Furnace Filter Direction: Which Way Does Air Filter Go Into Furnace?

“How to install an air filter? Which way should the arrow point?”

Furnace filter direction. We all know that the air filter has to be installed in the right direction. That’s why that arrow on the furnace air filter is added (saying ‘Point the arrow in the direction of the airflow’).

Despite that, many homeowners still struggle with which way an air filter goes into a furnace. Which side of the air filter faces out, and which way to put the air filter in the furnace are one of the most commonly questions about HVAC filters.

To help everybody out, we are going to explain all furnace filter directions in a structured to-the-point way (furnace itself and 3 furnace types, supply vents, return vents). First off, let’s stress that there are is 1 good thing about furnace filter directions and 1 very bad thing:

  • Good thing: All furnace filters have the same correct direction. It doesn’t matter if this is a Filtrete filter or 3M filter. It doesn’t matter if this is a Lennox, Carrier, Rheem, or any other furnace. The direction of the filter is always the same (you follow the “arrow system” every time).
  • Very bad thing: What happens if the air filter is backwards? All hell may break loose. If you put a furnace filter in backwards, the airflow will be reduced. This can lead to lower efficiency and capacity (least bad case) to furnace damage and eventual complete breakdown of a furnace that costs $1,000s (worst case scenario).

So, it’s safe to say that correctly putting an air filter into a furnace is very easy. If you mess it up, however, there are dire consequences.

Now, let’s have a look at which way does an air filter go into a furnace. It’s all about that arrow on the filter:

Which Way Should The Furnace Filter Arrow Point To?

In theory, figuring out the direction of a furnace air filter is simple: Just point the arrow on the air filter in the direction of airflow. However, that means that we have to know the direction of the airflow, and that’s not the same for all furnace filters.

Namely, we have 3 different furnace filters (and associated correct directions):

  1. Filter that goes into the furnace itself. These are MERV 6, 8, or even MERV 11 filters. We will look at which point the arrow should point in case of an upflow furnace, downflow furnace, and horizontal furnace.
    downflow furnace filter arrow direction
    Not all furnaces are the same. This is a downflow furnace with an air filter slot located at the upper part of the furnace. The airflow is downwards and therefore the filter arrow should also point downwards.
  2. Supply vent filters. These are usually located low on the wall and they intake the room air and deliver it into the furnace.
  3. Return vent filters. These are located high on the wall, ceiling, or even on the floor. They deliver heated air from the furnace back into our home.

Let’s break down the correct directions for each of these filter locations one-by-one, starting with the filters that go directly into the furnace:

Correct Furnace Filter Directions For Upflow, Downflow, And Horizontal Furnace

There are 3 main furnace types as far as airflow direction goes. As we know, the airflow direction is the key to adequately installing the furnace filter.

Here is how to install an air filter into each of these furnace types:

  • Upflow furnace. These are furnaces usually installed in the basements, appropriate for cold climates. The airflow is blown upwards and the air filter slot is located at the bottom part of the furnace. That means the arrow on the filter should point upwards in the upflow furnace. You insert the filter horizontally.
  • Downflow furnace. These are furnaces usually installed high up in the house (attic, for example), appropriate for warm climates. The airflow is blown downwards and the air filter slot is located at the upper part of the furnace. That means that the arrow on the filter should also point downwards in the downflow furnace. Of course, the filter is inserted horizontally.
  • Horizontal furnace. Horizontal furnace will not have up-down airflow; instead, it will have left-right airflow. The filter on a horizontal furnace is located on the left side (supply vent inflow), and the air flows to the right side of the furnace. That’s why the arrow on the filter should point to the right, since that is the direction of airflow on a horizontal furnace.
which way does an air filter go into a furnace
Horizontal furnace has an air filter slot on the left. The airflow direction is from left to right. That’s why the filter arrow should point to the right as well.

Note: It is important to note that you can rotate the filter whichever way you want. You can turn it by 90 degrees, 180 degrees, or even 270 degrees; it doesn’t matter. All that matters is that the arrow on the filter points in the direction of airflow.

Let’s look at what direction you should install the supply and return vent HVAC filters:

Direction Of Supply And Return Vent Furnace Filters

This one is easier. We just need to be aware that:

  • Supply vents suck in the air. That means that the airflow is channeled from the room into the wall. Accordingly, the arrow on the supply vent filter should point from your room into the wall (in the direction of airflow).
  • Return vents output the air. That means the airflow is channeled from the wall, ceiling, or floor (depending on where you have the return vents) into the room. Hence, the arrow on the return vent filter should point toward the room (toward you when you’re installing it).

As we can see, the only thing you need to know to choose the correct filter direction is the airflow direction. Once you figure that out (we have covered all the cases here), you simply point the arrow on the filter in that direction (you can rotate the filter all you want).

If you are still unsure which way the air filter goes into your furnace, you can use the comment section below. Tell us about your furnace (upflow, downflow, horizontal, etc.), and we can help you out.

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