This post overlaps with a fuller one: How Often to Change an HVAC Filter: One to Three Months.
Change a standard 1-inch pleated filter roughly every one to three months, sooner if it looks gray, and put it in with the arrow toward the furnace. That is the short version of our full air filter guide.
How often
- 1-inch pleated filters: typically every one to three months. Check monthly from May through October, when the system runs most.
- 4-inch and 5-inch media filters at the furnace: typically every six to twelve months.
- Washable filters: rinse monthly and let them dry fully before reinstalling.
Go sooner with pets, allergies, or after a smoke day. Around Pasadena, CA an October filter can be done in five weeks.
The signs it is done
- Gray and matted, and you cannot see light through it.
- Weak airflow at the registers, or a return grille that whistles.
- Longer run times, and rooms that never reach the setpoint.
- Dust building on the return grille and furniture faster than usual.
- The AC icing up, or the furnace shutting off and restarting on its own. Both are airflow problems until proven otherwise.
Choosing it and changing it
The size is printed on the frame. On MERV, the EPA says at least a MERV 13, "or as high a rating as your system fan and filter slot can accommodate." That second half is the part people skip: a MERV 13 in a 1-inch slot on an older blower can restrict airflow enough to ice the coil. On most older systems here, MERV 8 to 11 is the safe range in that slot; the way to run MERV 13 is a 4-inch media cabinet.
Turn the system off at the thermostat, note the arrow on the old filter, put the new one in with that arrow toward the furnace, seat it flat with no gaps, and date it.
If a fresh filter does not fix it, the filter was not the cause. A dirty coil, leaking attic ducts or a failing blower look the same from the living room, and need a maintenance visit or coil cleaning.
The full guide: How Often to Change an HVAC Filter: One to Three Months.
Our diagnostic is $69, quoted before we book and credited toward the work. Call Boost Services Heating and Cooling at 818-277-5056, 6am to 10pm, seven days a week, or book online.
Written with AI assistance by Boost Services, Heating and Cooling team, and reviewed by our licensed technicians before publishing. Boost Services Heating and Cooling, C-20 #1119892.