Furnace Filter Airflow Direction: 1 Simple Rule For Filter Arrow

“Which way does the air flow on a furnace?”

This is what we should always ask ourselves when installing a furnace air filter. Furnace airflow direction is very easy to figure out with just 1 rule that applies to all furnaces and all filters. You don’t need to stick your hand into the air filter slit or anything like that. Nor do you need a furnace airflow direction diagram to figure this out.

Here’s the deal with furnaces and installing air filters:

  • Every filter will have an arrow on it. Filtrete filters, 3M filters, Honeywell filters, all of them will have this arrow.
  • Important: Point the arrow on the filter in the direction of airflow. That’s exactly why we get so many questions about furnace airflow direction.
    Furnace Filter Airflow Direction on a downflow furnace
    Downflow furnace example: Filter slot on the upper part. Airflow from up to down. You should point the arrow down.
  • All furnace brands follow the same filter direction principle. Carrier, Lennox, Bryant, York, or any other furnace brand; the direction of the airflow is always for the same type of furnace.

Let’s first look at these 3 types of furnaces with different airflow directions (upflow, downflow, horizontal furnaces). We will explain which way the airflow flow on every type, just to illustrate how this works.

Below that, we will reveal the 1 simple rule that helps to properly install filters into any furnace every time (it’s really simple, easy to remember, and you can’t go wrong with it). Alright, let’s start with the 3 furnace types and airflow directions:

Airflow Direction On Upflow, Downflow, Horizontal Furnaces

Since the arrow on the filter has to align with the airflow direction, we need to figure out which way the air is moving through our furnace. Namely, there are 3 types of furnaces we all use, and they have different airflow directions:

  • Upflow furnace (located in the basement). The direction of airflow is down-up (from the basement to the 1st, 2nd floor, etc.). The supply air comes in from below, passes the air filter (with the arrow up), and enters the furnace itself.
  • Downflow furnace (located in the attic). The direction of airflow is up-down (from the attic to the lower floors). The supply air comes in from above, passes the air filter (with the arrow down), and enters the furnace. You can read more about upflow vs downflow furnace here, if you need to figure out which one you’ve got.
  • Horizontal furnace. Here, the airflow direction is left-right. Supply air comes in from the left, passes the air filter (with the arrow to the right), and enters the furnace.

As you can see here, the furnace air filter slot is always located between:

  • Supply air.
  • Furnace itself.
    furnace airflow direction diagram
    Furnace airflow direction diagram for a downflow furnace: Upper slot, airflow direction down towards the heart of the furnace.

This explains the following air filter slot location:

  • Upflow furnace has the filter slot located at the upper part of the furnace.
  • Downflow furnace has the filter slot located at the lower part of the furnace.
  • Horizontal furnace has the filter slot located at the left side of the furnace.
    direction of airflow on a furnace
    Example of a horizontal furnace: Filter slot on the left, airflow to the right, arrow should point to the right as well.

Based on this understanding, we logically come to that 1 simple rule for furnace airflow direction and, hence, which way the arrow should point on a furnace filter:

1 Rule For Furnace Airflow And Filter Arrow Direction

As we have seen above, the furnace airflow is always channeled from the supply vents – through the filter – into the center of the furnace.

That’s why whenever we are asking ourselves which way should the arrow point on a furnace filter, we can use this 1 simple rule:

  1. Filter arrow should always point to the center of the furnace.

That’s it.

The filter arrow must be aligned with the airflow, and the furnace airflow is always from the non-center of the furnace (up, down, left) to the center of the furnace.

Using this furnace airflow direction rule, you pretty much never have to Google this again. It’s such a simple rule to remember, and it works for:

  • All filters (Filtrete, 3M, Honeywell, etc.)
  • All furnace brands (Lennox,  Carrier, Bryant, York, Rheem, etc.)
  • All furnace types (upflow, downflow, horizontal).

We have also talked about what happens if you put the filter in backwards here (in short, make sure that that never happens).

dont install filter backwards opposing the furnace airflow direction
Air filters have only 1 right side. If you put it backwards, the furnace will “suffocate”. This can lead to all kinds of problems.

Hopefully, now the furnace airflow direction is crystal clear, and you know which way should the filter arrow point to. If you have any questions about this, you can use the comment section below, describe what’s the problem, and we’ll help you out.

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