Which Way Does a Furnace Air Filter Go? A Guide for Medina, WA, Residents

Which way does a furnace air filter go? Using the proper filter orientation involves understanding the direction of air intake for your system. It also depends on the type of filter you use and the design of the furnace.

Learn more about air filter installation, which you can do yourself or with the help of our technicians for heating services in Medina, WA.

How To Tell Which Direction the Furnace Air Filter Goes at Your Medina, WA, Home

The easiest way to determine the proper orientation of the furnace air filter is to look at its edge. Frequently, the filter manufacturer will print an arrow on the cardboard filter frame. Match this arrow to the direction that the air flows through the slot where you insert the filter.

In most furnace designs, the filter sits between the air return duct and the furnace. The air travels through the return and into the furnace, where the unit heats it. If this is the case, the arrow should point toward the furnace.

In most designs, the return duct enters the furnace toward the bottom along the floor. The slot for the filter should be near the floor, too. 

If you struggle to figure out which ducts handle air return and air supply, the supply ducts frequently extend out of the top of the furnace. Heated air rises, so it makes sense to have the supply ducts higher than the return ducts.

The exception would be if you have a downflow furnace, which is uncommon in Medina, WA. If you must install two air filters or a V-shaped filter, you might have a downflow furnace.

What If My Filter Doesn’t Have a Directional Arrow?

Which way does a furnace air filter go if it doesn’t have an arrow printed on it? Some reusable and washable foam furnace air filters won’t have arrows because it doesn’t matter which way you insert them. The foam has a uniform thickness, so the airflow direction is not a factor.

If the label with the arrow fell off your pleated fabric or fiberglass air filter, inspect the material. The side with thinner material should go toward the furnace, and the thicker material should face the air return.

Why Is the Orientation Important for the Furnace Air Filter? 

The furnace air filter removes airborne particles. As the air return duct brings air from throughout the home to the furnace for heating, it contains dust, pet hair, and other pollutants. The filter materials block and trap the contaminants while allowing air to pass through.

The filter’s orientation is important to trap pollutants. The pleats’ design works more effectively to trap dust and other items when the filter faces the right direction.

What If My Filter Is Inserted the Wrong Way?

Maintaining the energy efficiency of a furnace usually requires changing the air filter at least every three months. When you take out an old filter at your Medina, WA, home, you need to put in the new filter with the correct orientation.

Which way does a furnace air filter go, and will the furnace work if it’s the wrong way? The furnace should continue to work even if you’ve inserted the filter backward. However, some furnace models may not work without a filter or with an incorrectly inserted filter.

Even if the unit does work with a backward filter, it won’t work as efficiently. The filter could become clogged faster, inhibiting the clean air flowing into the heating chamber. You’d need to change the filter more frequently because of the clogs.

Your furnace will probably burn more fuel to operate, too. Because Medina, WA, has a cold and wet winter climate, furnace operational costs are important to many people. 

Low temperatures in Medina, WA, are regularly in the 40s and 30s from October through May. You probably run the furnace a lot. You don’t want to drive up your heating bills unnecessarily by inserting the filter incorrectly.