What Is Air Changes per Hour?

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by Sarah Miller
2 years ago

The ACH is an acronym of Air Exchanges per Hour or Air Changes per Hour, and it’s equally important as CADR rating. As the name suggests, ACH reflects the number of times an air purifier changes the air in a particular room per hour.

ACH represents the speed at which an air purifier will clean the environment.

RELATED: Learn more about CADR and how it is calculate.

Most air purifier manufacturers don’t indicate the ACH rating on their machines. As such, you have to do a little math to determine the ACH rating yourself.

Suppose you get an air purifier with 250 cfm (cubic feet per minute or ft3/min) and your intention is to use it in a 200 sq. ft. room with a standard ceiling height of 8 feet. How many air exchanges (ACH) will you get?

To begin your calculations, remember that ACH is measured per hour. This means that you need to multiply the cfm by 60 minutes. Subsequently, multiply your room’s floor size with the height; take the first answer and divide it with the latter.

How to calculate ACPH “Air Changes/Hour”

Formula:

ACPH

Parameters:

Q = cfm (Cubic Feet per Minute or ft3/min) of Air Purifier

V = Volume of the Room Length x Width x Height in (Cubic Feet)

Height of the room is assumed to be 8 ft

Solution:

Volume = 200 sqft x 8 ft = 1600 (ft3)

ACH or ACPH (Air Changes Per Hour) = [60 min x 250 (ft3/min)] / V (ft3)

ACH = 15000 (ft3)/ 1600 (ft3)

ACH = 9.375

This means that the unit will deliver 9.3 air exchanges per hour.

The ideal ACH for your space will depend on your home’s pollution level and the occupant’s health conditions. ASHRAE (The American Society of Heating, Refrigerating and Air-Conditioning Engineers) recommends a minimum of four air exchanges per hour for a patient’s room, six for ICU, and 20 for operating rooms.

Therefore, your machine’s ACH should increase in tandem with your need for clean air. The higher your unit’s ACH rating, the fresher your indoor air will be. An air purifier with a high ACH is highly recommended for homes whose occupants have respiratory issues like asthma or bronchitis.

Below table includes the minimum recommended ACPH for each type of space extracted from ANSI/ASHRAE/ASHE Standard 17022008, these values are used in ventilation design. And can be use to size an air purifier.

AIR CHANGES RATE / HOUR (ACPH) MINIMUM RECOMMENDATION
Basement Parking15–30
Residential Basement3–4
Bedroom5-6
Residential Bathroom6-7
Residential Living Rooms6-8
Residential Kitchen7-8
Residential Laundry8-9
Business Offices6-8
Business Lunch Break Rooms7-8
Business Conference Rooms8-12
Business Copy Rooms10-12
Computer Rooms10-14
Restaurant Dining Area8-10
Restaurant Food Staging Area10-12
Restaurant Bar15-20
Public Hallway6-8
Public Retail Store6-10
Public Foyer8-10
Church8-12
Public Auditorium12-14
Commercial kitchens & Restrooms15–30
Smoking rooms15-20
Laboratories6–12
Classrooms3–4
Warehousing3-10

Note: Just like the CADR rating, ACH is calculated on the highest fan speed. A unit’s ACH will be lower than expected when the fan speed is low.

The Bottom Line

ACH ratings represent an air purifier’s efficiency. They are meant to help shoppers make better choices when looking to buy a suitable unit for their specific application.

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