ACH or Air Changes Per Hour is a metric that tells us how many times an HVAC device can fill up the full volume of a room with air. This is especially useful when comparing different air purifiers or air conditioners.
Example: Let’s take an air purifier with 250 CFM airflow. We put it in a 200 sq ft room with the standard-height ceiling (8 ft). How many air changes per hour does the unit make?
Calculation: 250 CFM is 250 cubic feet per minute. In one hour (60 minutes), we get 60*250 = 15,000 cubic feet per hour. The whole volume of the room is 200 sq ft * 8 ft = 1,600 cubic feet. Such an air purifier is capable of changing the whole volumetric air in room 15,000/1.600 = 9,375 times.
Answer: ACH = 9.375
Note: This post has [comment_no] approved comments. If you have difficulty calculating the ACH with the calculator below, we can help you out in the comment section.
Here is a neat air change per hour calculator you can freely use. Just put in the area, ceiling height, and CFM of an HVAC device in question, and you can calculate ACH:
Air Changes Per Hour Formula (“How To Calculate Air Changes Per Hour”)
The formula of how to calculate air changes per hour from CFM is simple enough. Pretty everybody can calculate it using a digital calculator. All you need to know is the room area, height, and CFM.
This is the formula for ACH (air changes per hour):
ACH = CFM x 60 / (Area x Height)
where ‘Area’ is the area of the space where you intend to have an HVAC device, and ‘Height’ is the height of the ceiling.
The formula is basically ‘how many cubic feet of air can an HVAC unit provide every hour’ divided by the volume of the room.
Here’s how to calculate the ventilation rate of a room
We always get CFM, but that is the volume of air per minute. To calculate air changes per hour, we have to translate that into hours. Hence the multiplication with 60 in the equation above.
The volume of a room is calculated by length*width*height. Multiplying the length of the room with its width will give us the surface area (‘Area’). To get the volume, we have to multiply the area by height.
On this topic, we have published an insightful article about how to adequately size a bathroom exhaust fan. This calculation includes knowing bathroom dimensions and implementing the 8 ACH rule of thumb for bathrooms. You can check this article about how many CFM bathroom fan you need here (with included calculator and chart, based on 8 ACH).
You can also use the ACH calculator to size different types of fans. Here are some examples:
- Best wall-mounted fans with 8 ACH.
- Best high velocity floor fans can be used to achieve a 16 ACH in badly ventilated spaces.
How Many ACH Different Air Purifier Brands Use
The calculation of the recommended coverage area in the air purifier specification is based on CADR rating, maximum airflow, and ACH. Air purifier producers know how to calculate the air exchange rate.
Essentially, to calculate the recommended coverage area, different air purifier companies use 1-5 air changes per hour. The ones that use 5 ACH are very thorough when removing air pollutants that recommended room size, using 2 ACH less so. 5 ACH is recommended for allergy-prone people; we have written about that in our list of the best air purifiers for allergies here.
Here is a list of how many ACH different air purifier brands usually use to calculate the recommended coverage area:
- Medify Air purifiers use 4 ACH. These are very thorough price-performance units; you can check the Medify Air purifier reviews here.
- Alen BreatheSmart uses 2 ACH. Example: Alen BreatheSmart 75i – #1 ranked air purifier – has a 1,300 sq ft recommended coverage area. Its maximum airflow is 350 CFM. At 5 ACH, the recommended coverage area is 520 sq ft.
- Coway air purifiers’ coverage area is based on either 2 ACH or 5 ACH. Example: Big Airmega 400 has a 1,560 sq ft coverage area with a 350 CADR rating (2 ACH). The high-performance Coway AP-1512HH has a 361 sq ft coverage area with a 246 CADR rating (5 ACH).
- Molekule has a recommended coverage area but provides no ACH, CADR, or maximum airflow data. Molekule Air, for example, has a 600 sq ft coverage area, but it’s impossible to deduce how many air changes per hour does it make.
- Honeywell uses 5 ACH. Example: Honeywell HPA300 has a 465 sq ft coverage area with a 300 CADR rating (5 ACH).
- Levoit air purifiers are interesting; they use 3.33 ACH with their best model. Example: Levoit LV-H135 has a 463 sq ft coverage area and 360 CADR rating. Air is changed every 18 minutes; thus, the Levoit unit makes 3.33 air changes per hour.
- Okaysou uses 3 air changes per hour. Example: Their most popular Okaysou AirMax8L air purifier has a 500 sq ft coverage area, with a 210 CADR rating (3 ACH).
- Dyson is very shy about revealing the room sizes. That’s why it’s not possible to calculate the ACH for any Dyson air purifier.
Of all the HVAC devices, air purifiers are unique as far as ACH is considered because their job is the most closely rated to the ACH specification. In essence, ACH is the second-best determining factor that indicates how well air purifiers clean the air.
It is important to understand that the calculation of ACH is solemnly based on airflow. It is not a measure of how well the air purifier’s filtration system works; it does not measure the effectiveness of HEPA filters, activated carbon filters, or even ozone generator filters. High ACH doesn’t, for example, directly reduce the chances of mold growth (mold inspection and testing can attest to that).
There is another better determining specification valid for air purifiers that measure the effectiveness of the filtration system; the CADR rating. The CADR rating is proportional to both ACH as well as various filters air purifiers might employ. Because of this, ACH calculation and the consequent CADR calculation are most appropriate for air purifiers.
In larger rooms and in mold-infested rooms, it’s harder but important to keep the air changes per hour higher. You can check a list of the best air purifiers for large rooms here and a list of the best air purifiers for mold here. The key here is to keep ACH at at least 2 air changes per hour.
For more help if you are looking at SCFM units (Standard Cubic Feet Per Minute), you can consult SCFM vs CFM article here.
If you have any questions regarding the calculation of air changes per hour, you are welcome to ask us in the comments below.