Air changes per hour (ACH) measures how many times total air volume in a room is completely replaced with fresh air in 60 minutes. ACH ratings determine ventilation adequacy, indoor air quality, and contamination control effectiveness. Building codes and ASHRAE standards specify required ACH for different occupancy types.
Standards and Requirements
Residential spaces typically require 0.5 to 1.5 ACH, while commercial offices need 2 to 6 ACH. Healthcare facilities demand 6 to 12 ACH or higher depending on infection control requirements. Data centers require 10 to 15 ACH for equipment cooling. ASHRAE 62.1 specifies minimum ventilation rates based on occupancy density and space function.
Practical Implications
Higher ACH improves odor removal, reduces airborne pathogens, and dilutes pollutants and CO2 buildup. However, excessive ACH increases energy consumption and operational costs. HVAC system design balances ventilation requirements with energy efficiency, typically using demand-controlled ventilation to adjust ACH based on occupancy and CO2 levels.