Which carbon filter matches my fan?
Buy a filter rated for at least the air you actually push through it, with the same flange size as your fan. Bigger is fine: slower air through more carbon smells better and lasts longer. Hang it so the fan pulls through it: filter, then fan, then out.
Your result
How it works, in plain words
Match the airflow
Filter rating ÷ the airflow you run should be 1.0–1.5. Under 1.0, smell bleeds through and the carbon wears out sooner.
Contact time
Contact time = carbon bed volume ÷ airflow. A 10 × 16 inch filter with a 1.5-inch bed holds about 10.5 L of carbon; at 330 CFM air spends about 0.07 s in it. Slower air or more carbon means more time to grab the smell.
Layout
Filter → fan → duct → outside. Pulling through the filter keeps the whole run under negative pressure, so a leak sucks in rather than leaking smell. Use a pre-filter sleeve to keep dust off the carbon.
Every number this calculator uses
The same constants feed the Calculate it properly guides, so the tools and the articles agree. Rules of thumb are labelled as such.
| Constant | Value | What it is | Evidence |
|---|---|---|---|
FILTER_MATCH_LO | 1 × fan CFM | Filter rated airflow should be at least the airflow you actually run | Grower practice |
FILTER_MATCH_HI | 1.5 × fan CFM | Up to ~1.5× is better: slower air through the bed means longer contact and longer carbon life | Grower practice |
M3H_PER_CFM | 1.699 m³/h per CFM | Airflow conversion | Established |
Sources
- ASHRAE. 2019. ASHRAE Handbook: HVAC Applications, chapter 'Control of gaseous indoor air contaminants' (sorbent bed depth and residence time govern removal). https://www.ashrae.org/technical-resources/ashrae-handbook
- NIST. Guide for the Use of the International System of Units (SP 811), Appendix B: conversion factors (1 W = 3.412 142 BTU/h; 1 ft³ = 0.028 316 8 m³; 1 US gal = 3.785 412 L). https://www.nist.gov/pml/special-publication-811
