How to change a furnace filter

Check the filter every month, especially in summer and winter when the system runs most, and change it at least every 3 months. A clogged filter slows the airflow, and airflow problems can cut a system's efficiency by up to 15 percent. The job takes about 15 minutes.

What you need

• A new filter the same size as the old one. The size is printed on the old filter's cardboard frame, as three numbers in inches.
• A flashlight, for a filter slot in a dark basement or closet.
• A marker, to write the date on the new filter's frame.

Picking the rating

A MERV rating, short for minimum efficiency reporting value, tells you how well a filter captures particles between 0.3 and 10 microns. The higher the number, the more of those particles it traps. Use a filter rated at least MERV 13, or as high as your system's fan and filter slot can take.

Not every system can take the highest-rated filter, and a heating and cooling technician can tell you which works in yours. Ask at the next check-up, and write the answer down with the size.

Steps

1. 1

   Turn the system off at the thermostat

   The blower shouldn't run while the filter slot is open, or it pulls dust straight past the empty slot.

   1 min

2. 2

   Find the filter slot

   It is usually in the side of the furnace or air handler, where the return duct enters. Some houses have it behind a return grille on a wall or ceiling. The owner's manual shows where.

   3 min

3. 3

   Pull out the old filter

   Note which way its airflow arrow points before you take it out.

   2 min

4. 4

   Slide in the new filter

   Point its arrow the same way the old one pointed, toward the blower and away from the return duct. Push it fully into the slot so air can't go around it. Write today's date on the frame.

   2 min

5. 5

   Close the slot and turn the system back on

   Replace any slot cover or grille. Set the thermostat back to its usual setting.

   2 min

6. 6

   Check the airflow at a vent

   Hold your hand over a supply vent once the blower starts. Air should flow as strongly as before. Put the old filter in a bag before it goes in the trash, so the dust stays in.

   2 min

If something goes wrong

If the new filter won't slide in, check the size printed on its frame against the old one's. Buy by the printed size, not by a tape measure.

If the airflow at the vents feels weak with a fresh filter, the problem is somewhere else. Cleaning and adjusting the blower is part of a contractor's check-up, so book one.