Air Changes Per Hour (ACH) is a metric that indicates how many times an HVAC device can fill up the full volume of a room with air. Air Changes per Hour metric is most commonly used for hospital and lab buildings, where the ventilation rates are high for keeping the air free of harmful contaminants.

Slight change in ACH value can have a significant impact on the total Energy Usage Intensity (EUI) of the building. Also, for the ventilation of building type hospital, ASHRAE 170 standard mainly utilizes the Air Changes per Hour values for various zones.

Cove.tool utilizes the total CFM value (IP) or total L/s value (SI) for 'Mechanical ventilation supply air flow rate' calculations. Following formulas should help to convert the ACH value into CFM or L/s value desired by covetool.

## Formula for Air Changes per Hour - IP units

Fresh air flow rate (CFM) = (Space Volume (ft3) * ACH) / 60

## Formula for Air Changes per Hour - SI units

Fresh air flow rate (L/s) = (Space Volume (m3) * ACH) / (3.6)

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