How to Size an Extraction Fan and Carbon Filter for Your Grow Tent
Greens Horticulture ·
Every grow tent needs air pulled out of it: to take the heat of the light away, to bring in fresh air and to keep the humidity down once the plants are big. The extraction kit is an inline fan, a carbon filter to take the smell out of the air, and the ducting that joins them. This guide shows how to size the fan for your tent, how to match a filter to it, and which of the fans and filters we stock fit each tent size.
The rule: change the air every one to three minutes
The figure printed on a fan is how many cubic metres of air it moves an hour (m³/h). The widely used rule for a grow tent is to replace the whole volume of the tent every one to three minutes. We size to once a minute, the demanding end, because the tent will also have a lamp in it and a British summer outside it, and a fan can always be turned down.
- Work out the tent's volume. Length × width × height in metres. A 120 × 120 × 200 cm tent is 1.2 × 1.2 × 2.0 = 2.88 m³.
- Multiply by 60 for one air change a minute: 2.88 × 60 = 173 m³/h. (Multiply by 30 for one every two minutes, by 20 for one every three.)
- Add about 25 per cent for the resistance of the filter and the ducting. A carbon filter and a few metres of duct knock a real chunk off a fan's headline figure: CAN put it at roughly 15 to 20 per cent for the filter alone. 173 + 25% = about 216 m³/h.
- Pick a fan rated at or above that number, then a filter rated at or above the fan.
Go a size up if the tent will run a hot lamp (an HPS rather than an LED), if the duct run is long or has several bends, or if the room it stands in gets hot in summer. Too much fan is cheap to fix with a fan speed controller; too little cannot be fixed at all.
Fan and filter by tent size
The tents are the sizes we sell (Budbox Lite and Pro, Secret Jardin). The fan column names the Systemair RVK of that size and its rating from its product page; the filter column is the Rhino Pro or Rhino Hobby model (both ranges come in the same sizes) with its rating. Any fan of the same size and a similar rating does the same job.
| Volume | Air change a minute | +25% for filter & duct | Fan we'd fit | Filter to match | |
|---|---|---|---|---|---|
| 60 × 60 × 140-160 cm | 0.5-0.6 m³ | 35 m³/h | 45 m³/h | 100 mm (RVK 100, 211 m³/h) | 100/300 (350 m³/h) |
| 80 × 80 × 160-180 cm | 1.0-1.2 m³ | 70 m³/h | 90 m³/h | 100 or 125 mm (RVK 125 L, 230 m³/h) | 125/300 (500 m³/h) |
| 100 × 100 × 200 cm | 2.0 m³ | 120 m³/h | 150 m³/h | 125 mm (RVK 125 H, 368 m³/h) | 125/300 (500 m³/h) |
| 120 × 120 × 200 cm | 2.9 m³ | 175 m³/h | 220 m³/h | 150 mm (RVK 150 L, 423 m³/h) | 150/300 (600 m³/h) |
| 150 × 150 × 200 cm | 4.5 m³ | 270 m³/h | 340 m³/h | 150 mm (RVK 150 H, 737 m³/h) | 150/600 (900 m³/h) |
| 120 × 240 × 200 cm | 5.8 m³ | 345 m³/h | 430 m³/h | 200 mm (RVK 200 L, 827 m³/h) | 200/600 (1125 m³/h) |
| 240 × 240 × 200 cm | 11.5 m³ | 690 m³/h | 865 m³/h | 200 mm H or 250 mm (RVK 250 L, 863 m³/h) | 250/600 (1420 m³/h) |
For the smallest tents the sum comes out far below the smallest fan we sell, so any 100 mm fan is plenty: in a 60 or 80 cm tent it is the light's heat, not the air changes, that decides things.
The fans we stock
- Systemair RVK: the standard AC fan, made in Germany, in 100 to 315 mm, each size in a low (L) and high (H) output. Ratings run from 211 m³/h (100 L) to 1364 m³/h (315 L). Run it on a Variispeed or an SMSCOM controller.
