How Many CFM Does a Dryer Exhaust?

A dryer is a perfect appliance for removing moisture from your clothes and other textiles once they are washed.

One way of determining a dryer’s efficacy is by its rate of air volume flow, which is measured as cubic feet per minute (CFM). But how many CFM does a dryer usually exhaust? Typically, a dryer will exhaust around 100 to 250 CFM. This is assuming that your dryer’s capacity, ductwork, and other influencing factors are of standard value. But once these influencing factors change, the exact rate your dryer exhausts air will alter as well.

So, starting with the very basics, let’s see how these factors affect how many CFM your dryer exhausts, and also calculate the exact value for it.

What CFM Means, and Why It’s Important

Simply put, the CFM (Cubic Feet per Minute) value of your dryer indicates the amount of air it moves within a minute. Mathematically you can calculate it with the simple equation “Volume (in cubic feet)/ Time (in minutes)”. But is knowing the rate at which your dryer exhausts air really that important? The answer is yes, a dryer’s CFM value will affect many details such as its installation and energy consumption.

Generally, the dryer draws in the heated air in front of it and moves it outside through the ductwork. Once the heated air is removed, it is replaced with cooler air from the interior surrounding of your home. This process continues till your clothes are dry.

The vents and duct system connecting your dryer to the exterior should be installed in a way that allows the dryer to exhaust the most amount of heated air possible. So, knowing the CFM value of your dryer will allow you to install the ductwork correctly.

Improper duct installations can cause bad outcomes. For example, a vent system that’s too long can lead to a drop in pressure along the vents, causing the heated air to linger. This can sometimes increase the temperatures of different rooms in a home, causing a burden on the cooling system. The cooling system will then have to do more work, resulting in higher power consumption.

Another crucial reason for having proper ductwork is that it minimizes the risk of fires. Around 16,000 dryer-related fires are reported on average each year according to the National Fire Protection Association. To put it in context, clothes dryers make up 92% of the causes of house fires. Most of these fires were because of maintenance issues and poor ventilation.

So, knowing your dryer’s CFM value and setting up your ductwork accordingly will make your home safe as well.

Calculating the CFM Value of a Dryer

Calculating how many CFM your dryer exhausts can be a tricky matter. This is because the value can change depending on where you take measurements from and the structure of the outlet. For example, the exhaust rate at your dryer’s outlet is going to differ from that at the exit vent of your home. We will discuss the reason for this later.

As mentioned earlier the basic equation to calculate the amount of air moved per minute is “Volume (in cubic feet “cu. ft.”)/ Time (in minutes)”. Let’s tackle each part of the equation separately to better understand it.

1) Volume

The volume depends on the structure of your outlet. The general equation for volume is “Length x Breadth x Height”. This however is true for a rectangular structure.
Therefore, if you are measuring the exhaust rate out of a rectangular-shaped outlet, your go-to equation for volume is – “Length x Breadth x Height”.

But this equation changes if you are to measure the airflow at a circular outlet such as a tube. So the equation to measure the volume of a tube or any other cylindrical outlet would be “Length x 3.14 (pi) x radius squared”.

Although the breadth and height of a rectangular outlet or the radius of a cylindrical outlet are easy to measure, it’s difficult to measure the running length of the duct. We will come up with a solution for this shortly.

Keep in mind that the breadth, height, and radius should be measured in feet.

2) Time

Next, we have to measure the time a certain volume of air is exhausted. But doing so is fairly difficult, just like the running length of an outlet. So to make matters easy, we are going to combine the two variables, which would then give us velocity – “Length/Time”.

Finding the velocity of airflow is quite easy, all you need is a device called the Anemometer. Usually, the anemometer gives the velocity reading in “meters per second (m/s)”, if yours does this as well, use the following relationship to convert it to “feet per minute (ft/min)” – 1m/s = 196.85 ft/min

3) Simplified Equations

Now that we have all the required variables, let’s combine them and come up with a few simplified equations to make the calculations easier. Let’s go back to the basic equation used to measure the volumetric flow rate.

Volumetric flow rate = “Volume (cu. ft.)/Time (min)”

Using this equation lets derive a simple equation to deal with rectangular outlets.

Volumetric flow rate = (Length x Breadth x Height)/Time
Volumetric flow rate = (Breadth x Height) x Length/Time
Therefore the Volumetric flow rate = Breadth (ft) x Height (ft) x Velocity (ft/min)

Likewise, we can come up with another equation to measure how many CFM a dryer exhausts through a circular duct.

Volumetric flow rate = (Length x 3.14 x radius squared)/Time
Volumetric flow rate = (3.14 x radius squared) x Length/Time
Therefore the Volumetric flow rate = 3.14 x radius squared (ft) x Velocity (ft/min)

A Few More Factors Affecting The Exhaust Rate

It is clear that the dimensions of the ducts and the speed of airflow affect how many CFMs your dryer exhausts. But these are not the only factors that influence a dryer’s exhaust rate.
Although not directly calculated, the following elements contribute to the volumetric flow rate of your dryer.

The power of the blower fan-The blower fan in your dryer helps move the air. It is a vital part of the machine. The power the blower fan consumes directly relates to the amount of work it does. If the blower fan consumes more power, it will move a larger volume of air. As a result, the exhaust rate of the dryer will also increase.

The number of Elbows in your ductwork-If you are to measure the volumetric rate at the exit of a duct, you will most likely get a value lower than the actual exhaust rate. This is because of the elbows in your ductwork. Elbow joints act as restrictions to the airflow, hence reducing the volumetric flow rate.

Other airflow restrictions- Another airflow restriction is the accumulation of lint. This is also the primary cause of dryer-related fires. Lint gradually accumulates in the ductwork and the filters during a dryer’s active life span. As it builds up, the path air can flow through narrows which would eventually affect the dryer’s exhaust rate.

Home Maintenance