How often should you change your HVAC filter? (MERV 8 vs. 13)
Short answer: ENERGY STAR says to check your HVAC air filter monthly and change it as needed, at least every three months. Change it sooner if it looks heavily soiled. For filter rating, the EPA suggests at least MERV 13, or the highest MERV your furnace or air handler can accommodate. A technician can confirm what your system handles.
How often should you change your HVAC filter?
Check it every month. Change it when it looks dirty, and at least every three months. That is the wording in the ENERGY STAR Homeowner Manual.
ENERGY STAR’s maintenance checklist says to inspect, clean, or change the filter once a month. It warns that a dirty filter can raise energy costs and damage equipment.
The EPA’s Guide to Air Cleaners in the Home says manufacturers typically recommend replacement every 60 to 90 days. Follow the instructions printed on your filter or in your furnace manual first.
When should you change it more often?
The EPA says to consider changing filters more often if they look heavily soiled when you pull them out.
A practical check: hold the filter up to a light. If you cannot see light through the media, or the surface is gray and caked, replace it.
Treat these as rules of thumb, not fixed schedules. Filters tend to load faster when:
- You have pets that shed.
- Someone in the house has allergies or asthma and you want cleaner air.
- You are remodeling, sanding drywall, or doing other dusty work.
- Wildfire smoke or other outdoor smoke is getting indoors.
- The system runs hard, as in peak summer or winter.
During heavy smoke, the EPA specifically says to replace filters more often than the manufacturer recommends.
What does MERV mean?
MERV stands for Minimum Efficiency Reporting Value. It comes from a test method developed by ASHRAE. The EPA explains that MERV reports how well a filter captures particles between 0.3 and 10 microns. The scale runs from 1 to 16. Higher numbers capture a larger share of small particles.
MERV 8 vs 11 vs 13: what is the difference?
These are the minimum capture rates from the EPA’s MERV table.
| MERV | 0.3 to 1.0 µm | 1.0 to 3.0 µm | 3.0 to 10.0 µm |
|---|---|---|---|
| 8 | Not rated | 20% or more | 70% or more |
| 11 | 20% or more | 65% or more | 85% or more |
| 13 | 50% or more | 85% or more | 90% or more |
The biggest jump is in the smallest band. MERV 8 has no requirement for particles from 0.3 to 1.0 microns. MERV 13 must catch at least half of them.
Is MERV 13 too restrictive for my furnace?
Maybe not, but check before you switch. The EPA’s air cleaner guide says most furnaces and HVAC systems can accommodate a MERV 13 filter without equipment problems. It also notes that HVAC systems rarely list filter recommendations.
The EPA’s advice is to choose at least MERV 13, or as high as your system will accommodate. To find that limit, it suggests consulting a professional HVAC technician. Your fan and the filter slot set the practical ceiling.
If you notice weaker airflow at the vents or longer run times after an upgrade, call a technician.
Should you buy a 1-inch or a 4- to 5-inch filter?
Use the thickness your filter slot or cabinet was built for. The EPA notes that filters commonly come in thicknesses of 1 to 5 inches. Thicker media filters fit in a dedicated cabinet. You cannot put a 4-inch filter in a 1-inch slot.
| Thickness | Where it fits | Notes |
|---|---|---|
| 1 inch | Most standard return grilles and furnace slots | Most common and cheapest per filter |
| 4 to 5 inches | Dedicated media filter cabinets | More filter material in the same face area; follow the label for change interval |
The EPA stresses fit over everything else. The filter should sit snugly so air does not leak around it. You should never have to bend or crush a filter to make it fit.
How do you find your filter size and airflow direction?
The size is printed on the edge of the old filter. It reads width by height by depth, such as 16 x 25 x 1. The printed number is a nominal size. The actual dimensions are usually listed in smaller print and run slightly smaller.
To install a new filter:
- Turn the thermostat to off before you remove the filter.
- Note where the filter sits. It may be in a return grille on a wall or ceiling, or in a slot at the furnace or air handler.
- Find the arrow on the filter frame. It shows the direction of airflow.
- Point the arrow toward the furnace or air handler, away from the return grille.
- Slide it in until it sits flat, with no gaps at the edges.
- Write the date on the frame so you know when you changed it.
What filter should you use for wildfire smoke?
The EPA’s wildfire guidance says to use a high-efficiency filter, MERV 13 or higher, if your system can accommodate it. It also says to:
- Set the fan to “On” instead of “Auto” so air passes through the filter more often.
- Close a fresh air intake, or switch to recirculate, if your system has one.
- Replace filters more often than usual during heavy smoke.
- Make sure the filter fits snugly in its slot.
The EPA’s air cleaner guide notes that a furnace filter only cleans air while the system runs. In heating and cooling seasons, that is usually less than 25% of the time. Running the fan longer improves filtration but raises electricity costs.
Here is the math for one example. Assume a blower that draws 400 watts. Your blower may draw much more or less, so check the nameplate or ask a technician.
Energy per day = 0.4 kW x 24 hours = 9.6 kWh
Cost per day = 9.6 kWh x price per kWh
The EIA expects the 2026 U.S. average residential price near 18.2 cents per kWh. At 18 cents, continuous fan running costs about $1.73 a day.
| Electricity rate | Cost per day (24 h at 400 W) | Cost per 30 days |
|---|---|---|
| 12¢/kWh | $1.15 | $34.56 |
| 15¢/kWh | $1.44 | $43.20 |
| 18¢/kWh | $1.73 | $51.84 |
| 25¢/kWh | $2.40 | $72.00 |
| 30¢/kWh | $2.88 | $86.40 |
For other ways to cut household electricity use, see our guide to standby power and phantom load.
Safety notes
- Turn the system off at the thermostat before changing the filter.
- Don’t run the system without a filter in place.
- ENERGY STAR warns a dirty filter can cause the air handler to shut down. If the system keeps shutting off after a filter change, call a technician.
- Keep working carbon monoxide alarms near sleeping areas in any home with fuel-burning appliances.
Filter changes also fit well into a seasonal routine. See our winterize your house checklist. If dust is not your only indoor air issue, also check ideal indoor humidity in winter.
Quick answers
Is it OK to change an HVAC filter every 6 months? Not by ENERGY STAR’s guidance, which says to change it at least every three months, sooner if it looks dirty.
Can a MERV 13 filter hurt my furnace? The EPA says most systems can handle MERV 13 without equipment problems. Ask an HVAC technician if you are unsure about your system.
Which way does the arrow go on an air filter? Point the arrow toward the furnace or air handler, in the direction air flows. It should point away from the return grille.
Should I run my HVAC fan on “On” during wildfire smoke? The EPA suggests “On” instead of “Auto” during smoke, with a MERV 13 filter if your system allows it. Expect higher electricity use while it runs.