How to Size an Extraction Fan and Carbon Filter for a Grow Tent
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 sizes the fan and filter for your tent and shows which of ours fit. Matched pairs are under fan and filter kits; fans and filters on their own are under inline fans and carbon filters.
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 is to replace the tent's whole volume 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.
- 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. CAN put the loss from the filter alone at roughly 15 to 20 per cent. 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 for a hot lamp (an HPS rather than an LED), a long or bendy duct run, or a room that 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
Fans are the Systemair RVK, filters the Rhino Pro or Rhino Hobby (both come in these sizes); any fan of that size and a similar rating does the same job. The last column names a matched kit with an AIR AC or RVK fan; the RVK kit pages give no airflow figures, so ask us.
| Volume | Air change a minute | +25% for filter & duct | Fan we'd fit | Filter to match | Matched kit (fan m³/h) | |
|---|---|---|---|---|---|---|
| 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) | AIR AC100 kit or RVK 100A kit |
| 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) | AIR AC125 kit or RVK 125L kit |
| 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) | AIR AC125 kit or RVK 125A kit |
| 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) | AIR AC150 kit (500 m³/h) or RVK 150L kit |
| 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) | AIR AC150 kit (500 m³/h) or RVK 150A kit |
| 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) | RVK 200L kit |
| 240 × 240 × 200 cm | 11.5 m³ | 690 m³/h | 865 m³/h | 200 mm (RVK 200 H, 1073 m³/h) | 200/600 (1125 m³/h) | RVK 200A kit |
In a 60 or 80 cm tent the sum is below the smallest fan we sell, so any 100 mm fan is plenty: the light's heat, not the air changes, decides things.
The fans we stock
| Fan | Type | Sizes | Rating (m³/h) |
|---|---|---|---|
| Systemair RVK | AC | 100-315 mm, low (L); 125-250 mm also high (H) | 211 (100 L) to 1364 (315 L) |
| CAN Max Pro | AC, two-speed switch | 150, 200, 250 mm | 470/600, 793/1218, 1224/1660 |
| Revolution Vector Silenced V2 | EC, silenced; controller needed | 150, 200, 250, 250L, 315 mm | 764, 1345, 1787, 2102, 2224 |
| Rhino Ultra Silenced EC | EC; controller included | 150, 200, 250, 315 mm | 530, 1205, 1800, 2987 |
CAN say the Max Pro 150 suits intake; the 200 and 250 take a filter and up to 4 m of duct.
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) turn down quietly and use less power at low speed. 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, so its rating must be equal to or higher than the fan's: too small a filter lets air through too fast to be scrubbed, and both can 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).
- 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).
- 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. Pro and Hobby sleeves differ, so order by filter type.
How a carbon filter works, and the back-ups
The fan pulls air through a bed of activated carbon, which holds the odour, so the air leaving the filter is odourless; Rhino say their RC412 carbon works in high and low humidity. Air takes the easiest path, so the top of the filter wears out first. The Rhino Hobby can be turned round to use the whole bed: its page says every 3 months.
Back-ups for a stubborn smell:
- CAN Inline Carbon Filters: fit in the duct run, as the main filter if matched to the fan or as a belt-and-braces second one; lightweight carbon, around 18 months, 425 to 3000 m³/h; up to 36 kg.
- AirOzone Ozone Generator: inline after the fan with at least 5 m of duct after it; 600 to 4800 m³/h, never one rated below the fan. Ozone can be hazardous: its page says it must be vented outdoors, never indoors.
- ONA Gel: an open jar where the smell is, or in the duct run in a T-piece; ONA Blocks last about 4 to 6 weeks; ONA Spray is for quick touch-ups.
Ducting and noise
Keep the run short and straight: every metre of flexible duct and every bend costs airflow. 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
A slightly too powerful AC fan goes on a Variispeed; one fan on a thermostat: the SMSCOM Smart Controller 6.5A, or the Rhino 1EC for a Rhino EC fan; for a Vector, the G.A.S EC5 with G.A.S Cable pack 8. Controllers that read humidity are compared in our humidity guide; running two fans together is in our intake guide.
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.
How long does a carbon filter last?
The product pages say: Rhino Pro at least two years, Rhino Hobby 9 to 12 months, CAN Lite up to 18 months and CAN Inline around 18 months.
Hobby or Pro, acoustic or combi?
Hobby for a year on a budget; the Pro's thicker bed lasts about twice as long. Combi ducting is tough and flexible; acoustic is a baffle that cuts the fan noise, for a tent close to living space. Every kit above comes in all four combinations.
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.