What Are Furnace Filters?

What Are Furnace Filters?

Do you have two air filters in your central air system and wonder what their purpose and effect are? Don’t worry! We’ve got you covered! If you live in a big house, your central AC unit will require two or more return air ducts, and each duct needs a filter to prevent airborne particles from reaching your evaporator or cooling coil.

Or it could be due to the presence of two central AC units in your house. Have you got your answer? If you didn’t, no worries. No one can easily understand the complexities of an HVAC system (except professionals). If you need an answer to this burning question, keep reading this comprehensive blog, as it contains vital information on why some central air systems have two filters.

1. Types Of Furnace Filters: A Buyer's Guide

What are the best furnace filters you can install for daily use? Before deciding on a suitable furnace filter type, you must know the answer to the question, ‘ what are furnace filters made of?’ The material used to manufacture the filter affects its rate of filtration. The most commonly used materials to make these filters are polyester, fiberglass, cotton, carbon, foam, and pleated paper. The type of material chosen directly affects the furnace filter ratings.

What type of furnace filter should I use? The answer to this question varies according to

different furnace types. Here are some tips for choosing the best type of furnace filter per your requirements:

  • Look for the MERV rating on furnace filters – The Minimum Efficiency Reporting Value (MERV) rating plays a huge role in deciding the effectiveness of the filter furnace. The MERV rating judges a filter’s capability to trap particles between 0.3 to 10 microns. A furnace filter MERV rating of a higher value indicates greater efficiency. Most spun filters have a MERV rating of 4, and the high-quality electrostatic pleated types have a MERV rating of 12. A furnace filter rating based on MERV measures the filter’s ability to trap unwanted particles and their sizes. The higher the percentage of particles trapped and the smaller the particle size, the greater the MERV rating on furnace filters.
  • Check your furnace filter size – Look for the size printed on the edge of your old filter or its box/manual. The smallest standard furnace filter size is 10 X 10, and the largest is 25 X 25. The furnace filter price will vary according to the filter sizes. The filter you purchase must be the perfect fit for your furnace cabinet.
  • Check the filter thickness – Thicker filters generally last longer. The standard thickness of filters ranges from one inch to five inches. Check your furnace or HVAC system configuration to determine the suitable thickness.
  • Watch out for airflow restrictions – Choosing filters with high MERV ratings can cause more airflow restrictions. These filters may cause your furnace (especially the blower fan) to work harder, causing frequent burnouts or equipment failure. They also contribute to costlier electricity bills.

The two most common air filters for furnaces are fiberglass and pleated air furnace filters.

1.1 Fiberglass Air Filters

Fiberglass air furnace filters are made of spun glass and have a MERV rating between 1 to 4. They are known as ‘disposable’ or ‘throwaway’ filters as they last for around 30 days, after which you must dispose of them. They are the most commonly used filters for residential furnaces.


  • They are economical.
  • They solve the purpose of domestic furnace filtering. They trap large debris and dust particles.
  • They are highly porous and do not obstruct airflow.


  • They need to be changed every 30 days. Failing to do so may clog your furnace and hamper its efficacy.
  • They cannot trap small particles and allergens effectively.

1.2 Pleated Air Filters

Pleated varieties are a popular choice among air filters for furnaces because of their high MERV ratings and a wide variety of sizes to accommodate most HVAC (heating, ventilation, and air conditioning) systems. The pleated texture of these air furnace filters provides more surface area for trapping pollutants.


  • They have a greater surface area to trap more contaminants, thus improving indoor air quality.
  • Their tiny pores effectively trap small-sized particles and prevent bacterial growth.
  • They last for a longer time compared to fiberglass air filters.


  • Prolonged use may obstruct airflow.
  • They are expensive.
  • They consume more electricity.

2. Other Types Of Furnace Filters

Choosing a filter for furnaces depends on functionality and use. Consequently, you must know about the furnace filter types to understand which will suit your home. Besides the commonly used fiberglass and pleated filters, here is a list of more air filters for furnaces.

2.1 Electrostatic Filters

They are electronic filters for furnaces that use static electricity to trap contaminants as they pass through the filter. They work for both heating and cooling purposes. The dirty air goes through a layer of ionizers emitting charged ions to attract dust particles. The electrostatically charged particles of the filter drive the dust and dirt toward the collector, from where you can remove them. The carbon filter purifies the remaining air.


  • They save time and are cost-effective.
  • They act like permanent filters since you do not need to replace them often.
  • They have efficient MERV ratings.
  • They provide good indoor air quality.


  • They need monthly maintenance (cleaning out allergens and dirt).
  • They cannot trap ozone and mold spores. Hence, they are not suitable for people with allergies or asthma.
  • If not cleaned properly, these filters can hamper airflow.

2.2 Activated Carbon Filters

These air furnace filters eliminate foul odors and gaseous compounds. They consist of carbon embedded in a mesh-like material. Volatile organic compounds (VOCs) responsible for odor slip past regular air filters. But the porous activated carbon in these air furnace filters traps the VOCs through the adsorption process (molecules get trapped in the carbon lattice).


