Air filter guide

Which Way Does an Air Filter Go?

Point the arrow toward the furnace, away from the room. Here is how to get it right in any vent, closet or furnace.

The quick answer

The arrow on the filter's frame shows which way the air flows. Point the arrow toward the furnace or air handler, and away from the room and the vent the air comes from.

How air moves through your filter

Your heating and AC pull air out of your rooms through a return vent, push it through the filter, then send it back out clean. The filter sits between the vent and the furnace, so its arrow points into the system.

Diagram: air flows from your rooms through the return vent and filter, through the duct to the furnace or air handler, and back to your rooms. The filter arrow points into the duct. — Rooms → return vent and filter (arrow points into the duct) → furnace → clean air back to your rooms

Which way the arrow points, by where your filter is

Arrow points up
Ceiling vent

Air is pulled up out of the room. Point the arrow up, into the ceiling, toward the duct.
Arrow points into the wall
Wall vent

Air is pulled from the room into the wall. Point the arrow into the wall, away from you.
Arrow points at the furnace
Furnace or air handler

Point the arrow toward the blower. In most basement furnaces that is up. In a sideways attic unit, it is toward the unit.

Not sure where your filter is? Check the large vent grilles in hallways and ceilings first, then the slot beside the furnace in your basement, closet or attic.

No arrow on the filter?

Look for the side with the wire mesh or cardboard grid behind the pleats. That side faces the furnace, because it holds the pleats up against the moving air.
Check your old filter before you throw it out. Its arrow, or the dirtier side, shows the direction: the dirty side faces the room.
Still unsure? Look at the furnace door or the filter slot. Many have an airflow arrow printed on them.

What if it is in backwards?

It will not break anything right away, but it does not work as well. A backwards filter catches less dust, and the pleats can bend toward the blower because the support is on the wrong side. That can cut airflow and make your system work harder. If you find one in backwards, just flip it.

How to put in a new filter, in 5 steps

Turn the system off at the thermostat.
Slide out the old filter and note which way its arrow pointed.
Check the size printed on its frame, like 20x25x1. How to find your size
Slide in the new filter with the arrow pointing toward the furnace, then close the cover.
Turn the system back on and write today's date on the frame, so you know when to change it next.

Common questions

Does it matter which way an air filter goes in?

Yes. Filters are built for air to pass through them in one direction. Facing the right way, the filter catches more dust and keeps its shape. Backwards, it filters less and can bend toward the blower.

Which way does the arrow go on an air filter?

The arrow shows the direction the air moves. Point it toward the furnace or air handler, and away from the room or return vent the air comes from.

Which way does an air filter go in a ceiling vent?

Point the arrow up, into the ceiling. Air is pulled up from the room, through the filter, and into the duct.

Which way does an air filter go in a wall vent?

Point the arrow into the wall, away from the room. Air is pulled from the room into the vent.

Which way does an air filter go in a furnace?

Point the arrow toward the furnace's blower. In most basement furnaces that means up. In a horizontal attic unit, point it along the duct toward the unit.

What if my air filter has no arrow?

Look for the side with the wire mesh or cardboard grid behind the pleats. That side faces the furnace, because it holds the pleats up against the moving air.

Which side of the air filter faces the room?

The side the arrow points away from. The front of the pleats faces the room or the return vent, and the arrow points into the system.

How often should I change my air filter?

For a 1-inch filter, about every 2 to 3 months in most homes, or every 1 to 2 months with pets, allergies or heavy heating and AC use. Thicker 4-inch filters can last 6 months or more.

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