The arrow printed on a furnace filter frame points toward the blower, the fan inside the furnace. Energy Trust of Oregon says the arrows should point in the same direction as the airflow. Filtrete's installation guide says the arrow on the frame should point toward the unit's blower motor.
The arrow is printed on the filter, so you need to find the filter first. Energy Trust of Oregon says the filter is usually on the furnace itself, typically in the blower compartment near the bottom. It may instead sit in a door near the top or on the side. Consumer Reports says the existing filter is located inside the furnace or inside the return air vent.
The steps come from Energy Trust of Oregon, Consumer Reports and Filtrete. Energy Trust of Oregon and Consumer Reports give no steps for a furnace with no filter slot. The furnace manual or a heating, ventilation and air conditioning (HVAC) professional is the place to ask.
Point the Arrow Toward the Blower
The arrow points toward the blower. Energy Trust of Oregon says the arrows printed on the side of the filter should point in the same direction as the airflow, which is toward the furnace. Filtrete's guide gives the same direction for a furnace, an air conditioning system, or a wall or ceiling return.
Check the direction in three steps:
- Find the blower side. Note which side of the filter slot faces the blower before you pull the old filter, then draw the airflow direction on the outside of the furnace with a permanent marker, as Consumer Reports says.
- Match the arrow. Energy Trust of Oregon says to make sure the arrows point in the same direction as the airflow. Slide the new filter in with its arrow pointing toward the furnace.
- Confirm on a used filter. Run a finger across both sides of the filter already in place. Energy Trust of Oregon says the side with more dirt should be the side the air is coming from, so a dirtier side facing the blower means the filter is backwards.
Find the Airflow Direction from Where the Filter Sits
Wherever the filter sits, the arrow still points toward the blower. Energy Trust of Oregon says the filter is usually in the blower compartment near the bottom of the furnace. It says the filter may instead sit in a door near the top or on the side, but the source gives no separate orientation steps for those positions.
If you cannot find the filter, Energy Trust of Oregon says it may be behind a nearby return air grille, which a screwdriver can usually remove. Consumer Reports says to draw the airflow direction on the outside of the furnace with a permanent marker.
Turn off the heat at the thermostat before swapping the filter, as Energy Trust of Oregon says. Consumer Reports says to turn off the furnace before removing the old filter.
Where the Filter Sits and How to Orient It
The table lists each filter location the sources name and the orientation step each one supports.
| Filter location | What the source says | Orientation step |
|---|---|---|
| Blower compartment, usually near the bottom of the furnace | Energy Trust of Oregon: the filter is usually located on the furnace itself, typically in the blower compartment, near the bottom. | Arrow pointing toward the blower (Energy Trust of Oregon, Filtrete) |
| Door near the top or on the side of the furnace | Energy Trust of Oregon: the filter may instead sit in a door near the top or on the side. | Same arrow rule; the source gives no separate steps for this position |
| Behind a nearby return air grille | Energy Trust of Oregon: the filter may be behind a nearby return air grille, which a screwdriver can usually remove. | Remove the grille, then match the arrow to the blower side; no further steps are given |
| Inside the return air vent | Consumer Reports: the existing filter will be located inside the furnace or inside the return air vent. | Look for the markings that show which side of the filter should face the furnace |
| Wall or ceiling return | Filtrete: insert the filter into the designated opening in the furnace, air conditioning system, or wall or ceiling return. | Arrow on the frame pointing toward the unit's blower motor |
| Any location, filter has no arrow | Consumer Reports does not say what to do if a filter has no arrow. | Use the used-filter dirt-side check from Energy Trust of Oregon, or the furnace manual |
What to Do When the Filter Has No Arrow
Consumer Reports does not say what to do if a filter has no arrow. Its article says only to look for an arrow or other markings that show airflow direction.
Two routes are available. The first is the used-filter check from Energy Trust of Oregon: run a finger across both sides and treat the dirtier side as the side the air comes from. The second is the furnace manual.
The dirt check applies to a filter that has already been in use. The sources do not say how to orient a new filter that has no arrow. For a new filter with no arrow, the furnace manual is the remaining route.
