Grow Room Ventilation: Sizing & Installing an Extraction System
Grow Room Ventilation: Sizing & Installing an Extraction System
Once your tent is up, extraction is the next thing to fit — and it’s the single most important piece of kit in the room. Plants release moisture and consume CO₂; lights and ballasts produce heat. Without air exchange, temperature and humidity climb, CO₂ runs down, growth slows, and mould and pests get comfortable.
This guide covers the whole job: what the system does, how to size the fan properly, how to assemble and hang it, and how to run it once it’s in.
What a Ventilation System Actually Does
Ventilation is the process of pulling hot, humid, CO₂-depleted air out of the growing area while fresh, cooler, CO₂-rich air comes in. Get it right and you gain control over temperature, humidity, CO₂ levels, pest pressure and overall air quality in one go.
The parts you definitely need
- Extractor fan — moves air out of the tent.
- Carbon filter — scrubs odours before the air leaves.
- Ducting, clips and clamps — carries and directs the air, and joins it all together.
- Intake vents or an intake fan — lets fresh, CO₂-rich air back in.
- Air circulators — mix the air inside the tent so you don’t get hot spots or stratification.
- Fan speed controller — keeps conditions steady instead of running flat out.
- Hanging equipment — rope ratchets, or chain and S-hooks.
Our extraction kits are supplied with the fan, filter, ducting and clips together, so you’re only adding hanging gear and a controller.
Worth adding
- Bug blockers — stop pests being dragged in through the intake.
- Temperature and humidity meter — you can’t manage what you don’t measure.
- Grow room heater — extraction in winter can pull the room too cold.
- Humidifier or dehumidifier — for when ventilation alone can’t hold RH where you want it.
- CO₂ supplementation — ventilation caps you at roughly ambient CO₂ (500–600 ppm). Plants can use 1,000–1,200 ppm, but only in a sealed or semi-sealed room.
Sizing Your Extractor Fan
You need to know how many cubic metres of air to move every hour. Four steps.
Step 1 — Work out your volume
Measure your active growing area — the space actually covered by your lights. If in doubt, just use the tent’s dimensions.
Length × Width × Height = volume in m³
Step 2 — Multiply by your target air changes per hour (ACH)
This is where LED and HPS rooms differ:
- LED fixtures: 30–45 ACH. LEDs push less radiant heat into the canopy, so you can run lower.
- HPS fixtures: 45–60 ACH. More heat at the canopy means more air changes.
Step 3 — Add back the losses from your filter and ducting
A carbon filter and a run of ducting typically cost you around 25% of your fan’s rated airflow. To recover a 25% loss you need to multiply by 1.33, not 1.25.
If your ducting is long or goes round several bends, add another 20% (× 1.2).
Step 4 — Adjust for the room
- Warm attic: × 1.2
- Basement: × 0.85
- Air-cooled lights: × 0.7 – 0.75
South-facing rooms run warmer than north-facing ones — factor that in by eye.
Worked example
| 1.2 × 1.2 × 2.0 m tent, 1 × 600W LED, in a warm attic |
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The same tent under a 600W HPS at 60 ACH works out at roughly 276 m³/h — a useful illustration of how much extraction you save by switching to LED.
Suggested fan sizes for 600W LED fixtures
Guideline pairings. Step up a size if the room runs warm or the duct run is long and twisty.
| Tent & fixture | Volume (m³) | Target ACH | Required flow (incl. +30%) | Suggested fan |
|---|---|---|---|---|
| 1 × 600W LED in 1.2 × 1.2 × 2.0 m | 2.9 | ≈ 45 | ~170 m³/h | Mammoth TT 125 |
| 1 × 600W LED in 1.5 × 1.5 × 2.2 m | 5.0 | ≈ 45 | ~290 m³/h | Mammoth TT 150 |
| 2 × 600W LED in 2.4 × 1.2 × 2.2 m | 6.3 | ≈ 45 | ~370 m³/h | Mammoth TT 150 (TT 200 with long duct runs) |
| 4 × 600W LED in 2.4 × 2.4 × 2.2 m | 12.7 | ≈ 45 | ~740 m³/h | Mammoth TT 200 (TT 250 for hot lofts or summer) |
Sizing Your Intake
Now air is leaving, fresh air has to get back in. The rule that matters here is negative pressure: always draw in less air than you extract, by around 15–20%.
Get this wrong and the tent overfills. Unfiltered, smelly air then seeps out through joins, zips and port socks instead of going through your carbon filter — and in bad cases the tent visibly inflates.
Option A: passive vents
For a small room with a single light up to 600W, passive vents at the bottom of the tent are usually enough. To check whether yours are:
You need 0.09 m² of free air space for every 1,000 m³/h you extract.
| Example — one 200 × 200 mm grilled vent |
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So a single 200 mm grilled vent covers our 207 m³/h example comfortably. For every 1,000 m³/h extracted you’d need about four of them.
