Furnace and AC air filter: Guide

What it is

A clogged filter restricts airflow. Manuals from Goodman, Carrier, Lennox and Trane list blocked filters or restricted airflow among the causes of limit codes, and Carrier lists low airflow among the causes of its AC low-pressure faults.

The U.S. DOE says to clean or replace furnace filters once a month or as recommended, and ENERGY STAR says to inspect, clean or change them monthly.

How to choose

Size first: the nominal size is printed on the frame of the old filter (for example 16x25x1). Listings also give the actual size, which is slightly smaller. See the filter size guide.
Check the arrow on the old filter and fit the new one with the arrow pointing toward the furnace or air handler.
A higher MERV rating captures more small particles (EPA), but follow your manual for allowed filter types.

How it is tested

ENERGY STAR says to inspect the filter monthly; replace it when it is dirty.

How to use it

Switch the thermostat or furnace off, slide out the old filter, note the size and arrow, insert the new one with the arrow toward the unit and switch back on.

Safety firstTurn the furnace or air handler off before opening the filter slot. If you smell gas, leave the building and call your gas utility from outside.

Capacitors can hold a charge after the power is off. Safety terms.

Where to buy

These are Amazon links. Product page links go to one specific listing whose title we read on Amazon; we do not say it fits your model. Search links open search results, which change. Always confirm your model number and the part number in your manual.

Read the size printed on the old filter (for example 16×25×1). The searches below are by size. They are not a fit for your model.