Extraction is the one system that controls heat, humidity and odour all at once, so getting it right sorts most of your environment headaches in a single buy. It's also where beginners try to save a tenner on an underpowered fan and then spend three months chasing a smell that won't quit. Here's how to size an inline fan to your tent, pair it with the right carbon filter, and what we stock to do it properly.
How does grow tent extraction work?
An inline fan pulls air out of the tent through a carbon filter, which scrubs odour as the air passes through the activated carbon, then pushes it out via ducting. Because air is being drawn out, the tent sits at slightly negative pressure - the walls pull gently inward, so every bit of air inside, odour included, flows toward the filter instead of leaking out the zips and seams. Passive vents at the bottom let fresh air fall in to replace it. Turn the fan up and temperature and humidity both drop; turn it down and they climb. That's why it's a system, not a single part.
The pairing that does the work is an inline fan plus a matched carbon filter. We stock both browse inline fans here and carbon filters here.
What size inline fan do I need for my grow tent?
Do the maths the box won't. Take your tent volume (length x width x height in metres) and multiply by 60 for one full air change a minute - that's your bare minimum. Then add 20 - 30% for the carbon filter, because pulling air through carbon costs you airflow, and allow a little more for ducting losses.
A worked example: a 120 x 120 x 200cm tent is 2.88mB3 x 60 - 173m B3/h minimum, which climbs to roughly 220 - 250m B3/h once the filter takes its cut, so you'd spec a fan rated around 250mB3/h. That's a 150mm (6-inch) inline fan. A smaller 80 x 80cm tent runs happily on a 100mm (4-inch) fan. Don't bolt a 250mm fan onto a tiny tent - it just roars and folds the walls in.
- 60C60cm - 100mm (4-inch) inline fan
- 80C80cm - 100mm (4-inch) inline fan
- 100C100cm - 125mm (5-inch) inline fan
- 120C120cm - 150mm (6-inch) inline fan
If you're between two sizes, size up - a fan running comfortably is quieter than a small one flat out. Our inline fan range is sorted by diameter so you can match it to your tent.
How do I match the carbon filter to the fan?
The filter has to be sized for the same airflow as the fan, or it becomes the bottleneck. Match the filter's collar diameter to the fan (a 150mm fan wants a 150mm filter), and check the filter's rated airflow is at or above your fan's output. A filter that's too small chokes the system and lets odour leak; one sized correctly keeps the air moving and the smell contained.
Carbon filters also have a working life - the activated carbon saturates over months of use, and a tired filter is the usual reason a tent starts smelling again. We carry carbon filters in all the common collar sizes to pair with your fan.
Why is my grow tent smelling even with a carbon filter?
Nine times out of ten it's one of three things. The fan's too weak to pull air through the filter fast enough to hold negative pressure, so odour escapes the seams. The filter's saturated and past its working life. Or the ducting is fighting you - every 90B0 bend costs 10 - 15% of your airflow, so a kinked or coiled run quietly strangles the whole system. Keep ducting short and straight with gentle curves, not sharp angles.
Quick check: press gently on the tent walls. A slight inward pull means good negative pressure and contained odour. Walls ballooning out means your intake is beating your extraction. Slack, neutral walls mean the fan's too weak or the ducting's too restrictive - check for kinks and a clogged filter.
Where should the exhaust go?
Out of the room entirely - a window or wall vent. Exhausting back into the same room just recirculates warm, humid air through your intake, so the tent gets hotter and damper every cycle, and the odour never really leaves. Run insulated ducting on cold routes to cut condensation and dampen noise. And remember odour peaks in the last couple of weeks of a grow - exactly when a saturated filter or a loose duct joint will catch you out, so give the system a once-over regularly.
Where to buy inline fans and carbon filters in Ireland
If you're sizing a setup and not sure which fan and filter pair to your tent, measure your space, tell us the tent size, and we'll match it for you - we do this every day. DIG is Ireland's largest grow shop, based in Dublin, with collection available in store and delivery across Ireland.
Want the full science behind negative pressure, ducting losses and air changes? Our friends at Grow Good Bud have the free in-depth guide.
FAQ
What size inline fan do I need for my grow tent?
Tent volume in mB3 x 60 for the minimum, then add 20 - 30% for the carbon filter and a little more for ducting bends. Roughly: a 100mm fan suits an 80 x 80cm tent, a 150mm fan suits a 120 x 120cm.
Why is my tent still smelling with a carbon filter fitted?
Usually an undersized or weak fan that can't hold negative pressure, a saturated filter past its working life, or restrictive ducting choking the airflow. Check the wall suction, the filter's age, and the duct run for kinks.
How do I match a carbon filter to my inline fan?
Match the collar diameter (a 150mm fan to a 150mm filter) and make sure the filter's rated airflow is at or above your fan's output, so the filter doesn't become the bottleneck.
Can I just open a window instead of an extraction fan?
No. An open window is uncontrolled - you can't manage humidity, temperature or odour, and the smell goes everywhere. A properly sized inline fan with a carbon filter and passive intake gives you control.
Where can I buy inline fans and carbon filters in Ireland?
DIG (Dublin Indoor Gardening) stocks inline fans and carbon filters in all common sizes, matched to your tent, with collection in Dublin and delivery across Ireland.