Air Exchange (ACH).
Air Exchange Rate (ACH) Calculator
Compute air changes per hour from CFM and room volume — fan and exhaust sizing for indoor cultivation.
ACH targets
| Use case | Target ACH | Notes | ||
|---|---|---|---|---|
| Indoor cannabis (no CO₂ enrichment) | 30–60 | Heat removal + air movement | ||
| Indoor cannabis (CO₂-enriched, sealed) | 0–5 | Sealed room; HVAC removes heat | ||
| Greenhouse (passive) | 30–60 | Roof and side vents | ||
| Greenhouse (mechanical exhaust) | 40–60 | For temperature control | ||
| Mushroom fruiting room | 4–6 | Fresh air for fungi but not drying | ||
| Drying room | 2–4 | Gentle air movement; not dry-out-fast | ||
| Curing room | 0.5–1 | Minimal exchange | | Walk-in cooler | 0.5–2 | Slow; refrigeration handles temp |
Math
volume_ft³ = length × width × height ACH = (CFM × 60) / volume_ft³ time_per_change_min = 60 / ACH
The CFM input should be your effective combined intake and exhaust capacity. If you have separate intake and exhaust fans, use whichever is the bottleneck (typically the smaller of the two, since pressure cascades require balanced flow).
Why ACH not just CFM
CFM (cubic feet per minute) is fan capacity. ACH (air changes per hour) is the *outcome* — how completely the room's air is replaced each hour. A 600 CFM fan in a 1,500 cubic-foot tent achieves 24 ACH (good); the same fan in a 10,000 cubic-foot warehouse achieves 3.6 ACH (insufficient for indoor cultivation without CO₂ enrichment).
For un-enriched indoor cultivation, plants consume CO₂ during photoperiod faster than the room can re-supply through small leaks. Without sufficient air exchange, ambient CO₂ in the room drops below the global average (420 ppm) into 250-300 ppm range — a major growth limitation. 30+ ACH ensures adequate CO₂ resupply.
For CO₂-enriched sealed rooms, the calculus inverts — too much exchange wastes injected CO₂. Sealed rooms run 0-5 ACH and rely on HVAC for heat removal.
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