Grow Tent Ventilation: Passive Intake or an Intake Fan?

Grow tent ventilation is two jobs: an inline fan pulls warm, stale air out through a carbon filter, and fresh air has to get in to replace it. That intake is either passive, drawn through the tent's vents by the extractor, or active, pushed in by a second fan. Our fan and carbon filter guide sizes the extractor; this guide covers the intake side.

Passive intake: let the extractor do the work

Our grow tent setup guide gives the test: open a low vent, ideally opposite the fan, and the walls should pull in slightly; that negative pressure (see our glossary) stops smell leaking from the zips. If they pull in hard, open another vent or turn the fan down: a starved extractor moves less air, so the tent runs warmer.

Budbox Pro tents have passive vents with Velcro covers (two on the Budbox Pro 120cm X 120cm). Secret Jardin Dark Street tents have no pre-cut ports; each comes with one intake vent bracket and light baffle the same size as its extraction bracket, to fit where it suits the room. If the walls pull in hard with this vent open, that is the case for an intake fan.

As a rule of thumb, open more vent area than the extractor's duct, low down where the air is coolest. The tent gets the room's air: our humidity guide notes an extractor can only bring the tent down towards the room's humidity, and the same goes for temperature.

When to add an intake fan

Add a second fan when the walls pull in hard with every vent open, or when fresh air must come through a long duct from somewhere cooler. If the tent runs hot while the walls pull in only slightly, an intake fan from the same room will not fix it: the extractor is too small (see our fan guide) or the room is too warm.

The Budbox Pro 120cm X 120cm page says the tent suits up to 600 W of HID light or the LED equivalent; above that it recommends a high-powered 150 mm extractor as an absolute minimum, some control over air-intake temperature and, ideally, an active air-intake.

How big? Our advice: a fan one size smaller than the extractor, with a lower airflow rating on its page, on a speed controller so you can turn it down until the walls pull in slightly.

Fans that suit the intake

An intake fan has no carbon filter to pull through, so it needs less pressure. With a 150 mm extractor, our advice means a 125 mm fan from CAN RK Fans, Systemair RVK Fans or Air EC Inline Fans. CAN Max Pro Fans start at 150 mm, so they partner a 200 mm extractor.

What to fit on the intake

Check Bug Barriers regularly to keep them free of debris, as their page says; a clogged screen restricts the fan (our aphids guide covers pests that fly in).

Where the air comes from, summer and winter

A grow room works like a tent: extract high, let air in low on the opposite side, and keep extraction a little ahead of the intake. Take intake air from the coolest, cleanest place you have, and send the exhaust where it cannot be drawn straight back in: our tent size guide lists a window, loft, chimney breast or wall vent. An extractor blowing into the small room the tent breathes from is warming its own intake.

In winter, air from a loft, garage or outside can chill plants at lights-off. Draw from a heated room if you can, and fit Back-draft Shutters on ducts to the outside: their page says they stop cold air entering when the fans are off. They are spring-loaded, so on a passive intake check the walls again after fitting one. See our heaters guide for the dark period.

Running two fans together

Of our fan speed controllers, these suit two fans.

• SMSCOM Twin Controller 4.5A. Its page says it runs intake and extraction fans together on temperature and humidity, but not whether AC or EC: ask us which fans it suits.
• G.A.S AC-EC Controller. For AC or EC fans; its page lists balancing fans for negative pressure, day and night settings, and minimum and maximum speeds to 1%. EC cables are extra (G.A.S Cable packs).
• Several EC fans. The G.A.S EC5 Fan Controller and G.A.S. Enviro4 EC Fan Controller pages say each controls more than one fan, but not that the intake can run slower than the extractor: for balancing, use the Twin or AC-EC.
• By hand. A Variispeed Fan Controller turns one AC fan of up to 300 W down: fit it to the intake. Its page says most fans should run at 35% or more.

Set the extractor to hold your temperature and the intake slower, then check the walls with the light on and again at lights-off, when a thermostat may slow the extractor: slightly in is right; bowed out means the intake is winning.

Passive or active: what to do

1. Size and fit the extractor and filter first (or a matched fan and filter kit), ducted out short and straight.
2. Open low vents opposite it and check the walls pull in slightly.
3. Run a full day with the light on. Temperature holding, walls slightly in: stay passive. Running hot, walls only slightly in: fit a bigger extractor.
4. If the walls pull in hard with every vent open, or you need air from a cooler room, add an intake fan one size smaller (our advice), low down and blowing in, with a Bug Barrier, balanced on a controller.
5. Before winter, fit shutters and plan heat for lights-off.

Quick answers

Does a grow tent need an intake fan?

Not always: passive vents are often enough. Add one when the walls pull in hard with every vent open, or to duct in cooler air from elsewhere. A tent that runs hot with the walls only slightly in needs a bigger extractor, not an intake fan.

What size intake fan for a grow tent?

Our advice: one size smaller than the extractor, such as a 125 mm intake with a 150 mm extractor, on a speed controller so you can turn it down until the walls pull in slightly.

What is negative pressure in a grow tent?

Slightly lower air pressure inside than outside, because extraction beats intake. The walls pull in a little, so any leak is inwards and smell leaves through the filter.

More Growers' Guides

• Fan and carbon filter size
• How to set up a grow tent
• Grow tent humidity
• What size grow tent?

Not sure what you need? Call the shop on 0117 971 3000 or pop in to the Bristol store and we'll help you pick.