Air Change Rate (ACH) is a measure of how many times the total volume of air in a room or building is replaced with fresh air within one hour. It's a critical metric in assessing indoor air quality, ventilation effectiveness, and energy efficiency.
Why is ACH Important?
Indoor Air Quality: Higher ACH generally means better removal of pollutants like CO2, VOCs, dust, and odors, leading to a healthier indoor environment.
Ventilation Standards: Many building codes and health organizations specify minimum ACH requirements for different types of spaces (e.g., hospitals, offices, homes) to ensure adequate ventilation.
Energy Efficiency: While good ventilation is essential, excessive ACH can lead to unnecessary heating or cooling loads, increasing energy consumption. The goal is to achieve adequate ventilation without over-ventilating.
Contaminant Control: In sensitive environments (like cleanrooms or laboratories), precise ACH control is vital for preventing contamination.
How is ACH Calculated?
The formula for Air Change Rate is straightforward:
ACH = (Airflow Rate in CFM × 60 minutes/hour) / Room Volume in cubic feet
The result indicates the number of times the air in the room is completely exchanged in one hour. For example, an ACH of 10 means the air in the room is replaced 10 times every hour.
Factors Affecting ACH:
Mechanical Ventilation Systems: HVAC systems with fans and air handlers are the primary drivers of controlled ACH.
Natural Ventilation: Open windows and doors can contribute to air exchange, but this is less predictable.
Building Infiltration: Unintended air leakage through cracks and gaps in the building envelope.
Room Size and Occupancy: Larger rooms and higher occupancy generally require higher airflow rates to maintain desired ACH.
Understanding and calculating ACH helps building managers, HVAC professionals, and homeowners ensure optimal air quality and system performance.