AC filters go in with the arrow on the frame pointing toward the blower and away from the return duct, in the direction of airflow.
Rule
The arrow on the filter frame points in the direction of airflow: toward the blower (furnace or air handler) and away from the return duct.
Where
Filters sit in the return vent or at the air handler.
Replacement
Every 1 to 3 months.
Size
Check the size printed on the frame.
MERV rating
MERV 8 to 11 suits homes.
Tip
Write the date on the filter.
Mistake
A backward filter reduces efficiency.
Airflow direction
Air filters have an arrow printed on the frame that shows the direction of airflow. The arrow should point toward the furnace or air handler, and away from the return duct, so air flows through the filter in the direction of the arrow. Installing it backward reduces efficiency and may let the filter collapse.
- In a wall or ceiling return grille, the arrow points into the wall or ceiling.
- At the furnace, the filter slot is often on the side of the blower unit, and the arrow points toward the blower.
- Replace 1-inch filters every 1 to 3 months, thicker 4 to 5 inch filters every 6 to 12 months.
Sizing and rating
The nominal size is printed on the filter frame, such as 16x25x1. MERV ratings run from 1 to 16, and MERV 8 to 13 suit most homes. The EPA says MERV 13 or higher captures a high share of particles but check the HVAC manual, since high-restriction filters can reduce airflow. Never run the system without a filter.
Airflow direction
AC filters have an arrow printed on the frame that should point in the direction of airflow, toward the blower motor and away from the return duct. If there is no arrow, the more open, mesh side faces the incoming air. Install with the arrow pointing toward the furnace or air handler.
Sizing and rating
Use the size printed on the filter edge. MERV ratings run 1 to 16, and MERV 8 to 13 is common for homes. Higher ratings may restrict airflow in some systems. Replace every 1 to 3 months.
Installation steps
Switch off the system. Slide out the old filter, note the size printed on the frame (such as 16x25x1), and the arrow direction. Insert the new filter with the arrow pointing toward the blower or air handler and away from the return duct. For filters in the ceiling grille, the arrow points up into the ceiling.
Performance factors
MERV 8 filters catch particles over 3 microns, while MERV 13 catch down to 0.3 microns, including bacteria and smoke. Higher MERV can reduce airflow in systems not designed for them. A clogged filter raises energy use up to 15 percent, per the Department of Energy. Replace 1 inch filters every 1 to 3 months and thick media filters every 6 to 12 months.
Based on data from the US Department of Energy and ENERGY STAR