- CAN Max Pro: two-speed switch built in; 150 mm 470/600 m³/h, 200 mm 793/1218 m³/h, 250 mm 1224/1660 m³/h. CAN say the 150 suits intake and the 200 and 250 cope with a filter and up to four metres of duct.
- Revolution Vector Silenced V2: EC fan in an insulated case; 150 mm 764 m³/h, 200 mm 1345 m³/h, 250 mm 1787 m³/h. Needs an EC controller.
- Rhino Ultra Silenced EC: 150 mm 530 m³/h, 200 mm 1205 m³/h, 250 mm 1800 m³/h, 315 mm 2987 m³/h, with a 0 to 100 per cent speed controller included. Made to pair with Rhino filters.
AC or EC? AC fans (RVK, Max Pro) are the proven budget choice and hum a little when turned down. EC fans (Vector, Rhino Ultra, Q-Max EC, Cloudline) have a brushless motor that turns down silently and uses less power at low speed, and they take a smart controller that holds the tent at a set temperature. If the tent is in a bedroom or a room you sit in, EC and a silenced case are worth the money.
Matching the carbon filter
A carbon filter only works when the air spends long enough in the carbon. That is why the filter's rating must be equal to or higher than the fan's: a filter that is too small for its fan lets air through too fast to be scrubbed, and both wear out sooner. The flange must also be the same size as the fan (100, 125, 150, 200, 250 or 315 mm) so it joins straight on.
- Rhino Pro: Australian RC412 carbon in a 50 mm bed, rated for at least two years. Sizes from 100/300 (350 m³/h) to 315/1200 (3600 m³/h); the 150/600 is 900 m³/h and the 200/600 is 1125 m³/h.
- Rhino Hobby: the same carbon in a 30 mm bed, rated 9 to 12 months, in the same sizes up to 250/600 (1420 m³/h). Rhino suggest turning it round halfway through its life so both ends of the carbon are used.
- CAN Lite: light and good value, up to 18 months, from 1000 m³/h at 200 mm to 4500 m³/h at 315 mm.
- Secret Jardin filters: small filters of 100, 150, 250 and 350 m³/h for the smallest tents.
- Replacement sleeves: change the pre-filter sleeve when it clogs; the carbon itself when smell starts to escape.
Ducting and noise
Keep the run short and straight: every metre of flexible duct and every bend costs airflow, which is what the 25 per cent allowance is for. Aluminium ducting is the standard; stretch it out fully rather than leaving it bunched. Acoustic ducting wraps the duct in insulation and cuts the noise of the moving air by around 40 to 60 per cent, worth it from 150 mm up or anywhere you can hear the tent. Fix every joint with a clip and a wrap of tape, and put a back-draft shutter on the outlet.
Fan controllers
- Variispeed: a dial that turns an AC fan (up to 300 W) down; simple and cheap, some hum at low speed.
- SMSCOM Smart Controller 6.5A: holds a set temperature by speeding an AC fan up and down.
- Rhino 1EC: thermostatic control of one EC fan, powered from the fan.
- G.A.S. Enviro4: runs EC fans on temperature and humidity, more than one fan at a time.
Quick answers
Can I run the fan without a filter?
Yes, if smell is not a concern; the fan then moves close to its headline figure. Most growers fit the filter from the start, because the day you need it is the day the plants are biggest.
Where does the filter go?
Inside the tent at the top, where the warm air collects, with the fan pulling air through it and out through the duct. Hung this way the tent runs at a slight negative pressure and smell cannot leak from the zips.
Browse inline fans, carbon filters, ducting and fan controllers, or pick a matched fan and filter kit and skip the sums.
More set-up guides
- What size grow tent do I need?
- LED grow light wattage by tent size
- LED or HPS? Why the best growers use both
- Hydroponics: systems, kits and how it works
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.