How To Tell Which Way A Fan Blows

Fans are everywhere it seems! They’re critical for everything from cooling a computer’s CPU to keeping us comfortable or even helping with plant grow rooms. It’s honestly amazing how versatile they are.

But do you know how to tell which way a fan blows? If not, I’d love to show you how. Once you’ve got the basics down it’s not hard at all!

Infographic – Fan facts and airflow basics

How a fan blows and fan facts infographic

How to tell which way a fan blows – 3 basic methods

The great news is that generally speaking it’s easy to tell fan air direction if you know what to look for. Because of how they work (and how most are designed) there are great clues to help.

It also depends strongly on the fan’s intended use as well. Here are 3 ways to tell which way a fan blows:

  1. Using the fan angle and rotation direction
  2. By the type of fan and its intended use
  3. Fan directional markings

1. Fan blade angle and rotation direction

Fan air flow diagram

The direction a fan blows air is always determined by (A) the shape of the blades (sometimes called propellers) and (B) the direction they turn. If a blade is angled down and the fan turns in the same direction as the blade angle, it will blow forward. Likewise, if the blade is angled down and the fan turns in the opposite direction, the reverse is true. Air will blow in the other direction (towards the back).

These two characteristics of a fan are probably the easiest and most common way to tell which way it blows.

That’s because a fan always has the same basic design in one form or another: fans use angled and slightly curved blades to cut through the air and force it forward or backward depending on the direction of rotation.

Here’s an animated image I’ve put together to help show this.

In this image you can see exactly what I’m describing: the fan is rotating in the same direction as its blades which are angled downwards. This causes air to blow upwards (to the front of it).

If the fan’s direction is reversed, the air will go in the opposite direction as it would be forced downward (towards the rear) instead of the front.

An everyday example

CPU fan air flow direction example illustrated

Here’s a great example of how to decide which way a fan blows. This is a common small fan powered by direct current (DC) and used for many electronic cooling applications. Looking at the fan, we can see the angle of the blades and we know that most of these rotate clockwise (the same direction as a clock). Therefore, unless the power polarity is reversed, it will blow air forward.

The image above is a great example of a common fan you might run across. By looking at the fan and knowing which way the blades are angled, and knowing that it normally turns clockwise, I can see that it will blow air forward.

Of course, if for some reason the fan rotates in the opposite direction (unbeknownst to me) the air would be blown the other way.

2. The type of fan and its intended use

centrifugal fan vs axial fan comparison diagram

Fans generally only come in a few basic designs, most of which are axial or centrifugal types. Centrifugal fans normally only blow in the direction of the outlet as shown, while axial fans may be reversible in some cases. 

While many variations of fans exist, nearly all fall into just a few basic categories as the most important design principles are the same.

A fan’s direction is often fixed and largely determined by what it was designed to do:

  • Airplane propellers and jet engines always force air from the front to the rear
  • Ceiling fans are normally used for blowing air downwards into the room but can be reversed to force air to the ceiling
  • High-speed cooling fans of the centrifugal type blow forward
  • Home cooling fans normally blow forward and can’t be changed
  • Ventilation fans are mounted in a wall, window, or other assembly and spin to blow air out of a building

Lasko 3720 Weather Shield box fan in window

Most common home fans used for cooling like this window box fan are designed to blow only one direction: forward. You can see the blade shape in the picture which gives a clue about this as well. You’ll need to rotate the fan 180° to change the air direction.

Fans powered by alternating current (AC) such as common household 120V or 220V fans typically can only blow forward. However, direct current (DC) fans are often reversible.

That’s because DC power can change the operation of the electric motor inside the fan, causing the blades to spin in the opposite direction and force air to the rear.

3. Fans that have directional markings

Illustrated image of computer fan air flow direction

An example of some common fans with markings to show which way air blows. In both cases, there are 2 arrows: one to show the direction of rotation, and one to show the airflow direction. Left: This fan is the most common type, as it can be mounted backward if you need to reverse airflow. Right: This fan is less common but does have some uses where the airflow going to the rear is important.

While it’s not as common, some fans do have a marking indicating which way they blow. If you’re lucky enough to find that on one, it makes it even easier to tell!

Ordinarily, you’ll most likely find this on general-purpose fans like small DC-powered fans for cooling components like integrated circuit (IC) chips. However, from time to time it can be found on others too.

It’s really handy to remember method #1 above. That way you can tell which way a fan blows nearly 100% of the time and especially if one isn’t marked with arrows or a label to tell you.

Did you know? Great fan facts

Fans are typically not very expensive but can be very cost-effective in many applications. They’re helpful for many common uses and problems, but there are some things you might not know about them.

Here are some great fan facts to know. Be sure you’re using fans to improve your quality of life (and save money) as much as possible!

1. They’re very energy-efficient vs. air conditioning

Tower fan money saving badge imageElectric fans can run and help cool you and your room with up to an amazing 60% less power than air conditioning units!

That’s because fans don’t need nearly as much electric current to work. Air conditioning (AC) units not only have to power an electric fan but also an energy-thirsty refrigerant compressor motor. That adds up over time!

2. Fans cool you…but don’t actually cool the air!

Fan cooling forced convection diagram image

Fans can make you feel cool and comfortable as well as cycling the room’s air to prevent heat from building up. But did you know? They don’t work by chilling the air. They work off of the principle known as forced convection.

Here’s a cool (no pun intended!) fact – electric fans don’t cool a room by dropping the temperature. Air conditioning units do so by removing heat from the air.

Fans create a cooling effect by the principle known as forced convection. That just means that fans cool your body and your room by forcing air across surfaces, removing heat from you into the nearby air.

They also keep air circulating within a room, offering a continual effect that makes you more comfortable.

Because of that, it’s important to have a well-designed fan with great airflow and especially one with an oscillation feature to move back and forth, covering a large area in a room.

Lasko T42950 Wind Curve tower fan example

Tower fans are specially designed to produce a comfortable airflow. They’re different from other types as they produce a tall, vertical area of air and often oscillate (move side to side) to cool a room.

Tower fans are an excellent example of this.

3. They’re great for white noise

Lasko 3720 Weather Shield box fan front image

A simple but classic box fan produces a soothing and calming “white noise” that can improve your quality of life. A side effect of how they work is the noise that can “mask” (covers up) outside noises like neighbors, traffic, people talking, and more. It’s a great and cost-effective way to help you focus, relax, or sleep better.

Fans are also very helpful in some cases for blocking outside noises that cause us stress and interrupt sleep, studying, or working.

Models that produce a very high airflow rate and introduce a lot of turbulence near them as a side effect can produce a large amount of soothing white noise. Box fans are especially helpful for this.

Grant Williams

About the author

Grant is a professional engineer by trade and has experience with both maintenance and do-it-yourself home projects. He enjoys sharing his expertise & ideas with others to help them improve their comfort and quality of life. Read more »

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