The Air Exchange Rate (AER), commonly referred to as Air Changes per Hour (ACH), is a fundamental metric in building science and public health. It measures how many times the air inside a given space is completely replaced with new air within one hour. Understanding the ACH is important for assessing ventilation performance and indoor air quality. A proper ACH level relates directly to the health of occupants, the removal of airborne contaminants, and the energy efficiency of HVAC systems.
Understanding the Necessary Input Variables
Calculating the Air Changes per Hour requires two primary measurements: the room’s total volume and the airflow rate. The room’s volume represents the amount of air the space can hold. To find this, measure the length, width, and height of the room in feet and multiply the three numbers together (L × W × H). This calculation yields the volume in cubic feet.
The second measurement is the airflow, quantified in Cubic Feet per Minute (CFM). CFM represents the speed at which air is supplied to or exhausted from the room by a ventilation system or fan. In residential settings, the CFM rating is often stamped on the housing of exhaust fans, such as those in a bathroom or kitchen. For central Heating, Ventilation, and Air Conditioning (HVAC) systems, the total CFM is typically a specification provided by the manufacturer or an HVAC technician.
If you are trying to estimate the natural air exchange, or infiltration, that occurs through leaks and cracks, the measurement becomes much more difficult. This typically requires specialized equipment like a blower door test. However, for calculating the rate due to mechanical ventilation, the manufacturer’s stated CFM for the equipment is the most practical input for a general user. For rooms with multiple air sources, the total effective CFM is the sum of all air movement into or out of the space.
Applying the Air Exchange Rate Formula
Once you have the room’s volume in cubic feet and the airflow rate in CFM, you can calculate the Air Changes per Hour using a straightforward formula. This calculation converts the airflow from a per-minute rate to a per-hour rate, comparing that hourly volume to the size of the room. The standard formula is: ACH = (CFM × 60) / Volume.
The multiplier “60” is included because there are 60 minutes in one hour, converting the CFM value (cubic feet per minute) into Cubic Feet per Hour (CFH). This conversion makes the units compatible with the ACH result. For example, consider a small office with a volume of 1,000 cubic feet ventilated by an air purifier supplying air at 50 CFM.
Plugging these values into the formula yields ACH = (50 CFM × 60) / 1,000 cubic feet. The numerator (3,000 CFH) is the total volume of air moved per hour. Dividing 3,000 CFH by the room volume results in an ACH of 3.0. This means the entire volume of air in that office is theoretically replaced three times every hour.
Interpreting the Calculated Air Changes Per Hour
The numerical result of the ACH calculation directly assesses the ventilation effectiveness in the space. A higher ACH number signifies more frequent air replacement, which is generally better for indoor air quality. This frequent replacement removes airborne pollutants and pathogens more quickly. For instance, the Centers for Disease Control and Prevention (CDC) suggests aiming for at least 5 air changes per hour in many non-residential settings to reduce the concentration of airborne viral particles.
For typical residential spaces, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends a minimum rate of 0.35 ACH for the entire home, or at least 15 CFM per person, whichever is greater. Specific rooms require higher rates due to their function; kitchens and bathrooms need higher intermittent exhaust capacities to manage moisture and odors effectively. A low ACH, such as less than 0.5 in a home, suggests stale air, which can lead to a buildup of carbon dioxide and other contaminants.
Conversely, an excessively high ACH can indicate a potential problem with energy efficiency, as the system is working hard to heat or cool a large volume of incoming outdoor air. For spaces like bedrooms, a rate between 0.5 and 1.5 ACH is often recommended to strike a balance between fresh air and thermal comfort. Comparing your calculated ACH to these benchmarks helps determine if adjustments are needed to improve air quality or energy use.