Room Air Change Rate (ACH)
Room Air Change Rate (ACH)
Air changes per hour (ACH) is the standard shorthand ventilation engineers, building codes, and infection-control guidance use to describe how quickly a room's air is being replaced with fresh or filtered air — a single number that is far easier to specify and compare than raw airflow rates, since it automatically accounts for room size. ACH targets vary enormously by application: a bedroom might be designed for roughly 0.5–1 ACH, an office around 4–6 ACH, and an operating room or airborne-isolation hospital room 12 ACH or more, which is why this simple ratio is one of the first numbers checked when evaluating whether a space's ventilation is adequate.
The air change rate is ACH = Q/V, the ventilation airflow rate divided by the room volume. where Q is the ventilation (supply or exhaust) airflow rate and V is the room air volume.
Inputs
Q_vent := 300 m^3/hr=0.08 m^3/s
V_room := 150 m^3=150 m^3
Dividing the hourly airflow rate by the room's air volume gives how many times per hour that volume of air is completely replaced, the standard way ventilation adequacy is expressed and compared to code minimums.
ACH_room := Q_vent/V_room=5.56e-04 Hz
Results
An ACH of 2.0 is a modest ventilation rate suitable for a typical residential room but well below what would be required for offices, classrooms, or healthcare spaces under most ventilation standards. Because ACH is a ratio, the same 300 m³/hr airflow delivers a much higher ACH in a small room than a large one, so increasing ceiling height or floor area without adding airflow actually reduces the effective ACH — a common oversight in renovation projects that repurpose a space to higher occupancy without upsizing the ventilation system.
Assumptions & limitations
- Assumes fully mixed room air (uniform concentration, no short-circuiting between supply and return).
- Airflow rate Q assumed steady and continuously supplied.
- Does not distinguish outdoor (fresh) air from recirculated air unless Q is specifically defined as outdoor air.
- Room volume V taken as the full geometric air volume, unobstructed by furniture or partitions.