What is Air Change & How to Calculate Air Changes Per Hour

For HVAC technicians, understanding air change and how to calculate air changes per hour (abbreviated as ACH) can help you improve comfort conditions, as well as indoor air quality, for your customers. 

In the wake of the pandemic, calculating air change rate is particularly important for recommending air purification equipment that can help reduce the spread of airborne contaminants like the virus that causes Covid 19.

What is air change?

Air change is simply the number of times that the volume of air in a room gets replaced each hour. In the context of HVAC service work, it’s a measurement of the amount of air flow and ventilation in a room, as supplied by air conditioning and/or air purification equipment.

HVAC service technicians can use this information when recommending new or replacement equipment:

• Installing an air conditioning system in a new space
• Replacing a system after a renovation of space or for a new usage of a space
• Evaluating the need for HVAC design changes to improve comfort conditions
• Recommending air purification systems to improve indoor air quality

Measuring air change helps you to optimize the airflow for a specific space. It takes into account the volume of air in the room, as well as how the room is used. For example, the optimal air change rate is different for a kitchen than it is for a classroom, a retail store, or a warehouse (see below for more information about recommended air change rates for different types of spaces).

How does air change impact comfort and air quality?

Air change is a measurement of air flow, and proper air flow is essential for both comfort and indoor air quality. 

When HVAC equipment blows adequate conditioned air into a room, and the same amount of air leaves the room, comfort conditions can be maintained at ideal levels. Also, air quality is improved because the air is flowing through HVAC filters (or better yet, air purification equipment), frequently enough to remove contaminants. 

On the other hand, if air flow is lacking, your customer can experience the following problems:

• Inconsistent temperatures: areas that are often too hot or too cold.
• Stagnant and stuffy air: occupants feel like they can’t breathe properly or have to put up with lingering, musty odors. In a residence, air quality problems can affect health. In a workplace, they can also impact worker productivity.
• Air pressure imbalance: air flow problems can cause air pressure in a room to be too high or too low, causing doors to slam on their own, odors to migrate within a space (like from a restaurant kitchen to the dining room) as well as uncomfortable drafts.

How to calculate air changes per hour for a room

Here’s how to calculate the air changes per hour in a specific room or enclosed space. 

To begin with, you need to measure the following:

• STEP 1: The CFM (or cubic feet per minute) of air entering the room from the HVAC supply registers. Then, multiply the CFM by 60, to convert cubic feet per minute to cubic feet per hour. 
• STEP 2: The volume of air in the room. Measure the dimensions of the room, including the length (L), width (W), and height (H), in feet. Calculating the volume in cubic feet is a simple matter of multiplying L x W x H. 

Finally, divide the supply air number from step 1 by the volume number from step 2 to get the rate of air changes per hour.

Here’s the formula:

Air changes/hour (ACH) = (CFM x 60) / Room volume in cubic feet

For example:

1. Let’s say you measure 200 CFM coming from a room’s supply register. Multiply 200 by 60 to get 12,000 cubic feet per hour.
2. If the room is 16 ft x 16 ft and 8 ft high, multiply L x W x H to get 2048 cubic feet of volume.
3. Divide 12,000 by 2048 to get 5.86 air changes per hour.

Recommended air changes per hour for space usage

Here are some accepted air change rates for common residential and light commercial applications (as reported recently by Contracting Business). For more applications and detailed information, refer to the ASHRAE HANDBOOK – HVAC SYSTEMS AND EQUIPMENT.

RESIDENTIAL SPACES	MINIMUM ACH
Basements	3-4
Bathrooms	6-7
Bedrooms	5-6
Kitchens	7-8
Laundry rooms	8-9
Living areas	6-8
LIGHT COMMERCIAL SPACES	MINIMUM ACH
Offices	6-8
Conference rooms	8-12
Computer rooms	10-14
Break rooms	7-8
Hallways	6-8
Foyers	8-10
Retail stores	6-10
Restaurant dining areas	8-10
Food service kitchens	14-18
Public restrooms	10-12
Auditoriums	12-14
Medical offices	9-10

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