Which Way Does a Furnace Filter Go? Arrow Direction and Changing Tips

Quick answer: The arrow on the side of the filter frame points the way the air flows: toward the furnace (or air handler) and away from the return duct that brings air from your house. If the filter sits in a ceiling grille, the arrow usually points up; in a wall grille, into the wall. Many standard 1-inch filters need changing every 1 to 3 months, and more often with pets or renovations. A clogged filter is one of the most common reasons a furnace overheats and shuts off, so check it before calling for furnace repair.

It's one of the simplest jobs in home maintenance, and one of the most often done wrong. Here's how to find your filter, put it in the right way, and choose the right one.

Step 1: Find your filter

Filters usually live in one of these places:

Some homes have more than one return grille with filters. If you find a filter in a grille and another at the furnace, you usually only need one. Too much filtration can starve the system of air. Ask a technician if you aren't sure.

Before you open anything, turn the system off at the thermostat.

Step 2: Read the arrow

Every filter has an arrow printed on the cardboard frame, usually labelled "airflow". Follow the air:

1. Air leaves your rooms through the return grille
2. It travels through the return duct
3. It passes through the filter
4. It enters the blower inside the furnace or air handler
5. It's heated or cooled and sent back through the supply vents

So the arrow should point toward the blower and furnace.

If you still aren't sure, note which way the old filter's arrow points, or which side is dirtier. The dirty side faces your rooms, and the arrow points away from it.

Step 3: Get the size right

The size is printed on the filter frame, for example 16 x 25 x 1. That's width x height x thickness in inches. These are "nominal" sizes, rounded up from the actual size, so always buy by the printed nominal size.

• 1-inch filters are the most common and the cheapest, and need changing most often.
• 4- to 5-inch media filters fit special cabinets and usually last much longer.
• A filter that's too small lets dirty air bypass it. Don't force in the wrong size.

Write the size and the date on the furnace in marker, or keep a note on your phone.

Step 4: Choose the right MERV rating

MERV ratings run from 1 to 16 for home filters. Higher numbers capture smaller particles, but they can also restrict airflow.

A filter that's too restrictive for your system can cause the same problems as a dirty one. For more detail, see the air filter MERV guide. If you want cleaner air without hurting airflow, a thicker media cabinet or other indoor air quality options may be a better fit.

How often to change it

Check it monthly during heating and cooling season. If you hold it up to a light and can't see light through it, change it.

What happens if it's backwards or dirty

• Backwards: many filters are built to load dust from one side. Installed backwards, they may trap less dust or restrict airflow more, and some pleated filters can bend or pull loose.
• Dirty or clogged: airflow drops, and the furnace runs hotter. The limit switch may then shut the burners off, causing short cycling (see furnace keeps shutting off).
• In summer: low airflow can freeze the AC coil (see why your AC is freezing up).
• Over time: extra stress on the blower motor and heat exchanger (see cracked heat exchanger signs).
• Missing filter: dust coats the blower and coil, which costs efficiency and can lead to repairs.

Step-by-step: changing it

1. Turn the thermostat to Off.
2. Open the filter door or grille.
3. Slide out the old filter and note the arrow direction and size.
4. Slide in the new filter with the arrow pointing toward the furnace.
5. Make sure it sits flat with no gaps around the edges.
6. Close the door or grille securely. Some furnaces won't run if the door isn't closed.
7. Turn the system back on.
8. Write the date somewhere you'll see it.

Common mistakes

• Buying by the actual measured size instead of the printed nominal size
• Using the highest MERV available without checking the system
• Forgetting a second filter in a return grille
• Leaving the old filter's plastic wrapper on the new one (it happens)
• Leaving the blower door loose after changing an inside filter
• Changing it once a year

When to call us

A fresh filter fixes many problems. If your furnace still shuts off, airflow is still weak, or the AC still freezes, the cause is elsewhere: a blower motor, dirty coil, duct problem or failing part. See furnace blower not working and how to reset your furnace safely.

Every tune-up includes a filter check and an airflow check. Our $89 heating or AC tune-up is available (terms apply), and the Prime Comfort Club covers one heating and one AC tune-up a year for $19/month, with priority scheduling and 10% off repairs (terms apply).

A reminder system that works

Set a phone reminder for the first day of every other month, and keep spare filters next to the furnace so changing one takes two minutes. Members of the Prime Comfort Club get a filter check at every tune-up.

Tune-ups and repairs near you

We service every brand of furnace, heat pump and air handler across all 153 towns in Ocean, Monmouth, Burlington and Middlesex counties, including Brick, Lakewood, Piscataway and Moorestown.

Book a tune-up online or call (732) 779-7106.