What Is The Difference Between Furnace Filter And Return Air Filter?

Dust, smoke, odors from chemicals, and many other forms of contaminants can cause significant discomfort in indoor spaces. Such contaminants may arise from the usual activities that take place in the house as well as from outside. One fundamental way of dealing with solid contaminants is to install filters.

There are essentially two types of filters: furnace and return air filters. Whereas these two types of filters serve the same function of filtering the air that moves around your home, they differ from each other in various ways.

Therefore, understanding the similarities and differences between air filters and furnace filters is critical in helping you use them properly.

What is a furnace filter?

A furnace filter is a small type of filter found in the furnace’s blower compartment. This filter is standard in homes where the heating and cooling systems are not integrated with the air systems. The furnace filters clean the heated air before circulation.

What is a return air filter?

A return air filter is a type of air filtration device found in the slots at the return air vents in the house. The purpose of the return air filter is to protect the fragile HVAC system components by removing all the solid contaminants from the air.

What is the difference between furnace filter vs. return air filter?

The main difference between a furnace filter and a return air filter lies in their location. Whereas a furnace filter is in the blower compartment of your furnace, the return air filter is located in a slot behind the return air vent. Both filters perform the primary air filtration function in the premises where they are installed.

Pros and cons of furnace filter

Pros of furnace filters

  • It prevents airborne infections by removing allergens and bacteria from the air. Allergens can trigger an allergic reaction in some people who are sensitive to them. In addition, bacteria and other microorganisms may trigger airborne diseases and conditions in people if they are not removed from the air. A furnace filter effectively removes allergens, bacteria, and other forms of disease-causing microorganisms from the air in the home.
  • Optimizes the performance of the furnace. Your furnace requires a steady and uncontrolled flow of air in and out to ensure that it works conveniently. The furnace filter helps to optimize and maintain the furnace’s performance by enabling it to draw in the air without any inhibitions. The furnace then warms the air and pushes it through the ductwork and grates of your home.
  • Cleans the warm air that moves through the ductwork. A good furnace filter ensures that the warm air that circulates through the ductwork in your home is free of contaminants. Some contaminants that the furnace filter cleans from the warm air are dust mites.

Cons of furnace filters

  • Costly and regular replacements. Although furnace filters are not exceptionally expensive, the cost of replacing them regularly may mount over time. This may be particularly pronounced if the air in your house is filthy. The more the percentage of contaminants in the air, the faster the filter will turn grey and require to be replaced.
  • Time-consuming and technical installations. Installing your furnace filter may be a challenge if you are not conversant with the technology and the general procedure. In addition, some large filters may pose an equally high challenge when you have to install them.

Pros and cons of return air filter

Pros of air filters

  • Some air filters capture small particles that cause odors in the home. For example, activated carbon air filters with high MERV ratings are excellent in capturing the tiny particles that cause odors from the house. Thus, such air filters ensure that the home is free of odors and comfortable.
  • Some ionic filters are excellent in capturing microscopic particles that measure as small as 0.01 microns wide. Such ionic air filters are ideal for specialized areas, including hospitals and scientific laboratories. However, using such high-end air filters in residential properties may not be necessary.
  • They protect people who are allergic by removing allergens. Some of the most common allergens include mold and pet dander. Mold and pet dander particles are considered slightly significant because they are more comprehensive than 0.3 microns. Most air filters with mid-range MERV ratings can effectively trap such allergens and prevent them from circulating in the ductwork of your home.

Cons of air filters

  • Not all carbon filters effectively remove odors from cigarettes and other sources. You need an air filter with exceptionally high MERV ratings for you to rid your home of some common odors from cigarettes and other sources, including chemicals.
  • The higher the MERV rating, the more the air filter will impede airflow. You always need to balance between the filtration capacity of your air filter and the air circulation needs of your home. Choosing a filter with an exceptionally high MERV rating may interfere with the flow of air in your HVAC system.

Similarities between furnace filter vs. return air filter

  • Both furnace filters and air filters remove solid contaminants from the air in the room. The solid contaminants that the furnace filters or air filters can remove vary in size depending on the nature of the surface of the filters. Some of the most common contaminants include smoke, dust, and dander from pets.
  • Both the furnace filter and the air filters prevent allergens from the air in the home. Allergens may trigger severe asthmatic attacks in people who are sensitive to them. Air filters remove the allergens from the air that circulates in the home. Similarly, furnace filters remove the allergens from the warm air before the air moves around the ductwork.

An in-depth comparison between furnace filter vs. return air filter

Some people refer to furnace filters and air filters as ‘whole house filters.’ This description is inaccurate because it assumes that a furnace filter and an air filter are the same things. Although these two types of devices look similar and primarily serve the same function of filtering the air in your home, they are essentially different from one another.

