the door as a test. If that solves it, you need more passive area, not a fan.

When an active intake fan makes sense

• Bigger lights. AC Infinity says passive intake works best with lighting under about 400 W. Above that, you need more air, and passive vents may not keep up without very large openings.
• Air from somewhere else. If you are pulling cooler air from another room, a basement or a window through a long duct, the duct adds resistance that passive vents cannot overcome.
• Filtered intake. If you want to filter incoming air against dust, pollen or bugs with a fine filter, that filter restricts flow, and a fan helps push air through it.
• Bigger tents where one corner gets no fresh air, even with good vents.

What an intake fan cannot do: make the tent cooler than the air it pulls in. If the room is too warm, the intake fan just delivers warm air faster. Fix the source first. See grow tent too hot.

Setting up active intake the right way

1. Exhaust stays stronger. AC Infinity's guidance is plain: the exhaust fan should always be more powerful than the intake fan. If the intake wins, the tent goes positive, walls puff out, and unfiltered air leaks out of every seam. That is the smell problem in the opening of this post.
2. Set it by the walls. Start the intake low, bring the exhaust up to what your heat needs, then raise the intake until the walls are just slightly bowed in. Stop there.
3. Intake low, exhaust high, opposite sides. Cool air enters near the floor, sweeps past the plants, and leaves warm at the top.
4. Do not blow intake air straight at plants. Point it at the floor or a wall. Your clip fans handle canopy airflow.
5. Keep it out of the light path. Intake ducts and vents need a light trap or a bend so they do not let room light in during flower. Test with a light leak check.

Worked example: a 4x4 with a 6 inch exhaust

Say you run a 4x4 tent with a 450 W LED, a 6 inch carbon filter and a 6 inch inline fan, in a room at 74°F.

1. Exhaust opening: a 6 inch duct is about 28 sq in.
2. Passive target: three to four times that, so about 85 to 113 sq in of open intake.
3. What the tent has: say two low mesh flaps of 6 by 12 inches each. That is 144 sq in on paper, but mesh blocks part of each hole, so call it close to the target.
4. The test: run the exhaust at the speed your heat needs. Walls slightly in, no whistle, canopy within a few degrees of your goal? Passive is enough.
5. If not: open a third low vent first. Only if the tent still starves, or you want to pull cooler air from another room, add a small intake fan set below the exhaust.

At 450 W this tent sits right around AC Infinity's 400 W guideline, which is exactly where testing beats guessing.

Passive vs active at a glance

Humidity, pests and the room you breathe from

Intake air brings the room's conditions with it. If the room is humid in summer, a fast exchange brings that humidity in, and the exhaust alone cannot pull the tent below the room's moisture level. A dehumidifier in the room, not just in the tent, is often the better fix in flower. See how to lower humidity in a grow tent. The reverse is true in a dry winter room, covered in how to raise humidity in a grow tent.

Open vents are also a doorway for bugs. Keep the vent socks or mesh on, and keep the room around the tent clean. If you have had thrips or spider mites before, a fine-filtered active intake is one of the stronger reasons to add a fan.

Whichever you choose, log temperature and humidity at canopy height for a few days before and after a change. With a paired Bluetooth sensor, Canna GLAP keeps those readings end-to-end encrypted, so you have a record to compare instead of trusting memory.

Questions growers ask

Read next

• What size fan does a grow tent need?
• Carbon filters for grow tents: size, setup and when to replace
• Grow tent too hot? How to bring it down
• Grow tent setup checklist

Education only, for adults 21+ where home cultivation is legal. Updated October 15, 2026.