Can You Stack Furnace Filters, or Should You Use a Thicker One?

No, you shouldn't stack two furnace filters in one slot. Use one filter of the depth the slot or cabinet is built for. Two layers of filter media add resistance to the airflow your heating and cooling system was designed around, and filters are designed and rated to be used one per slot. If you want more filtration or fewer filter changes, a deeper filter is the better route, but only when the housing is made for that depth. If you're not sure what your system takes, the Air Filter Finder can help you match it.

Can You Stack Furnace Filters?

You shouldn't stack furnace filters, because two filters in one slot make air pass through two layers of media instead of the one the system was set up for. The fix is almost always one filter of the right depth.

What happens to airflow when two filters share one slot

When two furnace filters share one slot, the air has to pass through two layers of media, which adds resistance to airflow. The DOE Building America Solution Center (PNNL) describes filter performance as two things: how well a filter removes particles, and its resistance to airflow, called pressure drop. Your system has to accommodate that pressure drop, so check your system's manual before adding anything to the filter slot.

Can two 1-inch filters replace one 2-inch filter?

No, two 1-inch filters are not a substitute for one 2-inch filter. A 2-inch pleated filter is a single, deeper filter rated as one unit, not two thin filters pressed together. If your track is 2 inches deep, buy a 2-inch filter; if you're unsure of the size, you can find the right size furnace filter by measuring.

Two filter locations are not stacking

Having a filter at a return grille and another at the furnace is two filter locations, not stacking. Some homes are built with more than one return, and each location takes its own filter. Replace each one on its own schedule rather than adding a second filter to either spot.

The table below compares common stacking setups with the setup that should be used instead.

Table 1. Stacking vs the Right Setup

If you were thinking about stacking, these options do the job properly:

• Buy a single filter that matches your slot's full depth.

• Choose a higher MERV filter in the same depth, if your system can handle it.

• Have a contractor install a media cabinet if you want a much deeper filter.

Does Furnace Filter Depth Matter?

Furnace filter depth matters because a deeper filter holds more media in the same face size, which affects airflow and how long the filter lasts. Depth is a separate rating from MERV, and both are worth checking.

Why deeper filters hold more media

Deeper furnace filters hold more pleated media, and the EPA notes that thicker or wider filters generally create less static pressure and help improve airflow. The EPA's Indoor airPLUS guidance also says to use the highest-rated filter your system fan and filter slot can accommodate. A filter access panel should seal with a flexible, air-tight gasket to prevent air leakage.

Depth and MERV are different ratings

Filter depth and MERV measure different things: MERV rates how well a filter captures particles, while depth relates to media area and service life. MERV is based on test methods in ASHRAE Standard 52.2, according to the EPA. A typical 1-inch spun fiberglass filter rates MERV 1 to 4. For more detail, see what a MERV rating measures.

How long each depth lasts

How long a furnace filter lasts depends partly on its depth, and manufacturers rate deeper filters for longer intervals. Filtrete rates its 1-inch filters for a change at least every three months and its 4-inch-and-deeper filters for 12 months. ENERGY STAR recommends checking any filter monthly.

The intervals below come from the filter makers and ENERGY STAR, not from blended estimates.

Table 2. Filter Depth and Replacement Interval

Can You Use a Thicker Filter Than Your Slot Was Built For?

You can use a thicker filter only if the slot or cabinet is built for that depth. A filter that's too thick won't fit, and one that's too thin for its housing leaves a gap that air can go around.

A 1-inch filter in a slot with extra room

If your 1-inch filter sits loose in a deeper slot, measure the slot depth before buying a thicker filter. If the track is built for 2 inches, a 2-inch filter fits and is the right choice. Never fill the extra space with a second filter.

Using a 4-inch filter in a 5-inch cabinet

A 4-inch filter in a 5-inch cabinet leaves space for air to bypass the filter media. The EPA advises installing filters properly and minimizing air bypassing the filter, and access panels should seal. Use the filter your cabinet maker lists for that cabinet.

When a deeper filter means a new cabinet

Moving from a 1-inch slot to a 4- or 5-inch filter means installing a media cabinet in the return duct. That's a contractor job, not a filter swap. Learn how cabinets work in our guide to the media air cleaner.

Use this table to check whether the filter you want will fit the housing you have.

Table 3. Will It Fit? Slot by Slot

1" vs 2" vs 4" vs 5" Filters: Which Should You Buy?

The right furnace filter depth is the one your slot or cabinet was built for, and from there you choose the MERV rating. If your system already has a media cabinet, a deep filter means fewer changes.

When a 1-inch filter is the right choice

A 1-inch filter is the right choice for most homes that have a 1-inch filter slot. Pick the MERV rating that suits your home and system, then replace it at least every three months, per Filtrete. ENERGY STAR recommends checking it every month, especially in winter and summer.

When a 4-inch or 5-inch media filter is worth it

A 4-inch or 5-inch media filter is worth it when your system already has a media cabinet built for one. Filtrete rates its 4-inch-and-deeper filters for 12 months, and AprilAire rates its 210, 410 and 213 media filters for 12 months. That means one change a year instead of several.

Matching depth to your household

Your household can shorten any filter's life, so match your change schedule to how your home is used. Filtrete says to change filters more often when you have:

• Construction or sanding in the home.

• Pets.

• Smokers.

• A fan that runs continuously.

Using any filter longer than recommended may restrict airflow or cause the system to malfunction, according to Filtrete. Auto-Ship can deliver filters on your change schedule.

DFS stocks filters at each common depth. Here's where to start for yours.

Table 4. Shop by Depth at DFS

Frequently Asked Questions

Why are some furnace filters thicker?

Some furnace filters are thicker so they can hold more pleated media in the same face size. The EPA notes that thicker filters generally create less static pressure and help airflow. A thicker filter needs a slot or cabinet built for that depth, so check your housing before you buy one.

Do thicker furnace filters last longer?

Yes, thicker furnace filters are rated to last longer by their manufacturers. Filtrete rates its 4-inch-and-deeper filters for 12 months, compared with at least every three months for its 1-inch filters. Household factors such as pets or a continuously running fan can shorten either interval.

How long do 5-inch furnace filters last?

A 5-inch furnace filter lasts as long as its label or your air cleaner manual says, because ratings vary by brand. AprilAire rates its 4-inch media filters for 12 months, and Filtrete rates 4-inch-and-deeper filters for 12 months. For other 5-inch filters, follow the schedule printed on the filter.

Is a 2-inch air filter better than a 1-inch?

A 2-inch air filter is only the better choice if your filter track is built for 2 inches. Depth has to match the slot, so a 2-inch filter won't fit a 1-inch track. Compare MERV ratings separately; our guide to washing or recycling a filter covers what to do with the old one.

Does filter depth change the MERV rating?

No, filter depth does not change the MERV rating. MERV measures how well a filter captures particles, using ASHRAE Standard 52.2 test methods. Depth changes media area and service life, so a 1-inch filter and a 4-inch filter can share the same MERV rating.

Sources

• Filtrete (3M) FAQ (intervals, conditions, filter used past its life)

• ENERGY STAR Guide to Energy-Efficient Heating and Cooling (monthly check)

• EPA Indoor airPLUS filtration bulletin (filter box sizing, gasket, fiberglass MERV, ASHRAE 52.2)

• DOE Building America Solution Center (PNNL), High-MERV Filters (pressure drop)

• EPA, Residential Air Cleaners, 2nd ed. (filter bypass)

• AprilAire 210, 410 and 213 product pages (12-month interval)

• Resideo FC100A1037/U product page