Furnace Filters vs. Air Filters: What’s the Difference?

The central air heating and cooling system in a home is made up of many integral components. As a key component, air filters make it possible for your central air system to safely clean heated or cooled air as it makes its way through the rest of your house. In most cases, your central air system uses 2 separate filters to adequately filter out unwanted particles, allergens, bacteria, and more.

Since most people use the term “air filters” interchangeably with other terms like “ac filters” and “furnace filters,” there’s a misconception that they’re either the same thing, or in some instance, completely separate things. We often hear customer asking, “what is a furnace filter?”

When it comes to AC air filters and furnace filters, they’re not identical, but they’re also not completely separate products. There are very specific differences between the two.

Below, we cover the key differences between “air filters” and “furnace filters” in your central air system so you can choose the best options for your home AC.

Merv 8 rated air filters and furnace filters

The Key Difference Between Return Air Filters & Furnace Filters

a picture of the furnace filter and the air filter

Return air filters and furnace filters work in the same way. Both components clean air as it circulates through the home, removing contaminants before the air passes through the HVAC equipment to the rest of the house. Whenever your HVAC system is running, this process is happening.

It is important to note that because the filtration process is the same between air and furnace filters, some homes utilize a single filtration system or type of air filter for both heated and cooled air. There won’t always be a specific spot for a furnace filter in some central air systems.

When this is the case, the air filter is sometimes referred to as a “whole home air filter.”

Central HVAC systems and newer heat pump systems are often designed so that the heating unit and cooling unit share the same air handler and intake vent air filters.

That being said, air filters and furnace filters are not the same thing.

In many homes across the country, the heating and cooling systems aren’t completely integrated, so a separate filter is needed for the furnace.

The differences start to occur between furnace filters and AC filters in older homes, or homes that have special heating and cooling needs (like homes in New England where winter weather is much more prominent than here in Alabama).

While both the air filter and furnace filter serve the same function – to clean the air in your home – features like the filter location, lifespan, filtration level, and cost are the key differences.

AC Air Filters

typical pleated, fiberglass air filter located in the home

Here are features that are specific to home AC intake air filters.

Air Filter Location

AC air filters are often located in a slot behind the air return vent. The vent can be in several places in a home. Typically, they are along a staircase, in a hallway, or on the ceiling in a main room in the house.

Air Filter Function

The primary functions of an AC air filter include:

  • Clears contaminants, allergens, bacteria, viral particles, dust, mites, and debris from the cooled air
  • Helps the AC system run efficiently
  • Protects the AC equipment from harmful contaminants

Air Filter Lifespan

As we have noted in the past, the lifespan of an AC air filter is largely dependent on how often the cooling system is used, who lives in the home, and lifestyle. That said, typically air filters need to be replaced or cleaned every 1 - 3 months.

Air Filter Filtration Level

Filtration level is where AC air filters stand out. There is a huge filtration range from just larger particles of debris to microscopic viruses that are floating through the air. The filtration level is noted as a MERV rating from 1 to 20. The higher the MERV number the more the AC air filter will be able to clear out of the air. AC air filters usually have a rating between 8-13 MERV.

Air Filter Cost

Pleated, fiberglass AC air filters, which are recommended for most home AC systems, range in price from less than $6 up to $50 a piece. It all depends on the size, thickness and MERV rating.

Furnace Filters

a man replacing a furnace filter

Below are the key features that differentiate a furnace filter from an AC air filter.

Furnace Filter Location

Furnace filters are located in the blower compartment of the furnace. The compartment is usually located in a tray or door on the blower compartment. Usually, the air blower is located in a central location in your house, often close to your air intake vents.

Furnace Filter Functions

The core functionality of a furnace filter is nearly identical to that of an AC air filter, except they apply to “heated” air as opposed to “cooled” air.

  • Clears a range of contaminants from heated air
  • Helps the heating system run clean and efficiently, preventing clogs
  • Protects the furnace and heating equipment from harmful airborne contaminants and debris

Furnace Filter Lifespan

The thickness of the furnace filter makes a huge impact on the lifespan. Furnace filters that are 1-2” thick need to be replaced or cleaned every 1-3 months. Furnace filters that are 3-4” thick can last up to nine months before needing to be replaced. And a 6” furnace filter can last up to 12 months.

Furnace Filter Filtration Level

Furnace filters also use the MERV rating scale to denote filtration level. Residential furnace filters are usually between 4-12 MERV.

Furnace Filter Costs

There are a lot of considerations that go into the cost of a furnace filter, however, they are generally between $15 and $25 for a single filter. Choosing a furnace filter with a higher MERV rating or thickness will be more expensive.

Air Filters vs. Furnace Filters Quick Guide




SystemUsed for cooling systems and modern central air systems with integrated cooling & heatingUsed in heating systems that require dedicated filtration
Core FunctionalityCore FunctionalityClears contaminants, allergens, bacteria, viral particles, dust, mites, and debris from the air
LocationUsually at 2 points in a house behind the air intake ventLocated in a tray or door on the air handler box
Filtration LevelMERV Ratings 1 - 20MERV Ratings 4 - 12
Replacement TimeRecommended every 90 days, but sooner depending on usage, pets, and number of residentsRecommended 1 – 3 months for furnace filters up to 2” thick, up to 9 months for thicker furnace filters
Filter Cost Range$6 - $50 per filter depending on size and MERV ratingUsually between $15 - $25 per filter

Air Filters & Furnace Filters for Any Home

If you need a replacement air or furnace filter, shop with Filterbuy and get your new air filters delivered to your door. Filterbuy offers custom air filter sizes as well as air filter subscriptions.

Or you can start by selecting your preferred MERV rating below.