CFM 101: What is CFM And How To Calculate CFM
Alright, so we are going to tackle CFM. What the heck it is and what that has to do with air efficiency. After you have a firm understanding of what it is, we are going to tackle the how. How in the world do we calculate CFM for our uses.
For example, how to determine the amount of CFM you need for the size of the room you want better circulated. So what exactly is CFM?
In the world of HVAC, CFM stands for Cubic Feet per Minute. It is a measure of how much air can be moved in one minute. CFM is the amount of air that moves through an area in one minute. The higher the CFM rating, the more airflow there will be to cool or heat your space.
Let’s dive a bit deeper into what CFM is before we get into how you can use this number to figure out your own needs. Keep reading.
What Is CFM?
CFM or its elaboration “cubic feet per minute” is referred to as the amount of air that can be moved per cubic foot in a minute. Also referred to as “airflow”. When a fan is set to its maximum speed, the CFM is calculated by utilizing both the volume of air and the rate at which the fan blades move.
The measurement of CFM is of immense significance when choosing the right fans for your room to guarantee the perfect balance of airflow, helping you stay healthy and cool at all times.
Even the manufacturers regard the CFM as a key issue in product manufacturing to compete against different brands and come up with the most air efficient fans.
What Is the Correlation between CFM and Air Efficiency?
As we already mentioned, top-rated fan manufacturers focus on producing the most air-efficient fans by focusing on the CFM. But why is that? Does a higher CFM always mean that the fan is more air efficient?
For determining the airflow efficiency, the measured CFM is divided by the amount of energy (in Watts) that is used up when the fan is being operated at its maximum speed level. In such cases, the standard or minimum CFM to label a fan as an efficient one is considered to be 75.
A drawback to high CFM fans to be considered as an inefficient one is when it consumes high watt energy. So, air-efficient fans need to have both high CFM along with relatively lower wattage consumption. Try to keep both of these aspects in mind when purchasing a fan with the CFM you need.
Calculating CFM: How to Calculate CFM (Cubic Feet Per Minute)
Now that we know the significance of measuring the CFM, let’s move on to the next step – “how do we measure the measure the CFM?” Quantifying the CFM can be a bit tricky if you’re bad at math, but we have you covered! You just have to follow our instructions, and you’re good to go.
Here’s the detailed process on how to figure out the CFM easily:
Method 1: Using the Air Changes Calculation to Determine Room CFM
The air change rate is basically the measure of how long it takes for an enclosed area to replace the inside air with the outside air through ventilation.
The unit used to express this air change rate is the ratio of cubic meters per hour and the volume of air within the room. Simply put, it is the number of times the air is exchanged between the inside and outside of a space.
For instance, if the volume of air in a room is replaced twice in an hour, then space is said to have two air changes per hour or 2 ACPH.
Air Change Formula to Calculate CFM
Although a number of complex computer analyses can be done to calculate the air change rate, a much simpler mathematical formula can also be used for the same purpose. This formula is incredibly straightforward and does not require any expertise to determine the ACPH (air change per hour).
You can easily use the following equation to do so –
To calculate the air change in a room, the airflow is measured (CFM) and multiplied by 60 minutes per hour. It is then divided by the volume of the room you wish to measure (in cubic feet):
Air Changes / hr = (CFM x 60 minutes) / Volume of the room (cubic feet)
In simpler words, we change the CFM or Cubic Feet per Minute into CFH or Cubic Feet per Hour and divide it with the volume of the room. The volume of the room is determined by multiplying the room’s height, length, and width (Volume = Height x Length x Width).
An example to clear out your confusion-
Suppose the CFM of your room is 400, and the room measurements are as follows:
Height = 10 feet
Length = 20 feet
Width = 15 feet
Then the ACPH (air change per hour) would be –
Air change / hr = (400 CFM x 60 minutes) / (10’ x 20’ x 15’)
Air change / hr = 24000 / 3000
Air change / hr = 8
Therefore, we can say that the air change rate is 8 times per hour in that particular space.
How Frequently Do I Need to Exchange Air?
Frequency in the exchange of air differs due to differences in room sizes and types of the enclosed area. While a smoking zone requires considerably more air change rate, the ACPH for a basement is quite low.
Here is a list of the air change frequencies of some common spaces-
| Residential ACPH by Room |
Usual Air Change |
|
Basement |
3 – 4 |
|
Bedroom |
5 – 6 |
|
Bathroom |
6 – 7 |
|
Family Living Room |
6 – 8 |
|
Kitchen |
7 – 8 |
|
Laundry |
8 – 9 |
| Office ACPH by Room |
|
|
Business Office Room |
6 – 8 |
|
Lunch Break Room |
7 – 8 |
|
Conference Room |
8 – 12 |
|
Copy and Print Room |
10 – 12 |
|
Computer Room |
10 – 14 |
|
Smoking Zone |
13 – 15 |
| Restaurant ACPH by Room |
|
|
Dining Area |
8 – 10 |
|
Kitchen |
14 – 18 |
|
Bar |
15 – 20 |
| Public Area/Building ACPH |
|
|
Hallway |
6 – 8 |
|
Stores |
6 – 10 |
|
Church |
8 – 12 |
|
Restroom |
10 – 12 |
|
Auditorium | 12 – 1 |
|
Smoking Room | 15 – 20 |
Room CFM Formula
At times, the airflow or CFM of a particular space may be unknown to us. On certain occasions where you are confused about what CFM fans might be required for your room size or type, you can easily use the standard air change table above to calculate the airflow.
To do so, you simply change the equation or formula for the air change measurement. Hence, you can just calculate the volume of the room in cubic feet, multiply it by the desired air change per hour (our table can really come in handy!), and divide the product by 60 minutes.
Or in a more straightforward way, this is how to do it –
CFM = (Volume of the Room x Air Changes / hr) / 60 minutes
A sample to make it easier for you –
Suppose you want to help determine the CFM of your friend’s newly bought store. It necessitates an air change value of around 8 times per hour and has a volume of around 6000 cubic feet (10’ x 30’ x 20’). The required CFM would be –
Required CFM = (10’ x 30’ x 20’ x 8 ACPH) / 60 minutes
Required CFM = 48000 Cubic feet / 60 minutes
Required CFM = 800
This indicates that for proper airflow in the store, you would need to install a fan with around 800 CFM. This may vary in certain cases if you decide your air change rate to be a bit higher or lower.
Method 2: Using CFM Calculator
With the advancement of technology, you don’t really have to go all the way to use your math skills (lucky save if you’re bad at it!) and waste your time to determine the CFM that is needed for your space. Your savior is really just a CFM calculator!
What Is a CFM Calculator?
Calculators or, more precisely, CFM calculators help you to measure the airflow in a much more hassle-free way. Interesting, isn’t it? You can easily find such calculators online, which are, in fact, free!
How to Use CFM Calculator to Measure CFM
Using a CFM calculator is pretty easy and does not require any expertise at all. All you have to do is measure the height, length, and width of your enclosed space and input it into the virtual calculator along with the frequency of your desired air change rate. And voila! The calculator shows you the required CFM!
Some CFM calculators also come in with options to input the wattage consumption as well as the number of light sources in the room, which helps in a much more precise calculation.
Pretty amazing how technology can help you to always be at ease by just sitting back and relaxing, right?
Conclusion
Even though determining the CFM and air change rate may seem like an extra burden to most people, you now know how essential it really is to ensure appropriate air ventilation for residential or commercial spaces. Then why wait? Go and spread your knowledge to your friends and family now!
