Furnace Filter Replacement Guide: How to Change a Furnace Filter
Updated July 26, 2026
Changing a furnace filter takes about five minutes and needs no tools. Turn the system off, note the size and the airflow arrow on the old filter, slide the new one in with the arrow pointing toward the furnace, and restart. This guide walks through every step with photos and video, shows you where to find your filter, and explains how to pick the right replacement — including the one MERV rule most homeowners get backwards.
Key Takeaways
- Turn the system off at the thermostat first — it kills the suction and makes the filter slide out easily
- The airflow arrow points toward the furnace, away from the return duct
- Check every return grille — many homes have several filters, and they are often different sizes
- Filter depth, not the MERV number, drives airflow resistance — MERV 8–11 for a 1-inch slot, MERV 13–16 for a 4–5″ cabinet
- Actual size is not simply smaller than nominal — each dimension can round up or down, so match by part number on deep filters
- Reset the filter reminder on your thermostat so the next change isn't down to memory
Watch: How to Change a Furnace Filter
Furnace filters are also known as air filters, HVAC filters, AC filters, or air conditioner filters — in a typical split system they all refer to the same component, because the furnace and the AC coil share one air handler.
How to Change a Furnace Filter: Step by Step
- Turn off your AC or heat. Press the mode button on your thermostat until the display reads OFF; some thermostats simply have an off switch. This stops the blower, removes the suction holding the filter in place, and keeps the system from pulling unfiltered air across the coil while the slot is empty.
- Locate every filter. Your filter sits in the blower compartment of the furnace, in an attached cabinet or filter case, or behind a wall- or ceiling-mounted return grille. Walk the house and find all of them before you order replacements.
- Open the grille or access panel. Return grilles that hold filters usually have latches rather than screws — on the top, side, or bottom depending on installation. Hold the grille securely as you release the latches so it doesn't swing down on you.
- Note the size and the arrow before you pull it out. The nominal size is printed along the frame edge, along with an arrow showing airflow direction. Photograph it if the printing is faint.
- Slide the old filter out and the new one in. Keep it level so dust doesn't spill. Install the new filter with the arrow pointing toward the furnace or air handler and away from the return duct. It should sit snugly — gaps at the edges let air bypass the filter entirely.
- Close up and restart. Latch the grille or refit the access panel, then press the mode button back to your normal setting. In the example below the system was returned to AUTO.
- Reset the filter reminder on your thermostat, and write the date on the filter frame with a marker.
Installing a 20x20x1 filter? See our complete 20x20x1 installation guide for size-specific tips and troubleshooting.
Step 1: Turn off your AC or heat by changing the mode to OFF
When replacing a filter behind a return grille, turn the HVAC system off from the control on the wall. This shuts down the motor and takes the suction out of the system, making the filter easy to remove. Press the mode button until the display indicates OFF. For some thermostats, you may just have an off switch.
Notice that AUTO is selected in the image. Pressing the mode button moves the selection to the next operation — here you would press mode once to move it to OFF.
Where Is My Furnace Filter Located?
If you've wondered where your air filters are, you're not alone. Your filter can be in the blower compartment of the furnace, in an attached filter case or cabinet, or in a wall-mounted return air grille. Our guide to where the furnace filter is located in upflow vs. downflow systems covers the less obvious layouts.
Filters located behind return grilles
Filters are often located on a wall or ceiling behind a return grille. If yours are in the return grilles, you may have several filters to replace. Locate every return grille in the house — and don't assume they're all the same size. Check each one before ordering.
Sometimes you can see the filter through the grille. If the grille holds a filter, it probably has a latch instead of screws. The latch can be on the top, side, or bottom depending on how it was installed. The grille below has two latches on the bottom that swing out to unlock it.
Pull down on the latches to open the grille and replace the filter. Be careful that the whole grille doesn't fall — hold it securely as you open it.
Find your 1-inch air filters by size.
Once both latches are open, the grille swings down and the dirty filter can be swapped for a new one.
Filters located in a cabinet or filter case
Many furnaces have a compartment or cabinet for a whole-house media filter, like this Lennox filter in its own cabinet. These are the 4-inch and 5-inch deep-pleated filters, and they last far longer between changes than a 1-inch panel.
If this is your setup, our whole house air filter guide to choosing a 5-inch media covers fit, MERV, and brand cross-references.
Reset the filter reminder
Most thermostats have a setting to reset the filter timer. This creates an alert on the control at a set interval — often three months — when it's time to change the filter again.
On this control the filter reset is at the bottom left, in the red box. Pressing it clears the alarm so you'll get a fresh warning when the next change is due. A filter subscription is another good way to make sure replacements arrive before you need them.
Expert tip: run the fan on LOW between cycles
Leaving the blower running on low circulates air through the filter even when the system isn't heating or cooling. That means more of your home's air passes through filtration, more even temperatures between rooms, and better humidity control. The fan itself draws power, so the net saving depends on your equipment — worth asking your HVAC technician whether continuous fan makes sense for your system.
Find Your Filter Easily
Enter the filter part number or size from your current filter in the search below — for example, X8795 or 20x20x1.