Here is a detailed comparison of these two devices regarding their functions, location, durability, and air filtration performance.

Air filter function vs. return air filter

The primary functions of an air filter include the following: helping the AC system operate effectively, removing contaminants from the air, and protecting the sensitive AC equipment from the contaminants. Solid contaminants can easily damage the AC system. Using a high-quality air filter prevents damage because the surface of the filter attracts and traps the contaminants.

 In addition, the surface of the filter attracts and traps disease-causing microorganisms, including bacteria. Lastly, an air filter protects people from allergic reactions because they capture allergens. Allergens may be in spores, smoke particles, and even dust.

Return air filters have more or less similar core functions to air filters. This is why many people consider these two types of devices identical. However, it helps to remember that return air filters operate on air that has already been heated and is ready to circulate in the entire house through the ductwork and grates.

Nevertheless, return air filters protect the heating equipment from solid contaminants. By so doing, return air filters enable the equipment to work efficiently. In addition, return air filters clean the hot air of all the contaminants, including dust, pet dander, and smoke particles.

Air filter location vs. return air filter location

Furnace filters are located in the same furnace compartment containing the blower. A small door opens the blower compartment where the furnace filter, blower, and other components are found. The furnace’s blower is usually in a central position in your home.

 This positioning of the blower is necessary to ensure adequate and uniform circulation of the warm air around the home to raise the ambient temperature. Typically, you can find the blower near the air intake valves in your house.

The location of air filters is very different from that of furnace filters. Air filters are usually located in a small slot behind the return vent. There are no specially designated places where these slots can be found in the home. In many homes, you can find them on the ceiling of the house’s main room.

 In other homes, the small slots can be seen on the ceiling along with the stairs case. Once more, the location of the slots is supposed to ensure that the air filters work optimally. This is important for maintaining the indoor air quality of your home.

Air filter lifespan vs. return air filter lifespan

Generally, an air filter lasts anywhere between one and three months. However, the actual lifespan of your air filter is a function of many factors. Notably, if the air in your home is notably highly contaminated because of the activities in the house, the chances are that you will have to replace your air filter earlier than it would be if the air in the house were clean.

In addition, the rate of use of the cooling system has a direct impact on the lifespan of the air filter. The more frequent the air cooling system is used in the home, the less the lifespan of the air filter. Conversely, if the air cooling system is hardly used, the air filter will remain helpful for a long time.

A furnace filter will last up to 12 months if it is 6 inches thick. Furnace filters that have a medium thickness of about 4 inches tend to last for an average of six months before needing to be replaced.

Thin furnace filters of less than 3 inches tend to last for about three months. The thick furnace filters tend to last longer than the thin ones because they can take up many contaminants before they need to be replaced. Thus, if you want to use your furnace filter for a long time, go for the thickest ones.

Air filter filtration level between furnace filter and return air filter

The filtration level of furnace filters refers to the composite percentages of particles of different sizes that the filter can successfully attract and trap. The surfaces of furnace filters are designed to attract and trap solid contaminants. Typically, MERV ratings of furnace filters range from four to 12.

The overall MERV ratings of all filters range from one to 16. The higher the MERV rating, the more small particles the filter can successfully attract and trap on its surface. However, high MERV ratings come with the additional problem of interfering with the airflow.

Air filters have exceptionally high MERV ratings of over 13. The high MERV rating is enough to ensure that the air filters trap small contaminants very well. Thus, you can rely on an air filter with a high MERV rating to remove microorganisms and other similarly small particles from the air in your home.

Air filter vs. return air filter cost

The price of air filters is determined by their size, MERV rating, and thickness. For example, air filters made from glass and of average MERV rating may cost about $30. Similarly, a single furnace filter may cost an average of $30.

Air filter vs. return air filter maintenance

Cleaning is a necessary maintenance procedure for both the furnace and air filters. Cleaning entails removing all the large contaminants that usually stick on the surface of the contaminants.

What varies between the air filter and furnace filter regarding cleaning is the frequency that one may need to carry out the process.

Generally, thick furnace filters may take a long time of about six months before they get dirty enough to warrant being cleaned. However, the need to clean your furnace filter will be determined by the overall level of cleanliness of the air in your home and the general activities in the space.

An air conditioner that is thick enough may continue functioning for even three months before it becomes filthy enough to warrant being cleaned.


There are fundamental differences between furnace filters and air filters. Air filters are found in integrated cooling and heating systems in modern buildings. Furnace filters are found in buildings where the cooling and heating systems are not integrated with the central air systems.

Thus, furnace filters are used to clean up the air heated by the furnace before the air is circulated through the house. In addition, whereas air filters are located at the slot on the air intake valves, furnace filters are confined to the blower compartments of the furnace.

You can find them located at strategic positions to maximize the air filtration performance and generally improve the home’s indoor air quality.