Where Is the Lennox Furnace Filter Located and How to Replace It

The location of a Lennox furnace filter varies by model and installation, but knowing where to find and properly change it is essential for airflow, efficiency, and indoor air quality. This guide explains common Lennox filter locations, step-by-step removal and replacement, recommended filter types and MERV ratings, troubleshooting tips, and maintenance intervals for U.S. homes.

Common Lennox Unit	Typical Filter Location	Filter Type
Lennox Air Handler (Indoor Unit)	Return plenum or blower compartment, behind access panel	Disposable pleated or washable
Lennox Furnace (Upflow)	At bottom of unit in filter slot or return duct	1″–5″ pleated filters or media
Lennox Furnace (Downflow)	Top of unit or return grille in ceiling	1″–4″ pleated filters, cabinet media
Lennox Horizontal Furnace	Side panel or filter drawer in unit side	1″–4″ pleated filters or media

How Lennox Filter Location Varies By System Type

Lennox manufactures furnaces, air handlers, and packaged units with different airflow configurations: upflow, downflow, and horizontal. Filter placement depends on the configuration and whether the installer added a filter rack in the return duct. Understanding the system layout helps locate the filter quickly.

Upflow Furnaces

In upflow furnaces the air enters through the lower return and moves upward across the heat exchanger. Filters are commonly installed at the bottom of the cabinet or in a filter slot inside the return plenum. Some installations place a filter behind a return grille in a wall or floor near the unit.

Downflow Furnaces

Downflow systems pull air from the top and deliver heated air downward. Filters often sit on top of the unit under an access panel or in a ceiling return grille. In attics or closets, a filter drawer may be mounted on the furnace top or side.

Horizontal Furnaces and Air Handlers

Horizontal installations, often in attics or crawl spaces, position the blower sideways. Filters are typically located in a side-access panel or a dedicated filter compartment. Some Lennox air handlers include a removable filter tray near the blower housing.

Common Locations To Check First

When searching for a Lennox furnace filter, start with the most likely locations. This saves time and prevents unnecessary disassembly.

• Return Air Grille: Wall, floor, or ceiling return grilles often hold the primary filter behind the grille frame.
• Furnace Bottom Panel: For upflow units, remove the lower access panel to find a filter slot or framed filter holder.
• Top Of Furnace/Air Handler: For downflow units or some air handlers the filter is on top beneath a cover.
• Filter Drawer/Side Panel: Horizontal units usually have a side filter compartment or drawer.
• Return Plenum: The duct closet or plenum immediately before the unit may contain a filter rack where the filter slides in.

Step-By-Step: Remove And Replace A Lennox Furnace Filter

Changing the filter is straightforward. Follow safety precautions and keep replacement filters on hand that match size and MERV recommendations.

Tools And Supplies

• Replacement filter sized to the unit (check existing filter for dimensions)
• Work gloves
• Screwdriver set (if access panel screws are present)
• Vacuum or microfiber cloth for light cleaning

Procedure

1. Turn Off Power: Switch off the furnace or air handler at the thermostat and the breaker to prevent the blower from starting.
2. Locate The Filter: Check the return grille, bottom panel, top of the cabinet, or side drawer depending on system orientation.
3. Open Access Panel: Remove screws or slide out the filter drawer to access the filter. Keep screws in a safe place.
4. Note Filter Direction: Observe the airflow arrows printed on the filter frame and on the unit. Install the new filter with arrows pointing toward the furnace/blower.
5. Remove Old Filter: Slide out the used filter carefully to avoid dislodging dust. Inspect for excessive dirt, which may indicate restricted airflow or long overdue replacement.
6. Clean Housing: Vacuum dust from the filter slot and surrounding area. Wipe panels with a damp cloth if needed.
7. Install New Filter: Insert the new filter into the slot with proper orientation. Ensure a snug fit without forcing.
8. Reassemble And Power On: Replace panels or grills, restore power, and reset the thermostat if necessary.

Choosing The Right Filter Size And Type

Using the correct filter size and type is essential for performance and equipment longevity. Lennox systems can accept a range of filter thicknesses, but compatibility depends on the cabinet and blower capacity.

Filter Sizes

Filters are typically measured in inches: width x height x depth. Common sizes include 16x25x1, 20x20x1, 16x20x1, and deeper media options like 20x25x4. Measure the existing filter or the slot to confirm dimensions. Never compress a thicker filter to fit a slot designed for 1-inch filters.

