Which Way Does Air Filter Go

No matter how easy it may seem, changing the air filter of certain air purifier systems is actually quite a delicate task. As a beginner, you probably have no idea which way does air filter go!

The direction your air filter should face in a system can be determined via an arrow sign on the filter body. Well, the sign doesn’t necessarily have to be an arrow, though, as different brands may use different ones. The orientation also depends a great deal on the unit itself.      

To be honest, there is no one-line answer to this question. That’s why after a lot of scouring, I’ve managed to prepare a straightforward guide for you. This guide may help you to find out the right orientation of air filters in different systems.

Why Setting the Filter Correctly Is Important?

There are various kinds of air filters available in the market. Some are tightly woven and are effective for capturing small pollutants. For instance, HEPA filters have very narrow fibers and are very tightly woven.

A HEPA filter can trap pollutants of surprisingly small sizes. Some filters can trap anything greater than 0.3 microns, which is pretty impressive, in my opinion.

But for such a delicate filter to function properly for a long time, it can’t be exposed to the atmosphere directly. Like a HEPA filter is typically accompanied by a pre-filter that traps the large pollutants first.

After that, the HEPA filter performs its own sophisticated filtering. Direct exposure to dust and other larger common pollutants, will render the HEPA filter dirty and not functional really fast.

While this is just one example, most units comprise multiple layers of air filters, and their roles are not fungible. So, the filter must be set accurately for the proper purifying of air and sustenance of the filter’s health. 

Which Way Does the Air Flow Arrow Go?

To help you properly set up your air filter, manufacturers typically provide an arrow sign on the filter box. Sometimes there can also be a text saying, “Air Flow.”

Which way does the arrow go.

But what do this sign and text actually indicate? And, more importantly, in which direction the airflow arrow point should on my air filter?

Obviously, the arrow wants you to face it toward the direction of airflow. Here, the tricky part is determining which way the unit’s path of air flow is.

Most of the time, facing the arrow toward the unit works. But that might not always be the case.

In a couple of upcoming sections, I will mention how you should approach some specific units and systems with a new air filter.  

HVAC Air Filter Direction – Which Way Does the Air Filter Go?

An HVAC unit is usually responsible for the heating, venting, and air conditioning of an entire house. Some systems are built more complex, and they can manage the entire atmosphere of a large building.

Installing the air filter for an HVAC unit can mean 2 things. You’re directly installing the filter at the unit, or you’re installing it in a vent. Let’s take a look at both.

  • At the HVAC Unit

For this, you’ll have to find out where the airflow arrow is marked first. For installing directly at the HVAC unit, the arrow must be pointed to the unit.

To be specific, the arrow drawn on the filter box must be pointed to the blower of the unit, as the direction of airflow depends on the unit’s blower.

  • In a Vent on the Ceiling or the Wall

When installing the filter in a vent on the ceiling or the wall, point the arrow towards the ceiling or the wall. Don’t point it towards the room.

Here is a pro tip that can save you from this sort of hassle in the future. Once you correctly determine which way does air filter go in a vent, purifier, or any other unit, draw an arrow on the unit’s body yourself.

This way, next time, all you’ll have to do is just match both arrows. 

Which Way Does the Air Filter Go in an Air Purifier?

When replacing the air filters on an air purifier, make sure you maintain the sequence between them. The carbon filter’s job is to remove odors and unwarranted chemicals from the air. So, it doesn’t matter if it sits first or last.

But things can get a little perplexing when it comes to installing the pre-filter and the HEPA filter. Both these filters should have the usual arrow marks on their body. 

Find out these marks first and then locate the blower of the machine. The arrow on the filters should be facing the blower, as that is the direction of airflow. Make sure to put the HEPA filter in between the blower and the pre-filter.

Frequently Asked Questions

  • Which side of the air filter goes up?

First, locate the arrow sign on your air filter. Virtually all filters in the market have it. This arrow will face upwards or towards the ceiling when installing the filter in a vent or a furnace.

But what if your filter has it missing? In such cases, the side with a wireframe should be facing outwards. This means if you’re installing the filter on a ceiling, the wireframe side should be facing downwards, and the side without a wireframe should go up.

  • What happens if the air filter is backward?   

Air filters are always designed to be highly functional in only one direction. The backside of the filter has comparatively loosely weaved mesh than the front side. This is because the air is supposed to enter through the back and get purified in different layers.

It’s not that putting an air filter in the wrong direction won’t work. It’ll just be way less efficient. Besides poor filtration, the wrong orientation of the filter will significantly diminish the filter’s lifetime. It’ll also increase your energy bill. 

  • How do I know which way the airflow filter is?

To avoid such a hassle, you can always match the arrow of your previous filter with your new filter. Another pro tip is to mark the direction once you’ve successfully figured it out. This way, you can avoid such confusing situations in the future.


Determining which way does air filter go is indispensable when you’re installing a new filter because the disadvantages of installing a filter the wrong way are just too vicious.

I hope this article will help clear all your confusion away regarding the orientation of air filters during setup. If you’re still not sure about how it should go, you can always ask for professional help.

Last Updated on February 22, 2023

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Billy J. Weber

Hi. It’s Weber, founder and author of this site Currently you are reading. I am dedicated to provide valuable insights and practical tips to air enthusiasts and anyone interested in improving their indoor air quality.

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