What filter should I get?

Question from a fellow grower:

My first grow skunked the entire neighborhood and nearly caused a
robbery, despite an excellent brand new carbon filter and ventilation
system. I’m sure there is a relationship between humidity level and
maximum effectiveness of carbon filtration, but I don’t know what that
level is. My wife hates the smell of growing marijuana plants,
especially near harvest time, which grow in our basement and migrate
smell upstairs, and I must solve the smell problem before starting
another grow. I’m considering a UVonair machine within the ventilation
system to help with eliminating smell, but have no idea if the machine
will work. There is a run of 20’ of 8” flexible ventilation tubing
between veg and flower rooms to the outside.

Can filters are rated on a per light basis
A 33 for instance is rated for 1 light or roughly a max of 9 plants a 50 roughly 18
I use overkill and run a 100. The biggest thing with a carbon filter is when they fail they fail entirely so you would totally know it no longer works since air travels through easiest escape route meaning the stink would be constant.
That being said a very common issue is not having enough negative air pressure in your grow room, it should be a vacume air should be pulled into the room without an intake fan or in a sealed room which constantly is under vac. If your extraction is working right will pull the door shut behind you a carbon filter only works if all air is pulled through it to exit as a trick when I cut or trim I turn off intake fan so vac is stronger. Try walking around outside your room with insence if the smoke isn’t pulled into it through any of the cracks around the door or other protrusions you don’t have enough neg pressure and need to reduce intake fan speed or increase exhaust fan speed.
hope this helps

Here is some info from Better Grow Hydro in regards to carbon filters:
Temperature and humidity are the factors that primarily affect the efficiency and performance of carbon filters. As either increases, the efficiency and the odor removal capabilities drop. As temperatures get into the 90s, performance really starts to suffer. Humidity around 10 percent is best. However, most grow rooms are going to have at least 30 percent to 50 percent humidity, which is fine. But as humidity increases to 70 percent or 90 percent, the tiny pores in the carbon start to fill up with moisture, making the filter ineffective.

For the carbon to work properly, air needs to pass through it at a certain rate. If the air passes too quickly, there is not enough contact time and the chemical reaction that neutralizes the odor isn’t complete. If the air flows too slowly, odor control becomes inefficient. But with temperature and humidity within range, and with a properly matched fan and filter combination, growers can achieve sufficient odor control.

Growers must combine the correct size of filter and fan to ensure air flows through the carbon at the right rate. To find the right match, first calculate the size of the grow room in cubic feet (LxWxH). To bring in fresh air and adequately filter the exhaust, air in the grow room should pass through the filter every one to four minutes, with one- to two-minute exchange times being optimal. So that means in a 1,000-cubic-foot grow room (about 10’ x 12’ x 8’), 250 to 1,000 cubic feet of air should pass through the filter each minute.

However, an appropriately sized carbon filter reduces a fan’s CFM rating by about 20 percent. So, to changeover the air in a 1,000-cubic-foot room within the correct time frame, the grower would need a centrifugal fan with at least a 310 CFM, such as the 6” Max Fan, 6" Can Fan HO or 6" Vortex to achieve a roughly 4-minute exchange, or an 8” centrifugal fan with around 700 CFM to exchange the air in about 2 minutes. A 12” fan with roughly 1200 CFM will clear the room in about one minute.

With that calculation in hand, it is easy to match a fan with the appropriate size filter. Can Filters provides data on suitable CFM rating for their filters, giving ratings on the CFM required to remove odors from exhaust or for scrubbing the air in a sealed grow room. Can Filters also offers lab-tested data on the CFM their Can Fans will produce when paired with their filters, which can be used to help choose the right size fan. BetterGrow Hydro posts those ratings along with Can Filter listings on our Web site.


Well answered I always apply overkill and keep humidity mid 50’s

I agree! Would much rather get a fan rated to high and use a speed controller than not have enough airflow :smiley:

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