Air Changes per Hour (ACH)

How many times the full volume of air in a room is replaced or processed in an hour — a measure used for ventilation and air purifier sizing.

Air changes per hour (ACH) describes how many times a room's entire air volume passes through a system — a fan, an air purifier, an HVAC system — in one hour. It's most commonly used for air purifier and ventilation sizing rather than dehumidifier sizing directly, though the underlying idea (matching equipment throughput to room volume) is the same principle behind dehumidifier capacity calculations.

As a rough rule of thumb for air purifiers: length × width × height × 5 gives an approximate ACH-based sizing figure — a higher multiplier for rooms with heavier particulate load (smog season, smoking, pets).