What is ACH?

Curious C: Hey, what is “ACH” ?

Designer: ACH is an abbreviation of “Air Change per Hour.”

Curious C: Why is knowing “Air Change per Hour” important?

Designer: Understanding “Air Change per Hour” is crucial is because it is how we measure the effectiveness of indoor air being filtered.

Many regulatory bodies recommend specific air-change-per-hour rates for different facilities to ensure optimal air quality. For example, you can often see “ACH” at ASHRAE’s publication. Such as this one from ASHRAE 62.2–2019 (Ventilation and Acceptable Indoor Air Quality in Residential Buildings): Table 5–2 Continuous Local Ventilation Exhaust Airflow Rate at the enclosed kitchen is 5 ACH based on kitchen volume.

Curious C: Oh, that’s so specific…By the way, can you tell me how to know the ACH in my room?

Designer: Let’s say you have a product with 250 cubic-feet-per-minute and put it in a 300 square-feet room with a ceiling height of 8 feet. Here is how ACH in your room is calculated.

250 cubic-feet-per-minute times 60 minutes equal to 15,000 cubic-feet-per-hour. Your room volume is 300 square feet times 8 feet, equal to 2400 cubit feet. 15,000 cubic-feet-per-hour divided by 2400 cubic-feet equals 6.25 air change per hour.

That means every 9 minutes and 36 seconds, the air in your room filters through the machine once.

Designer: By the way, the ACH is a aggregated value. Let’s say if your HVAC system provides 2.2 ACH of recirculated air and 2.6 ACH of outdoor air, then the total air change rate at your space is 4.8 ACH. If you wish to reach a 6 ACH condition at your location, then you need to get a device (for example, a portable air cleaner) that will provide another 1.2 ACH to the space.

Curious C: Oh. I didn’t know that improving the air quality also involves math. Thanks! I am curious no more.