Quick answer: An activated carbon filter removes odours and some gases from air passing through it by adsorption. In greenhouses and grow rooms it is usually fitted on the exhaust so odour does not leave the building, or on recirculation inside. Size it so the fan's real airflow (after the filter's resistance) matches the filter's rated airflow, add a pre-filter to stop dust, and keep the air through it below about 60–70 % RH: water vapour competes with odour molecules for space in the carbon, so humid air shortens filter life and reduces performance. Carbon filters do not remove particles well on their own, and they do not dehumidify.
Quick answer
- Match fan and filter airflow; check the fan curve at the filter's pressure drop.
- Keep RH through the filter below about 60–70 %.
- Replace when odour breaks through; typical intervals range from months to a couple of years.
What carbon filters do and do not do
| Removes | Does not remove (well) |
|---|---|
| Odour compounds (terpenes and other VOCs) | Particles, dust, pollen (use a pre-filter or particle filter) |
| Some gases, depending on carbon type | Moisture (it cannot dehumidify) |
| CO₂ |
The California Air Resources Board and US EPA explain how air cleaners that remove gases and odours work, and their limits (CARB; US EPA, Guide to air cleaners).
Sizing
- Airflow needed: for an exhaust filter, the room's exhaust rate (for a grow room or tent, often one air change per minute or as set by the cooling design).
- Filter rating: choose a filter rated at or above that airflow. Running more air than rated reduces contact time and odour removal.
- Fan curve: the filter, pre-filter and ducts add resistance. Check the fan delivers the airflow at that pressure; filters often reduce fan output by 20–40 % compared with free air.
- Placement: on the exhaust side (fan pulls through the filter) is common; recirculating filters inside sealed rooms are another option.
Humidity and carbon life
Activated carbon adsorbs water vapour as well as odours. At high RH, water takes up adsorption sites, so:
- odour removal drops;
- the carbon saturates sooner;
- dust plus moisture can clog the bed.
Keep the air through the filter below about 60–70 % RH. In grow rooms and greenhouses, that usually means dehumidifying the room, especially at lights-off and at night, when RH peaks. See grow room dehumidifier sizing and our ceiling-mounted and ducted dehumidifiers.
CO₂ and exhaust
Exhausting air through a carbon filter also exhausts CO₂. If you enrich CO₂, every cubic metre exhausted is enrichment lost. Sealed rooms with recirculating carbon filters and dehumidification keep CO₂ in; vented rooms trade CO₂ for cooling and humidity control. See CO₂ uptake and plant growth.
Maintenance
| Task | Interval |
|---|---|
| Clean or replace pre-filter | Monthly or when visibly dirty |
| Check odour at the outlet | Weekly |
| Check airflow (fan speed, pressure drop) | Monthly |
| Replace carbon | When odour breaks through; often 1–2 years in moderate conditions, sooner in humid or dusty rooms |
Verifying performance
- Smell test at the exhaust outlet at peak odour times, ideally by someone who has not been in the room.
- Airflow check against the design value.
- For regulated sites, follow any monitoring method the authority requires.
FAQ
Frequently asked questions
How do carbon filters work in a greenhouse?
Air passes through activated carbon, which adsorbs odour molecules before the air leaves the building.
Does humidity affect carbon filters?
Yes. High humidity reduces odour adsorption and shortens carbon life. Keep air through the filter below about 60–70 % RH.
How long do carbon filters last?
Often 1–2 years in moderate conditions; less in humid or dusty rooms.
Do carbon filters remove humidity?
No. Use a dehumidifier to control humidity.
Sources
- California Air Resources Board, Indoor air cleaners: removal of odors and gases.
- US EPA, Guide to Air Cleaners in the Home.