  • They can filter out smog, hazardous gases, and air pollutants.
  • They remove tiny particles responsible for the foul odor.


  • They are not suitable for removing dirt, dust, and pollen.
  • They require regular replacement.

2.3 Reusable Washable Air Filters

These filters typically constitute polypropylene. They have electrostatic charges to trap more particles. As the name suggests, you can wash and reuses these furnace filter types. How often to change furnace filters? These reusable filters do not need frequent changing but need periodic cleaning depending on their MERV rating and the environment.


  • Since they are washable and reusable, they are an eco-friendly option.
  • The HVAC system efficiency increases once you clean and wash these filters.
  • You do not need to purchase frequent replacements.
  • They can effectively remove pollens, allergens, and dust.


  • The initial investment is a bit expensive.
  • If not dried properly after washing, these filters may attract mold growth.
  • They do not help much with pollutants such as smoke, dander, and hairspray.

2.4 HEPA Air Filters

High-Efficiency Particulate Air (HEPA) filters are pleated mechanical air furnace filters having a considerable thickness to trap particulate matter by forcing air through a fine mesh. The randomly arranged dense fiber pleats help catch particles of varied sizes, both small and large. The air filtration in these types of filters takes place through three processes – diffusion (to trap molecules less than 0.1 microns), interception (moving particles stick to the fiber), and impaction (particles directly embed into the mesh).


  • They can remove up to 99.99% of contaminants.
  • They are effective against pollen, dust mites, dander, mold spores, and smoke and can bring allergy relief.
  • The resulting clean indoor air increases productivity and prevents the spread of infections.


  • They cannot eliminate or trap particles such as VOCs, bacteria, and viruses.
  • They are expensive.
  • The density and thickness of these filters may cause airflow obstruction after repeated use.

3. What Does A Furnace Filter Do?

A furnace filter’s principal function is to remove unwanted particles from the air before they can enter your HVAC system. How does that help? Trapping these particles protects the sensitive parts of your furnace, such as the blower fan, and improves indoor air quality. Here is a list of things a furnace filter does:

  • It prevents dirt, dust particles, dander, hair, and other air-borne allergens from entering your furnaces.
  • It reduces energy bills by avoiding clogging of the components.
  • The filter furnace helps keep allergies at bay and maintain general health by improving indoor air quality.
  • It safeguards the interior elements of your HVAC system from overuse and damage.

4. How Often Is Changing the Furnace Filters Necessary?

Before learning how often to replace furnace filters, you must know

  • How to install furnace filters?
  • How to change furnace filters?

While installing a new filter, it is crucial to locate the filter furnace’s airflow direction. Look for arrows that indicate the direction of furnace filters. Remove the panel covering the area where you need to place the filter. Install the air filter with the arrow pointing toward the correct direction. Put the cover back in place. To replace the air furnace filters, you need to know ‘which way does the furnace filter go?’ First, switch off the furnace, slide out the old filter and replace it with the new one. The filter usually is between the return air duct and the primary furnace unit. Make sure you get the filter furnace’s airflow direction right. Although the steps seem easy to attempt a DIY, it is better to employ professional filter furnace changing services for a seamless and hassle-free experience.

Now, coming to the question of how often to change furnace filters, the general recommendation is that you need to replace them once every 90 days. However, factors like the filter’s thickness, the environment, and the size of your home affect the change frequency. Many furnace manufacturers recommend changing the furnace filters at the beginning of each new season. Consider the following factors while planning when to change the filters:

  • The denser the filter, the more often you should change it.
  • Change filters more frequently if you have allergic individuals or pets at home.
  • Flat filters clog faster than pleated ones.
  • If you get renovation work done at home, you should change the filters on the job completion to eliminate additional dirt and debris.

Remember, the primary purpose you have furnace filters installed is to trap unwanted particles from entering your HVAC system. If you let those materials build up inside the filter for too long without cleaning or replacing it, you are inviting potential health hazards.

5. More Factors That Affect Frequency Of Changing Furnace Filters

Deciding how often to change furnace filters depends entirely on the type of your HVAC system. Of the many components in a furnace, such as a gas valve, sensor, and burner, the part that needs frequent change and maintenance is the filter. Air furnace filters prevent particulate matter from entering the furnace’s internal components. A filter’s cleanliness can affect the air quality and the efficiency and longevity of your filter furnace system. Understanding the factors impacting the filter’s cleanliness status will help you decide the correct frequency of changing it.

5.1 Home Occupancy

The smaller the house, the less air pumping is required to maintain a comfortable temperature. Consequently, the frequency of changing filters is lower in small-sized homes compared to big-sized homes. You can get a filter change every 4-5 months for a single-person household. Changing the filter every 90 days is advisable in the case of a large household with more family members. If you have a farmhouse, cabin, or vacation home, you may change the filter every six months to one year, depending on the usage.

