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

Changing the air filter in your AC or furnace can seem like a simple task.

But there is one critical thing you need to know – which way does the air filter go?

The arrow on your air filter should point to the blower in your HVAC unit. If the filter is in the wall or ceiling in your home, then the arrow on the filter should point to the wall or ceiling.

In this article, I’ll help you figure out which way the air filter goes in your home’s HVAC unit. I’ll also go over how to check the airflow direction in your AC or furnace.

Which way does the arrow go on an air filter?

Most air filters have an arrow printed on their side that indicates the direction of airflow.

The arrow should point in the same direction as the airflow in your HVAC unit.

  • If you are installing the air filter at the HVAC unit, the arrow should point to the blower.

  • If you are installing the air filter in a ceiling or a wall, the arrow should point to the ceiling or wall.
Air filter arrow direction

Here’s a tip for you: After figuring out which way the arrow should point, draw an arrow on the filter slot. Now you can just match up the arrows next time you change your air filter!

What happens if you put the air filter in backward?

If you put the air filter in backward, then you run the risk that the filter will implode and damage your HVAC unit.

The air filter is not designed to be put in backward since the filter material only filters properly in one direction. There are multiple layers of mesh in the air filter.  These layers increase in density towards the downstream side of the air filter, which allows a higher filtration efficiency.

You’ll suffer from reduced filtration efficiency in your HVAC unit if the filter is backward.

In addition, the metal mesh that holds the filter together only works if the filter is installed in the correct direction. If the filter is installed backward, you risk the filter imploding on itself when your HVAC unit turns on.

Which side does the air filter’s metal mesh face?

The air filter’s metal mesh should face the blower in the HVAC unit. If the filter is in the wall or ceiling, then the mesh should face the inside of the wall or ceiling.

The metal mesh should always be on the downstream side of the airflow in your HVAC system.

What is the purpose of the air filter’s metal mesh?

The metal mesh on a pleated air filter is used to keep the air filter together. When air flows through your HVAC system, the air filter is subjected to strong pressure. The metal mesh holds the air filter’s filtration media together.

Pleated air filters are made of a pliable material such as polyester or cotton. The soft air filter material would stretch out if it didn’t have anything to hold it in place. The metal mesh also prevents the filter material from fluttering while air passes through it.

Which way does the air flow in an HVAC unit?

In an HVAC unit, the air flows from the return duct, through the HVAC unit, to the supply duct. In a residential central AC or furnace, the air almost always flows from the bottom to the top of the HVAC unit.

How do you figure out which way the air flows in an HVAC unit? Here are some methods:

One method is to inspect the ductwork attached to the unit:

  • The supply duct will sometimes have insulation around it. If one duct is insulated but the other one is not, the insulated duct is most likely the supply air duct.

  • If none of the ducts are insulated, you can put your hand on them to feel the temperature. If you are running your furnace, then the supply duct will be hot. If you are running your AC, then the supply duct will be cold.

  • If you have central AC, then the AC evaporator coil is located in the supply duct.

Once you know which duct is which, then you can figure out which direction the air flows.

Another method is to examine the location of the air filter slot in the HVAC unit. The air filter is always located in the return air duct. The airflow direction is from the air filter slot to the HVAC unit. 

Furnace air flow direction diagram

4 thoughts on “HVAC Air Filter Direction – Which Way Does the Air Filter Go?”

    • Hi Ronald,

      It depends on what is causing the ticking sound. A quiet ticking sound from a blower spinning usually isn’t of too much concern, but it might be worth getting it checked out by an HVAC professional to ensure that everything is working correctly.

      Cheers,
      -Trey

      Reply

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