What Is ACH and Why Is It Important?

Contributed by Dwyer Instruments
Written by Andrew Goldschmidt

If you’ve ever spent time in a hospital, you may have noticed a device similar to a home thermostat mounted on the wall with a display stating “ACH”. What exactly does “ACH” stand for and why is it important in a building or hospital?

What is ACH?

Air changes per hour (abbreviated as ACPH or ACH) or air change rate is a measure of the air volume added to or removed from a space divided by the volume of the space. Simply put, air changes per hour are the number of times you change over the total volume of air in a defined room or space. Air changes per hour are calculated by first determining the volume of the space you will be exchanging air in. This is calculated by multiplying the length by width by height of the room.

To calculate the air changes per hour, multiply the incoming or supply air flow rate (Q) in units of cubic feet per minute (CFM) by 60 minutes per hour, and then divide that number by the volume of the room.  One “air change” results when all of the air in the room has been replaced.

ACH = (60 × Q) ÷ Vol

Q = Supply air flow in CFM

Vol = Space volume calculated by L × H × W in cubic feet

Why is ACH important?

Homes and workplaces are full of potential contaminants ranging from VOCs (Volatile Organic Compounds) to common dust. Proper ventilation and the exchange of air is the best way to reduce the presence of these pollutants, maintaining a healthy living and working condition.

Dating all the way back to 1973, ASHRAE has provided a set of standards for minimum ventilation requirements in commercial and residential applications that is found in ANSI / ASHRAE 62.1 and 62.2. These standards are incorporated into the design and size requirements of air handling equipment to ensure the space classification is seeing an adequate amount of air changes.

In commercial office spaces, it is common to see air changes per hour of 6 to 8. On the other hand, hospital operating rooms and patient rooms can have air changes per hour of up to 25. You may also notice thermostat-style devices are continuously indicating air change rates in these rooms, allowing nurses and doctors to quickly identify safe working and operating conditions for patients.

To monitor the air changes per hour, individuals can measure the supply air entering the room or space, as well as the exhaust air or return air. With both of these readings, it’s easy to determine how many air changes are occurring. This can be temporarily accomplished with test and balance instrumentation, such as by using air flow hoods to monitor the air flow coming out of diffusers, or by using anemometers or Pitot tubes to monitor the air flow in supply duct work or return air duct work.

For more permanent installations, products such as averaging Pitot tubes, air flow stations, or hot wire anemometer transmitters can be installed to provide a building management system with real time air velocity measurements. Dwyer offers several air flow monitoring instruments to simplify the calculation of air changes per hour. These products can ensure a comfortable environment for individuals occupying the space.

Shop Dwyer air flow products here.

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