I just purchased a new filtration system for my room works like a charm, My question is, should the air flow circulate from the carbon filters to the hepa or from the hepa to the carbon? I know it matters because the particles become smaller. Also where is the best placement in a closed closet? up high? down low? behind the fan? in front of the fan?

Thanks!

The HEPA filter would be for keeping bad stuff away from your plants so that would be an intake filter. The carbon filter is for keeping the smell under control so that would be at the end of your exhaust fan. Exhaust high and intake low.