How to Change Your HVAC Filter

Go to your thermostat and switch the system to OFF (or set the fan to OFF). This prevents the blower from running while the filter is out, which would pull unfiltered dust and debris straight into the system and ductwork. It also keeps loose dust from blowing into your face when you remove the old filter. You'll turn the system back on after the new filter is installed.

HVAC filters are in one of three places: behind a large return-air grille on a wall or ceiling (the biggest vent in the house — usually in a hallway or near the thermostat), in a slot on the side of the air handler or furnace unit (typically in a utility closet, basement, or garage), or in an external filter rack mounted on the return duct. If you have a return-air grille, unlatch the clips or remove the screws to swing the grille open. If the filter is at the air handler, look for a metal or plastic door with a handle that pulls off or hinges open.

Slide the old filter straight out of its slot. Handle it carefully — a heavily clogged filter will shed dust if you shake or tip it. Immediately slide it into a trash bag to keep the dust contained. Look at the frame of the old filter for the printed size — it will be three numbers like 16x25x1 or 20x20x4 (width x height x depth in inches). Write this size down or snap a photo. You'll need the exact size to buy the replacement; even 1/2 inch off means the filter won't seat properly and air will bypass it.

Match the exact size from the old filter. For filter quality, look at the MERV rating (Minimum Efficiency Reporting Value) — MERV 8 is the minimum recommended for residential use, MERV 11-13 is ideal for homes with pets or allergies, and MERV 13 is the highest rating most residential systems can handle without airflow issues. Don't go above your system manufacturer's recommended MERV rating — a filter that's too restrictive forces the blower to work harder, increases energy costs, and can damage the compressor. Pleated filters ($8-20) outperform flat fiberglass filters ($2-5) and last longer.

Every disposable HVAC filter has an arrow printed on the frame that indicates the direction of airflow. The arrow must point toward the air handler or furnace — that is, toward the duct that leads away from the room and into the equipment. For wall or ceiling return vents, the arrow points into the wall (away from you). For filters at the air handler, the arrow points toward the blower. Slide the new filter into the slot, making sure it sits flat and flush with no gaps around the edges. A filter installed backwards still works, but it clogs faster and reduces efficiency.

Swing the return grille closed and re-latch the clips or replace the screws. If the filter is at the air handler, slide the access door back into place. Go to the thermostat and switch the system back to your normal setting (AUTO or the previous heating/cooling mode). Listen for the system to start up normally — the blower should run smoothly with no unusual whistling or rattling. A whistling sound at the vent can mean the filter is too restrictive or not seated properly.