How to Buy the Right Replacement Filter
Getting the size right: nominal vs. actual
The size printed on your filter — 20x25x1, 16x25x5 — is the nominal size. It's a label, not a measurement, and the actual dimensions are usually different by a fraction of an inch.
Here's the part that causes most wrong-size orders, and it's the opposite of what many guides claim: actual dimensions do not simply run smaller than nominal. Each dimension can round up or down independently. A filter labeled 20x25x5 might actually measure 19¾ × 24¾ × 4⅜, and some brands run wider than nominal. Never assume, and never buy from a tape measure alone.
This matters most on 4-inch and 5-inch whole-house filters, where one nominal size maps to several distinct actual sizes tied to specific cabinets — and where manufacturers label the same physical filter differently. The reliable method is to match by cabinet or filter part number, then confirm the actual dimensions. Use the search box above, our nominal vs. actual filter size guide, or the complete list of 1″, 2″, and 4″ filter sizes with actual dimensions.
Choosing a MERV rating: depth matters more than the number
MERV (Minimum Efficiency Reporting Value) rates how well a filter captures particles. You've probably heard that a high MERV rating will restrict airflow and damage your furnace. That advice isn't wrong so much as misapplied — it's a 1-inch-filter rule repeated as though it covers every system.
Resistance depends on how fast air is forced through the media, which comes down to how much media there is. Push your system's full airflow through the thin face of a 1-inch filter and air moves through at high velocity, so resistance climbs quickly as you go up the MERV scale. Spread that same airflow across the deeply folded media of a 4-inch or 5-inch filter and it passes through far more slowly — lower velocity, lower resistance — even though each pleat is doing more filtering.
| Filter | Initial airflow resistance |
|---|---|
| 1″ MERV 8 (representative) | ~0.12 in. w.c. |
| 1″ MERV 13 (representative) | ~0.30 in. w.c. |
|
Atomic 4″ MERV 16 deep-pleated (20x25x4, Lennox X6675-compatible) |
0.07 in. w.c. at a full 1,024 CFM (independently tested to ASHRAE 52.2) |
Read that last row again: a deep-pleated MERV 16 tested at lower resistance than a thin MERV 13, and lower than a cheap 1-inch MERV 8. The MERV number went up and the resistance went down, because roughly 118 square feet of media gives air far more room to pass through. The same test measured 95% capture of the finest 0.3–1.0 micron particles. We break the full airflow curve down in our guide on whether a MERV 16 filter restricts airflow.
So the rating you should run depends on your slot depth, not on a blanket ceiling:
- 1-inch slot only: MERV 8 to MERV 11 is the sensible range. A thin MERV 13 is where the airflow warning has real teeth, especially with an older blower or undersized return ducting.
- 4-inch or 5-inch cabinet: MERV 13 is a solid baseline, and MERV 15–16 is realistic if you're managing allergies, asthma, pets, or wildfire smoke.
If you only have a 1-inch slot and want cleaner air, adding a deep-pleated media cabinet is usually a straightforward retrofit — our guide to installing a filter cabinet for a deep-pleated filter covers what's involved. For more on the ratings themselves, compare MERV 8 and MERV 11 or MERV 11 and MERV 13.
Choosing a filter type
Pleated filters are far more effective than flat fiberglass panels because the pleats pack much more surface area into the same frame, so they trap more dust, pollen, mould spores, and dander — and they last longer between changes. Fiberglass earns its place only in narrow cases, such as a cheap pre-filter during renovation dust. Our comparison of fiberglass vs. pleated air filters covers where each one makes sense.
Common Mistakes When Changing a Furnace Filter
Installing the filter backwards
One of the most frequent errors. Filters are built to pass air in one direction, and there's an arrow on the frame showing which way — make sure it points toward the furnace and away from the return duct. A backwards filter restricts airflow and reduces filtration efficiency.
Leaving gaps around the filter
A filter that rattles loose in its slot is only filtering some of the air passing by it. Bypass air is the most underrated filtration killer — air takes the path of least resistance straight around the media. If the fit is loose, recheck the size before running the system.
Buying a deep filter on nominal size alone
On 4-inch and 5-inch filters, the same nominal size covers several different actual sizes tied to specific cabinets. Match by part number, not just the printed size.
Stretching the replacement interval
1-inch filters generally need changing every 30–90 days depending on pets, allergies, and how hard the system runs; 4–5″ deep-pleated filters typically run 6–12 months. A clogged MERV 8 restricts airflow far more than a clean MERV 13 ever will, so an overdue filter is the real risk — not an efficient one.
Not turning the system off first
Always switch the HVAC off at the thermostat or breaker before opening the filter compartment. Otherwise the blower pulls unfiltered air across the coil while the slot sits empty.
Forgetting the other returns
Plenty of homes have two or three filters in different returns, often in different sizes. Changing one and forgetting the rest leaves most of your air unfiltered.
Ready to Reorder?
Atomic Filters makes USA-made replacements compatible with Carrier, Lennox, Trane, Honeywell, and other major systems, typically 30–50% below OEM pricing. Use the search box above if you know your part number, or browse all furnace filters, 1-inch filters by size, or whole-house deep-pleated filters.
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