How to Tell a Filter Was Installed Backwards
Energy Trust of Oregon gives one direct test. Turn off the heat at the thermostat, pull the filter, and run a finger across both sides. The side with more dirt should be the side the air is coming from. If the dirtier side faces the blower, the filter is installed backwards.
Consumer Reports says installing the filter backwards could block airflow instead of cleaning the air. The sources do not say how long a reversed filter takes to cause harm, and they do not describe damage to the furnace.
If the check shows a reversal, turn off the heat at the thermostat, reinsert the filter with the arrow toward the blower, and mark the direction on the furnace.
Mark the Cabinet and Set the Next Change Date
Consumer Reports says to draw the airflow direction on the outside of the furnace with a permanent marker. It also says to record the date, so you know when the filter needs changing next.
Energy Trust of Oregon says the replacement schedule is usually printed on the filter. It adds a visual check: if light cannot pass through the filter, replace it.
Energy Trust of Oregon says most furnace filters need to be changed every one to three months. Thicker filters, 3 to 6 inches, can last up to six months. Consumer Reports says filters need replacing every few months to maintain efficiency. Energy Trust of Oregon also says to change filters more often during wildfire season or when pets shed heavily.
When Orientation Is Not the Problem
Size is a separate check. Consumer Reports says the size is printed on the cardboard frame. The HVAC filter sizes page on this site covers reading a filter size, the difference between nominal and actual dimensions, and measuring the slot.
Thickness is also separate. Consumer Reports says disposable filters are typically 1 or 2 inches thick, and thicker filters may require an HVAC professional to modify the furnace.
Washable filters follow their own rules. Energy Trust of Oregon says some filters are washable and should be cleaned according to the manufacturer's guidelines. Consumer Reports says reusable filters with plastic frames should be cleaned periodically with a vacuum and water, and must be fully dry before they go back in.
The furnace filter page covers the change interval.
Who the Rule Covers and What to Do Next
The rule applies to a furnace or air conditioning system with a blower. Energy Trust of Oregon, Consumer Reports and Filtrete all describe that rule, in arrows or in markings.
Consumer Reports gives no steps for a filter with no arrow. Energy Trust of Oregon's used-filter dirt-side check and the furnace manual are the routes left. Energy Trust of Oregon and Consumer Reports give no steps for a furnace with no filter slot, so the furnace manual or an HVAC professional is the place to ask.
Before the next filter change, turn off the heat at the thermostat, match the arrow to the blower side, and draw the direction on the furnace with a permanent marker.
Common questions
In which direction should the arrow on the air filter point?
Toward the blower. Energy Trust of Oregon says the arrows should point in the same direction as the airflow, toward the furnace. Filtrete's installation guide says the arrow on the frame should point toward the unit's blower motor.
How to tell if a furnace filter is backwards?
Turn off the heat at the thermostat, pull the filter, and run a finger across both sides. Energy Trust of Oregon says the side with more dirt should be the side the air is coming from, so a dirtier side facing the blower means the filter is backwards. Consumer Reports says a backwards filter could block airflow instead of cleaning the air. The sources do not say how long that takes to cause harm.
How do I install the furnace filter correctly?
Consumer Reports says to turn off the furnace before removing the old filter, slide the new filter in, replace any cover, and record the date so you know when to change it next. Energy Trust of Oregon says the arrows should point in the same direction as the airflow, toward the furnace, and Filtrete's installation guide says the arrow points toward the unit's blower motor. Consumer Reports says the size is printed on the cardboard frame.
My furnace doesn't have a filter slot, what should I do?
Neither Energy Trust of Oregon nor Consumer Reports gives steps for a furnace without a filter slot. The furnace manual or an HVAC professional is the place to ask.
How often should furnace filters be changed?
Energy Trust of Oregon says most furnace filters need to be changed every one to three months, and thicker filters of 3 to 6 inches can last up to six months. The replacement schedule is usually printed on the filter. Consumer Reports says filters need replacing every few months to maintain efficiency.
Which way should the air flow go when installing a Filtrete air filter?
Toward the blower. Filtrete's installation guide says to insert the filter with the arrow on the frame pointing toward the unit's blower motor. That holds whether the opening is in a furnace, an air conditioning system, or a wall or ceiling return.