The Correct Direction to Install a Furnace Filter (Check Arrow)

Which Way Does an Air Filter Go In?

A furnace filter should be replaced at least after every 90 days (ideally at the beginning of each season). While changing an air filter is easy, you need to do it the right way otherwise it can turn out to be a quite expensive blunder.


Luckily, furnace filters have on arrow on the side. The arrow should guide you on the right way to put in the filter.

The correct way to install a furnace filter is in way that the arrow on the filter points towards the direction of the furnace and away from the return air duct. If the filter is on the ceiling, the arrow should point towards the ceiling. For filters on the wall, the arrow should point towards the wall.

In a nutshell, a furnace filter should be installed in such a way that the arrow on its side is pointing towards the direction of the airflow (from the house to the furnace). For future reference, you can draw an airflow direction arrow on the return air duct or furnace so that you always get it right.

If your air filter does not have an airflow arrow, the side of the filter with a wire mesh or metal grid support should be the one facing the furnace/air handler. The metal frame/grid acts as support to prevent the filter material from curving in.

If you install the furnace filter the wrong way (backwards), it will restrict the flow of air to the furnace forcing the blower to worker harder. The effects of that is increased energy bills, poor temperature regulation and even a reduction in the lifespan of the furnace.

How to Tell Which Way Air Flows in an HVAC Unit

In an HVAC system, air flows from the house to the furnace/air handler through the return air duct and back to the house through the supply duct after heating or cooling.  That forms a continuous loop but since the indoor air is bound to become stale, outdoor (ventilation) air is also introduced into the system.

For the sake of this, post, let us do away with ventilation air for now.

When the furnace kicks in, the blower starts to pull air from the house. As you already know, the indoor air contains dust, pollen, lint, dander animal fur among other impurities.

The function of the air filter is to trap these impurities and therefore prevent them from clogging the furnace and importantly being circulated back to the house. For that reason, the air filter is usually installed somewhere on the return air duct.

The return air duct is the one responsible for carrying air from the house to the furnace. While the filter is mostly located just before where the return air ducts terminates to the furnace, it could also be located just behind the return air vent grill.

After the air has been cleaned, it is heated or cooled in the furnace/air handler and then sent back to the house via the supply ducts.

In short, the airflow direction is from the house to the return air duct to the furnace/air handler to the supply air duct and lastly back to the house.

Which Direction Does the Air Filter Arrow Point?


Depending on where you air filter is located, it is very easy to figure out which way the air filter arrow should point. Here is cheat sheet:

  • If the filter is located next to the furnace or air handler, the airflow on the filter should point toward the furnace/air handler. That could at the bottom of the unit or on the side (usually left-hand side) depending on whether you have a vertical or horizontal HVAC unit respectively.
  • If the filter is located on the ceiling, the arrow on the filter should point towards the ceiling (away from you). That is because that is the direction of air flow (from the house to the furnace via the return air duct).
  • If the air filter is on the wall, the arrow on the filter should point towards the wall (again, away from you and out in direction of the air flow?

Note: A house will have a return and a supply air vent grill. To tell them apart, you can feel a suction force if you place your palm on the return air grill (when the furnace is on) and is usually bigger in size than the supply air grill.

What if an Air Filter Has No Arrow?

The arrows on air filters are there to guide homeowners on the correct direction to install them but experienced HVAC technicians don’t need them. They can always tell the way the filter should go in even without looking at the arrows.

Although not very common, you can still find air filters without an airflow arrow. So, how do you put in an air filter that has no arrow?

If you check both sides of the filter, you will notice that one side is plain while the other one has some sort of support like a wire mesh or even a metallic grid (depending on the size of the filter and the brand as well).

If your air filter has no airflow arrow, the filter’s side with the metal mesh should be the one facing the furnace/air handler. If the filter is on the ceiling or wall, the side with the metal mesh should face into the ceiling or wall respectively (away from you).

If you think about it, that design makes sense. Since air is flowing from the house to the furnace/air handler, having the metal mesh on the backside of the filter provides support (the furnace pulls air from the house forcefully) thereby preventing the filter from collapsing/bending.

What Happens if Air Filter is Installed Backwards?

There are several things that will happen if you put the air filter backwards. The first thing is that the filter will restrict the flow of air to the furnace/air handler, forcing the blower to work extra hard.

When the blower is working harder than it should, the whole HVAC systems becomes quite ineffective. The end result is that you will have higher energy bills.

A filter that is installed backwards will also not trap particles and they might even end up flowing backward in the house resulting in low quality indoor air.

Lack of proper filtration will mean that family members who suffer from allergies and asthma will have a harder time being indoors since those conditions will worsen. Reduced air flow will also cause headaches, dizziness and some respiratory infections.

As the blower motor works harder than usual to offset the air deficit created by the wrongly installed air filter, it will overheat on many occasions leading to wear and tear. The lifespan of the furnace/air handler will therefore be greatly reduced.

Wrap Up

And basically that is how to tell which direction the arrow on the furnace filter should point. I hope that these guide was helpful