Option B: an active intake fan
Once you’re extracting a serious volume, vents won’t keep up. Choose an intake fan rated 15–20% below your extractor’s rated output, or run a matched fan at reduced speed.
For our example: 207 m³/h × 0.8 ≈ 165 m³/h of intake.
Assembling the System
Two common kit configurations. Both follow the same principle: the fan must pull air through the filter, never push.
Mixed-flow fan + filter, joined with ducting
- Plug the fan in and check it runs before you assemble anything.
- Lay the fan and carbon filter out on the floor. Check the arrow on the fan housing points away from the filter.
- Cut a short piece of ducting to sit between the two.
- Connect that short piece to the filter’s spigot with a duct clip.
- Connect the other end to the intake side of the fan. Re-check the arrow direction.
- Connect the remaining length of ducting to the fan’s exhaust with your last clip.
Fan + filter, joined with a fast clamp
- Plug the fan in and check it runs.
- Line the fan and filter up, arrow pointing away from the filter.
- Use the fast clamp to join the fan’s intake directly to the filter spigot — no ducting between them.
- Duct-clip your ducting to the fan’s exhaust end.
A fast clamp gives a tighter, more efficient join than ducting and a clip, so use it where the kit supplies one.
Matching the filter to the fan
Use a carbon filter rated for at least your fan’s airflow. If you’re between sizes, go up — an oversized filter is less restrictive and quieter.
Hanging It Safely
Fans and filters are heavy. Spread the load across more than one support bar and never hang both off a single point.
Using chain and S-hooks:
- Loop two lengths of chain around the carbon filter and resize them to form a cradle, high enough to sit near the top of the tent.
- Move the tent’s support bars to where you need them and get the filter cradle into position.
- Create a small loop of chain on the support bar to take the fan.
- Make a hanging point on the fan with a short piece of chain.
- Attach an S-hook to that hanging point.
- Move the assembled system into the tent and line it up under the hanging gear.
- Lift carefully and hook the fan onto its chain loop.
- Manoeuvre the filter through the two chain loops so the cradle takes its weight.
- Push the ducting out through the tent port from the inside.
- Pull it taut from the outside so it sits flush.
- Connect the fan’s power lead, feed it out through a small port, and plug it into the mains or your controller.
- Tighten the drawstrings on every port sock.
Rope ratchets do the same job with easier height adjustment if you’d rather not use chain.
Positioning
- Intake low. Vents or intake fan at the bottom of the tent.
- Extraction high, and opposite. Extractor fan near the top, towards the back, on the opposite side to your intake. This drags air diagonally across the whole space instead of short-circuiting.
- Circulators aimed at the corners — and at ballasts, reflectors and chillers where heat pools. Never point them straight at the canopy; that causes windburn.
Running It Day to Day
Use a fan speed controller
A basic controller holds a set speed. A temperature-triggered or EC controller speeds the fan up when the room warms and slows it when it cools — quieter, steadier, and far less hands-on.
Set a minimum speed of 10–30%. Extraction isn’t only about temperature: even in a cool room you still need to shift humidity, replenish CO₂ and hold negative pressure.
Targets to aim for
Temperature
- Lights on: 24–27 °C
- Lights off: 21–24 °C
Relative humidity
- Propagation: 70–85%
- Vegetative: 65–75%
- Flowering: 45–65%
Under LEDs, leaf temperatures sit slightly lower than under HPS. Don’t be afraid to raise ACH through late flower to dry the room out after lights-off.
Check for negative pressure
The tent walls should suck very slightly inwards while the system runs. That’s your confirmation that odours are going through the filter rather than escaping round it.
Top Tips
- Vent well away from the tent. Ideally through a drilled hole into the loft, up a chimney, or straight outdoors. Failing that, to an open window or a separate room with one. Putting a tent in a room and shutting the door just recirculates the same air.
- Take intake air from inside the house. More stable temperature, more CO₂, fewer pests.
- Fit a bug blocker on the intake fan or intake ducting. Anything that stops pests reaching your plants is worth the effort.
- Keep ducting short and straight. Every extra metre and every bend costs you airflow. Use metal bends where corners are unavoidable, and a fast clamp where you can.
- Run fans 24/7. With a controller you can raise and lower speed for lights-on and lights-off rather than switching off entirely.
- Wash the pre-filter. The sleeve on your carbon filter catches particles before they reach the charcoal. Machine wash it with no detergent whenever it looks dirty.
- Replace carbon filters on schedule. Under normal conditions, a BAY6 filter lasts around 9 months and a CarboAir 50 around 18 months. Prolonged high humidity shortens both — another reason to keep the climate under control.
- Check the impeller. Dust build-up on the fan blades quietly kills airflow over time.
Extraction sized and installed properly is what makes every other decision in the room work. Get this right first and the rest of the grow gets a lot easier.
What’s Next?
Next in this series: Step 4: Lighting Configuration: Ballasts, Reflectors, LEDs & Timers
This blog is part of our How to Grow Indoor Plants – Your Complete Guide series, designed to tell you everything you need to successfully grow indoors.
