Air changes per hour (ACH) is a measure of airflow compared to a space's volume. Comparing the airflow to a space's volume is done to infer the number of times air within that space is replaced. Knowing the age of air within a space helps in understanding the systems effectiveness at removing contaminants from the space. As such, ACH is commonly used to defined requirements for healthcare and laboratory buildings.

The definition for ACH in imperial units uses airflow in CFM and volume in cubic feet:

ACH = (60*Airflow) / Volume

The definition for ACH in metric units uses airflow in L/S and volume in cubic meters:

ACH = (3.6*Airflow) / Volume

ACH will have a significant impact on the total energy use for a building due to the inclusion of volume in the definition.

A useful rule is 1 CFM/SF = 6 ACH for a space with a 10ft ceiling

This means that for any spaces with taller ceiling heights, the the equivalent CFM/SF will be higher. When modeling a building it's important to consider the impact of the volume on the airflow calculation.

cove.tool utilizes people and area based inputs for the mechanical airflow calculation. If using an ACH requirement for your project you can convert this to the area measures used, which are CFM/SF value (IP) or total L/s/sqm value (SI):

### ACH to CFM/SF

CFM/SF = ACH * height (ft) / 60

### ACH to L/S/sqm

L/S/sqm = ACH * height (m) / 3.6