Air Changes per Hour (ACH)

ACH normalises ventilation to room size: six air changes per hour means the supply air adds up to six times the room's volume every hour. In imperial terms it is ACH = 60 × CFM ÷ ft³, since cfm counts minutes and ACH counts hours. Codes lean on it heavily — 2 ACH for a general office, 6–10 for a commercial kitchen, 12 for an airborne-isolation hospital room, 15+ for a laboratory fume hood space — and residential blower-door tests report ACH50, the leakage at 50 Pa, where a 1970s house measures 10–15 and a Passive House must beat 0.6.

The trap is treating ACH as if the room mixed perfectly. It does not: a poorly placed diffuser can short-circuit straight to the return, so the air at desk level is stale while the meter says six changes an hour. Ventilation effectiveness factors exist precisely because of this. Worked example: 200 cfm into a 20 × 15 × 8 ft bedroom (2,400 ft³) gives 60 × 200 ÷ 2,400 = 5.0 ACH. Run it backwards to size equipment — a 60 m³ room at 6 ACH needs 360 m³/hr, or 6,000 L/min, which is the number you take to the fan schedule.