Check a standard 1-inch furnace filter every month and replace it at least every 3 months. With pets, allergies or heavy winter and summer use, plan on every 1 to 2 months. Thicker 4-inch media filters usually last 6 to 12 months, but if any filter looks gray and caked, change it now.
How often to change each type of filter
The schedule depends mostly on how thick the filter is. Thicker filters have more pleated material, so they hold more dust before they clog.
| Filter type | Typical replacement | What the guidance says |
|---|---|---|
| 1-inch pleated | Every 1 to 3 months | ENERGY STAR says to check it monthly, especially in winter and summer, and change it at least every 3 months. |
| 4-inch or 5-inch media filter (in a cabinet next to the furnace) | Every 6 to 12 months | AprilAire says its MERV 10 to 13 media filters typically last about a year. Check them every 6 months if your fan runs all the time. |
Write the install date on the filter frame with a marker. It takes the guesswork out of the next check.
What makes a filter clog faster
Three months is the longest you should go, not the target. The Pacific Northwest National Laboratory (PNNL) building guide says to replace filters sooner in homes with:
- Pets
- Four or more people
- Indoor smoking
- Allergy or asthma sufferers
- Nearby construction or a remodel in progress
- A system that runs a lot, including a fan left on "On" instead of "Auto"
In the Chicago suburbs, check more often during the coldest and hottest weeks of the year, when the system runs almost nonstop.
MERV 8 vs. MERV 11 vs. MERV 13
MERV (Minimum Efficiency Reporting Value) rates how well a filter catches particles from 0.3 to 10 microns. Higher numbers catch more of the small stuff. Based on the capture rates published by EPA and PNNL:
- MERV 8: Catches over 70% of larger particles (3 to 10 microns), like dust and pollen. A solid basic filter.
- MERV 11: Catches over 85% of large particles, over 65% of mid-size particles (1 to 3 microns) and over 20% of the finest (0.3 to 1 micron).
- MERV 13: Catches at least 90%, 85% and 50% across those same three size ranges.
The catch is airflow. EPA recommends at least MERV 13 "or as high a rating as your system fan and filter slot can accommodate," and suggests asking an HVAC technician what your system can handle. A dense filter in a 1-inch slot can restrict airflow on some systems.
Thickness helps. A 4-inch media filter has much more surface area, so it usually resists airflow less than a 1-inch filter with the same rating. In one PNNL example, a 4-inch filter had about 40% less pressure drop than a 1-inch filter with the same MERV. PNNL also notes that a higher MERV does not automatically mean more resistance.
Signs your filter is clogged
- It's gray or caked with dust, and you can't see light through it.
- Airflow from the registers feels weaker, or some rooms won't warm up or cool down.
- The furnace or AC runs longer than usual to reach the set temperature.
- More dust settles on furniture between cleanings.
- The filter is bowed, sucked into the slot, or whistling or banging.
- It's damp or moldy.
- In summer, ice forms on the indoor coil or the copper refrigerant line. Restricted airflow is one common cause.
PNNL says to replace a filter right away if it has collapsed, makes a banging noise, or is damp or moldy. A wet filter also points to a moisture or drain problem worth having checked.
How to change it safely
- Set the thermostat to Off, or turn off the furnace's service switch, so the blower isn't pulling air while the slot is open.
- Find the filter. It's usually in a slot between the return duct and the furnace, in a cabinet beside the furnace, or behind a return-air grille in a wall or ceiling.
- Read the size printed on the old filter's frame, such as 16x25x1 or 20x25x4, and buy that exact size.
- Slide the new filter in with the airflow arrow pointing toward the furnace, away from the return duct.
- Close the filter door fully and turn the system back on.
Don't run the system without a filter. Unfiltered air carries dust straight to the blower and the AC coil. The filter slot is the only part of the furnace a homeowner should open. Leave the burner compartment and other panels to a technician.
When to call a pro
Call a technician if a new filter clogs within a few weeks, if you're not sure your system can handle a MERV 13 filter, or if you'd like to switch from a 1-inch slot to a 4-inch media cabinet. Yogi Heating & Cooling in Schaumburg can look at your setup and recommend a filter that fits it. Call (814) 431-1709 or book online. You can also ask about filter options during a tune-up visit.
More questions
Can I run my furnace without a filter for a day?
It's best not to. Unfiltered air sends dust straight to the blower and AC coil. If you're out of filters, leave the old one in until you can get a new one.
Is a higher MERV filter bad for my furnace?
Not always. EPA recommends MERV 13 when your fan and filter slot can handle it. Some systems with 1-inch slots lose airflow with dense filters, so ask a technician.
Do I need to change the filter in summer too?
Yes. A furnace and central AC share the same blower and filter, so the filter works all year.
What size filter do I need?
The size is printed on the frame of your current filter, such as 16x25x1. Match all three numbers, including thickness.
Related
- Indoor air quality
- Maintenance plan
- What humidity should my house be in winter?
- Is a furnace or AC tune-up worth it?