Filter Types And MERV Ratings

Filter choices include fiberglass, pleated disposable, high-efficiency pleated, and washable/reusable types. MERV (Minimum Efficiency Reporting Value) rates filtration effectiveness. Common guidance:

• MERV 1–4: Basic fiberglass; minimal particle removal.
• MERV 5–8: Better pleated filters; good household dust, pollen capture.
• MERV 9–12: Higher efficiency; captures finer particles and some pet dander.
• MERV 13–16: Hospital-grade filtration; may restrict airflow on some residential blowers.

For most Lennox residential systems, MERV 8–11 balances air quality and airflow. Consult the Lennox manual or HVAC contractor before using MERV 13+ filters to avoid reduced performance or shortened equipment life.

When To Replace The Furnace Filter

Replacement intervals depend on filter type, household conditions, and system usage. Typical schedules:

• 1-inch pleated filters: Replace every 1–3 months.
• 4-inch media filters: Replace every 6–12 months.
• High-efficiency or pleated filters with heavy loading: Check monthly and replace when noticeably dirty.

Factors that shorten filter life: pets, smokers, high dust, renovations, and households with allergy sufferers. Inspect filters monthly for the best balance of air quality and system protection.

Troubleshooting Filter-Related Problems

Many airflow and performance issues trace back to filters. Identifying symptoms and fixes is critical for efficient operation.

Restricted Airflow And Reduced Heating

Symptoms: weak warm airflow, longer run times, uneven heating. Cause: clogged or undersized filter. Solution: replace with correct-size filter and consider a deeper media filter for longer service life.

Unusual Noises

Symptoms: rattles or vibration after filter change. Cause: loose access panel or incorrectly seated filter. Solution: reseat filter, tighten panels, ensure grill is secure.

Dirt On Blower Or Coil

Symptoms: visible dust accumulation on blower or evaporator coil. Cause: filter bypass or wrong filter type. Solution: ensure proper filter fit, inspect for gaps, and clean coils during maintenance.

Maintenance Tips To Extend Filter Life And System Efficiency

Regular maintenance supports cleaner air and lower utility bills. Combining homeowner tasks with professional service yields the best results.

• Keep Replacement Filters On Hand: Stock common sizes to change promptly.
• Seal Gaps: Use foam or metal strips to seal gaps where the filter meets the cabinet to prevent bypass.
• Schedule Annual Tune-Ups: A qualified Lennox technician should inspect, clean coils, check airflow, and confirm filter compatibility.
• Use Equipment Alerts: Smart thermostats and some Lennox systems display filter change reminders.
• Consider Zoned Filtration: For larger homes, add return grilles with dedicated filters or whole-home media filters to reduce frequency of changes.

Compatibility And When To Call A Professional

Some Lennox models have specific filter racks or require professional access for filter changes. If the filter slot is inaccessible or the unit is installed in a tight attic or crawl space, schedule a professional service to avoid damage or safety risks.

Call a certified HVAC technician when:

• There is no obvious filter access point and the owner’s manual is unclear.
• Frequent clogging occurs despite timely changes—this may indicate duct contamination or system imbalance.
• The system uses high-efficiency media and filter replacement requires disassembly beyond homeowner capability.

Frequently Asked Questions

How Can One Identify Filter Size If Labels Are Missing?

Measure the old filter’s frame from edge to edge for width and height and measure depth for thickness. Round up to common commercial dimensions if needed and verify slot measurements before purchase.

Can Any Filter Be Used In A Lennox Furnace?

Filters must match cabinet dimensions and be compatible with the blower capacity. Using a filter with too high a MERV can restrict airflow. Always confirm recommended filter types in the Lennox manual.

Are Washable Filters A Good Option?

Washable filters can save money but often have lower efficiency and require thorough cleaning to avoid mold. Disposable pleated filters are typically better for consistent performance.

What If The Filter Keeps Falling Out Or Doesn’t Fit Securely?

Check for the correct orientation and any broken retention clips. Use a properly sized filter frame or add a simple retainer bracket. If the filter slot is damaged, contact an HVAC professional for repair.

Resources And Next Steps

Owners should consult the Lennox owner’s manual or model-specific documentation to confirm filter size and placement. Lennox also provides online manuals and a dealer locator for service. Keeping a small toolkit and a few spare filters at home reduces downtime between replacements.

Key Takeaway: Locating and replacing a Lennox furnace filter is usually straightforward when the system configuration is known. Proper filter size, correct airflow orientation, timely replacement, and occasional professional inspections keep the system efficient and improve indoor air quality.