When you are buying an air purifier, you will come across the term Air changes per hour meaning the number of times the air in the room is exchanged with air from an air purifier, with filtered air.
ACH means air changes per hour. There is no hidden meaning to it. When you switch on the air purifier, it pulls the air in the room inside and over its HEPA filters. They filter all the contaminants and then it pumps the now purified air out.
It pulls in more air, purifies and releases again. With every air change, there is less polluted air in the room. It continues doing this until all of the air in the room has been changed many times over.
How does an air purifier work?
While in this post we are looking at air changes per hour for air purifiers, it’s applicable in other settings too. For instance, ACH also applies to the number of times air enters a room to fill it.
Back to how air purifiers change air, the ACH rate will be indicated somewhere on its description. If the manufacturer says the unit has an ACH rate of 4 times in an hour, it means that it can clean the air in the room once every 20 minutes.
As you buy your unit, you must also look at the space rating of the air purifier. If the manufacturer says to use it in a 250 sq. ft. room, do not use it in a bigger space. So, the ACH rate must go hand-in-hand with the space rating.
To remove the pollutants from the air, the appliance can use HEPA filters, activated carbon, washable filters and others. The most popular are the HEPA filters, which capture more than 99% of the air pollutants.
ACH Calculator and Formula
There is a simple way to calculate the ACH. This can help you to calculate the times that a HVAC system must work to fill a room with air. Remember, you also need to know the air changes per hour for your whole house air purifier and air conditioner.
To calculate the ACH rate, you must know a few things. One of them is the CFM – Cubic Feet per Minute (cubic feet of air moved per minute).
Secondly, you also need to know the area of the room, that is, the length multiplied by the width. Thirdly, you need to know the height of the room (from floor to ceiling).
Thus, the formula for calculating the ACH of your appliance looks like the following:
In this case, the number 60 is for the minutes in one hour, since CFM is cubic feet of air moved by your appliance per minute.
The room volume means the area of the room (in square feet) multiplied by the height of the room (floor to ceiling).
CFM is the rate of airflow of the air purifier. It will be indicated on the appliance.
Also read: Why your air purifier smells like ozone?
How to calculate the ACH of your air purifier
To make things easier, we can do a practical example to see how you calculate the ACH of your appliance to see how effective it’ll be:
Let’s assume you need to purify the air in a 200 square foot room. The height of the ceiling is 8 feet. If the CFM is 100, we can do some simple arithmetic here.
So, the ACH will be (100 * 60) ÷ (200 * 8) = 3.75
This means that your air purifier has an ACH rate of 3.75. In one hour, it’ll complete 3.75 air changes, which is about one air change every 16 minutes. This is fast enough even for heavily polluted areas.
You won’t need to calculate the ACH of your appliance manually like we did here. Just ensure that the manufacturer has indicated it on the appliance.
How many air changes per hour do I need?
The number of air changes per hour you need depends on many things.
First, you can go by what the American Society of Heating, Refrigerating and Air-Conditioning Engineers advises for HVACs. They say that a home should get 0.35 air changes per hour.
However, this is the rating for a home with regular air, not polluted air. For an air purifier, there’s need for more because you’ll already have identified the pollutants in your indoor air.
Since here we are looking at air purifier air changes, this may not apply to the air changes from a HVAC system.
If you have asthma, you need an air purifier with a higher ACH rate. You need to remove all the pollutants from the air so that they don’t irritate your airways.
The ACH rate also depends on the climate zone, especially for the HVACs. For instance, in Michigan, you can’t have more than four air changes per hour.
In most places though, the ACH rates range between three and five times in an hour. In any case, it is also not possible to find a portable air purifier with an ACH rate that’s higher than five.
If you have a whole house air purifier integrated in your HVAC system, you need to have a professional check it over regularly.
That way, you’ll know whether you need to fix the ventilation, whether you’re losing heated or cooled air and so on.
If you have a portable air purifier, use it to purify air in the recommended space. If you don’t adhere to the space rating, it is not going to work as effectively as it should. Thus, it will take longer than necessary to purify the air in your space.
Also read: Can air purifier help with mold spores?
What is Air Purifier Air Changes Per Hour? FAQ
An air purifier with an ACH rate of 2 can change the air over two times, or once in 30 minutes. However, many things are involved when considering the air change per hour that you need. Here are some questions and their answers, which will shed more light on this topic:
What’s the recommended air changes per hour for air purifiers?
According to the CDC and ASHRAE, the recommended air changes per hour rate is two times. This is a guideline, so you can find a Honeywell HPA300 HEPA Filter Air Purifier, which has a rate of 4.8 times and a space rating of 465 square feet.
Further, ASHRAE says the ACH rate will differ from room to room. For instance, bedrooms should have ACH of up to 6 times, kitchen up to 8 times and laundry rooms up to 10 times. However, these are rates for the HVAC system, not a portable air purifier.
How long does it take an air purifier to purify the air?
This depends on the size of the room, again, to reiterate, this is why air purifiers have a space rating. Large rooms can take up to 5 hours to purify the air. For smaller rooms, it can take as few as 30 minutes to purify the air completely. Also, more polluted air takes longer to clean.
How many hours a day should an air purifier run?
Portable air purifiers are not designed to run continuously for days on end. However, a good quality one can do up to 12 hours. You can stop running it when it has purified all the air in the room. Remember, open windows might bring in more polluted air from outside thus needing the air purifier to run even longer.
Why is air change per hour important?
Air change per hour is so important because it helps you determine how long to run an air purifier. For a HVAC system, good ACH helps a lot with humidity because the whole house dehumidifier will remove the moisture from the air with every exchange. The higher the ACH rate, the cleaner the air will be indoors.
What is a good air changes per hour?
It depends on the size of the room and the size of your air purifier. Some of these units come with up to 5 ACH. Some can even come with more than that. HVAC systems have 2 times ACH or even less. The best ACH is one that meets your needs in as short a time as possible.
What is 10 air changes hourly?
This means that the air in your house has gone through the purifier up to 10 times in one hour, which is once every six minutes. That is a very high ACH rate and many portable air purifiers might not achieve it.
How many times should air exchange per hour?
There is no golden rule to this as there are so many variants. For HVAC systems, exchanging air three times in 60 minutes or once every 20 minutes is considered good. Portable air purifiers can come with ACH rates of up to 5 times an hour or more.
The air purifier changes per hour for your unit determine how fast you can have clean air at home.
Here, we recommend you get one with at least 5 ACH. It will change air in the room up to 5 times in 60 minutes, taking it over the HEPA filters and removing all the pollutants.
You can calculate how much ACH rate you need in your room using the formula that we put out here. Thanks to the modern air purifiers, they already come with ACH rating. Just to trust the manufacturer and go with that.
It is also good to know the things that might affect the ACH of your unit. One of them is the size of the room, the size of your unit as well as how polluted the air is.
Since modern air purifiers can remove all types of pollutants from your indoor air including Formaldehyde gas, perhaps you need to buy a VOC sensor.
This device detects the level of VOC (volatile organic compounds) in your indoor air and it lets you know when you need air changes.
Most VOC sensors are simple, easy to interpret and use.
Also find out the types of pollutants your unit can remove from the air. If you have dust at home, get one that can remove such.
If you have VOC, get one designed to remove such and the same applies to mold spores.