5.2 Furry Family Members

Having pets at home can add to the dander and fur quantity in the air. Pets may bring in more particulate matter like dust, pollen, dirt, and hair from outdoors. They also shed fur, invariably causing your furnace filters to get clogged quicker than usual. Consider changing your filters once every 60 days if you have one pet. If you are the proud parent of multiple pets, you should get the furnace filters changed every month.

5.3 Indoor Air Quality

Your home environment is relevant while considering the frequency of filter change. If your home is subject to high dust and debris exposure, it lowers indoor air quality. It means your furnace filter needs to work harder to remove the particulate matter from entering the innermost components of your furnace. Due to this, the filter gets congested sooner and would require frequent replacement. The presence of smokers at home also affects the air quality. The lower the indoor air quality, more is the frequency of filter changes.

5.4 Allergies

If you or any other individual at home is sensitive to allergens, mold spores, moisture, and dust, getting a filter replacement every 30-45 days is a good idea. If you continue with the same filter for more days, the bad air quality can lead to itchy eyes and nose, headaches, sore throats, and complicated respiratory disorders. Cleaning or replacing filters at the beginning of every season is beneficial for people with allergic conditions.

5.5 Heating & Cooling Usage

The regularity with which you use your HVAC systems can affect the decision of how often to replace furnace filters. If you live in a place with moderate climates and use filter furnaces for only a few hours daily, the filter can last almost six months. However, if you live in a place with extreme climates where HVAC systems have to be on a constant ‘switch-on’ mode, the frequency of filter change increases.

6. Dirty Furnace Filter Disasters

A dirty furnace filter not being replaced on time is a potential hazard. The clogs can damage the heat exchanger and blower fan, causing the furnace system to shut down. In severe cases of filter congestion, the filter itself may get sucked inside the vent leading to short circuits and fire. Moreover, running a furnace with a dirty filter increases the risk of dispersing unwanted dust, pollen, bacteria, spores, and allergens all over the home.

What does a dirty furnace filter look like, you may ask? If you notice a dark grey filter with significant clumps or a thick cover of dirt/dust and moldy growth, you need to replace the filter immediately. A brand-new filter appears white. After some days, it turns brownish or grey with a thin film of dirt. Remember, running a furnace with a dirty filter is worse than running one with no filter. Ensure periodic filter changes to avoid the following potential disasters caused by a dirty filter:

  • High electricity bills due to decreased efficiency
  • Damage to the internal components of the furnace
  • Poor indoor air quality
  • Faulty temperature settings
  • A shutdown of the furnace or HVAC system
  • Risk of ventilation fire

6.1 HVAC System Damage

Running a dirty furnace filter can cause a breakdown of your HVAC system, entailing costly repairs. Let us discuss how dirty filters can affect your HVAC system.

  • Increased energy bills – A blocked and dirty filter can increase energy consumption by up to 15%. Dirt in the filter will cause your furnace system to work harder to cool or heat the air, raising the energy bills.
  • Reduced airflow – If the heating and cooling coils get coated with thick layers of dirt, it will obstruct the airflow and cause poor ventilation. Consequently, the furnace blowers will generate less hot or cool air resulting in uncomfortable temperatures inside the house.
  • Low efficiency – Running a furnace on dirty filters may cause frequent episodes of short cycling (the HVAC system fluctuating between off and on modes). It reduces the overall efficiency of the filter furnace system.
  • System failure – The restricted airflow places a heavier load on the blower or fan motor of the HVAC system. The wear and tear of internal components occur and lead to overheating of the system. This additional heat and strain may cause total equipment failure. Repairing this will incur more expenses than the actual cost of replacing a dirty furnace filter. It is better to invest 50$-80$ for a filter replacement than to spend hundreds of dollars repairing mechanical issues in the furnace system.

6.2 Poor Comfort & Air Quality

Using dirty air filters increases the chances of dust, dander, and allergens entering your air ducts. It lowers indoor air quality drastically. These particles then distribute themselves all over the house, and your family will start breathing this air. The growth of mold spores and microorganisms in the duct due to a dirty air filter contributes to poor air quality, possibly leading to severe respiratory illnesses.

The reduced airflow due to blocked air filters does not allow the heating and cooling mechanism to work as it should. This faulty control leads to poor temperature settings. The rooms do not reach the required temperature, causing an overall feeling of discomfort.


Most equipment and machinery around us use filters, for instance, cars and water purifiers. Using furnace filters for HVAC systems and furnaces solves two primary purposes:

  • They improve the air quality.
  • They prevent the internal components from overheating and damage.

As discussed in the article, most modern-day furnaces use disposable filters you need to replace every three months. The failure to do so causes unnecessary health and equipment hazards. One should remember that fitting furnace filters are not optional. It is a necessary part of running furnaces and HVAC systems. Furthermore, these filters are vital to climate control and need frequent maintenance and replacement. While purchasing air furnace filters, remember to check the brand for proper accreditations and HEPA certifications. A thorough and periodic upkeep of home furnace filters directly affects your furnace’s longevity and your family’s overall health.