Furnace Filter Direction – The Right Way to Replace an Air Filter

What is the right furnace filter direction? Which way does the filter go? Which side up?  

Which Way Does the Furnace Filter Go In?

Here is the answer:

The furnace filter, aka air filter, should be placed so that the arrow points in the direction air is flowing within the air handler, furnace and/or ductwork.

Quick Way to Find Air Flow Direction in a Furnace or Air Handler

It’s one thing to say the arrow should “go with the flow,” by pointing in the direction that air is moving. For example, if air is flowing from the left to right, then the arrow points right. In an upflow furnace, the arrow should point up. In a downflow furnace – yes, the arrow should point down.

Every furnace filter frame has a printed direction arrow on the side of it.

  1. Turn the filter around until you find the arrow.
  2. Then, with the filter removed, not in the furnace, turn on the blower by choosing the Fan option of the thermostat and setting it to the On position.
  3. Within a few seconds of the blower starting, you should be able to feel which direction the air is going by putting your hand, a few fingers or even a slip of paper (hang onto it tightly) into the place the filter goes.

If air filter direction is still unclear, read on. We’ll “start from scratch,” and explain everything including which direction an air filter goes in each common type of heating and air conditioning system.

What Happens If You Put a Furnace Filter In Backwards?

Maybe nothing bad – or maybe something expensive!

The most common problem if the filter is backwards is reduced airflow, which can lead to poor efficiency and higher energy costs or equipment that works harder than it should and breaks.

Is your furnace acting funky after filter change? Specifically, does it turn off before the temperature you set the thermostat to is reached? You want it 74, but it keeps shutting down and the house is 72? This is called short cycling. The reduced airflow caused by the backwards filter isn’t letting out the heat. A sensor in the furnace gets overheated, and a switch shuts down the furnace to prevent major damage.

Locating the Filter and Filter Direction Tips

There are three basic types of furnaces, and the filter is located in different places depending on the furnace. But in all types of furnaces, the filter is always located near the intake air blower fan and positioned so that the air flows through the filter.

Upflow Furnace

An upflow furnace is installed in an upright position, taller than it is wide, and takes in air through the side or bottom of the unit, warms it in the heat exchanger, and then blows it upward into the ductwork of your home. Furnaces in the basement are upflow furnaces.

With an upflow furnace, the filter will be located near the bottom of the unit in the blower compartment next to the cold air return. Which side goes up? The arrow should be pointing up.

Here is a video on changing a filter in an upflow furnace.

Downflow Furnace

A downflow furnace takes the air in at the top, warms it, and then blows it into your ductwork. The intake air is moving downward so the filter will be located at the top, in the upper blower compartment. Most attic furnaces are downflow furnaces. If it is in the attic or upstairs of your house and taller than it is wide, it is a downflow furnace. The arrow should be pointing down.

Some downflow furnaces have a V-shaped filter system and need two filters placed in the compartment in a V-shaped angle.

Here is a video on changing a downflow furnace with two filters:

Horizontal Furnace

A horizontal furnace draws air in on one side, warms it or removes heat from it to cool it, and blows the treated air into the ductwork on the opposite side. Most horizontal furnaces will have a slide-in filter inside the unit. It will be located on the side that draws in the cold air. The filter will generally slide into a rack or slot. The arrow points in the direction the air is flowing.

Quick Tip

Reminder: The arrow on the filter always points in the direction that the air flows through the unit.

If you remember that the arrow should always point toward the heating compartment of the furnace you won’t need to worry about which way to install the filter or which side of the filter goes up or down.

How to Properly Dispose of a Used Filter

Since the old filter is full of dirt, dust and pollen, don’t just throw it in your garbage. Take a plastic garbage bag with you when you change the filter. Put the old filter in the bag and tie or tape it shut. Try not to shake or bang the old filter as this might release the particles back into your home’s air. Place the garbage bag in a trash can outside or in the garage.


Following are some frequently asked questions about furnace filters:

Why do furnaces have filters?

Furnace filters are essential in keeping the furnace clean and running at the highest efficiency. When the blower draws air in it passes over the filter removing dirt, dust, pet dander, pollen, and other particles that can eventually damage the furnace. The filter also helps to keep the air inside your home free of these pollutants.

When should a furnace filter be replaced?

An extremely dirty furnace filter will block airflow through your HVAC system making it work harder, use more energy, and eventually damage the system.

Fiberglass filters should be changed every 1 to 2 months and pleated paper filters changed every 3 to 5 months. Electrostatic filters should be washed every 1 to 2 months.

You may need to change or wash the filters sooner if you have furry pets, run the system most of the time, or open the windows often. You can always check the filter to see if it is dirty.

What happens if the filter is installed backwards?

If the furnace filter is installed backwards, the fibers won’t properly filter the air which can eventually cause damage to the unit. It also will diminish airflow through the furnace causing it to work harder and use more energy.

What if I have an air return with a filter on my ceiling or wall?

If you have an air return on the ceiling or wall that uses a filter, the arrow on the filter will point to the ceiling or wall. In these cases, the arrow always points in the direction that the air is flowing into the return duct.

Can I run my furnace without a filter?

It is possible to run a furnace without a filter but should not be done for more than a few minutes. Running a furnace without a filter will allow dirt and dust to get inside your furnace and eventually cause harm by clogging the compressor coils. It will also create poor air quality within your home.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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