The air filter is essential to any forced air or central HVAC system. You will also find air filters in many other HVAC units, such as window air conditioners and ductless mini splits. In a central HVAC system, the primary purpose of the air filter is to trap dust and debris to prevent the system’s components from getting damaged or clogged. Air filters can also potentially help to improve indoor air quality.

The efficiency of an HVAC air filter is usually measured in terms of the Minimum Efficiency Reporting Value or MERV. Whenever you need to replace your air filter, choose a filter with the right MERV rating for your HVAC system. If the filter isn’t efficient enough, it won’t be all that effective and could lead to numerous issues with your heating and cooling. Choosing an overly efficient filter is also an issue because your HVAC blower will struggle to draw air in through the filter. To make it easier to understand which filter you should use, here is everything you need to know about air filters and how they are rated.

The MERV Rating Scale

The MERV scale looks at how effective an air filter is at filtering out particles of various sizes. The more effectively a filter traps small airborne particles, the higher its MERV rating will be. To calculate the MERV rating, filters undergo a test to see what percentage of different-sized particles they can trap. Specifically, filters are tested to see how effective they are at trapping particles that range between 0.3 and 10 microns.

The scale goes from MERV 1 (least efficient) to MERV 20 (most efficient). Anything from MERV 17 to MERV 20 is considered a High-Efficiency Particulate Air or HEPA filter. We will look at HEPA filters in the next section because they are only used in a select number of indoor air quality units.

Air filters designed for residential central HVAC systems typically range between MERV 5 and MERV 12. Anything below MERV 5 is not effective enough to keep dust and debris out of the system, which is why MERV 1 to MERV 4 filters are used mostly in window units or portable air conditioners.

These lower MERV filters can only trap larger particles around 10 microns in size, which include things like dust, pollen, and mites. MERV 5 to MERV 8 filters can typically trap most particles down to three microns, such as cement dust, mold spores, and lint. MERV 9 to MERV 12 filters can trap most particles down to one micron, which includes some bacteria and viruses.

MERV 13 to MERV 16 filters are usually found in commercial or industrial HVAC systems and should generally never be used in residential systems. The reason is that these higher-rated filters are so efficient that it is extremely difficult to draw air in through them. This isn’t typically an issue for commercial or industrial HVAC systems because they tend to have much more powerful blowers that can draw air through a high MERV filter.

However, the blower on a residential system is typically not nearly powerful enough, which means that a high MERV filter would significantly restrict the airflow in the system. Using a high MERV filter in a residential system would basically have the same effect as using a filter clogged in a thick layer of debris. If you were to use a high MERV filter, your blower motor and possibly the furnace might quickly start to overheat. Your system would also produce very little heat or cool air because the filter would drastically limit how much air the blower draws in.

HEPA Filters and Their Uses

HEPA filters are generally only used when indoor air quality is a serious concern or in environments that need to be extremely sterile. In fact, even hospitals rarely use HEPA filters outside surgical rooms, labs, and ICU units. In most cases, a hospital will use a filter rated from MERV 13 to MERV 16 for inpatient rooms and MERV 10 to MERV 12 for the other general areas.

HEPA filters are highly efficient and can trap up to 99.97% of particles as small as 0.3 microns. Even though the airflow through a HEPA filter is normally far too restrictive for a residential HVAC system, there is still a way you can upgrade your home with HEPA filtration. Most whole-home air purification systems have a HEPA filter and work well for people with severe allergies, respiratory issues, or anyone who is highly concerned about indoor air quality.

HEPA filters are typically installed onto a central HVAC system using a bypass. The system is mounted next to your air handler and has one duct that brings air in from the HVAC system. A second duct and an internal fan blow the clean air back into the existing ductwork. This sequence allows HEPA filters to eliminate almost all airborne particles without restricting the air flowing through the HVAC system.

A whole-home air purification system will typically remove almost all airborne allergens, contaminants, and pollutants from the building within the first 24 to 48 hours of use. These systems are also a great way to keep your family healthy and prevent illnesses from spreading because they filter out a high percentage of bacteria and viruses. In addition, most of these systems have a secondary carbon filter that removes odors to keep your home smelling fresh and clean.

How To Know Which Air Filter Is Best for Your HVAC System

Which air filter you should use in your HVAC system usually comes down to how much you’re willing to pay and your specific priorities for air purification. If your primary concern is ensuring that dust and debris can’t clog your heating and cooling system, anything rated between MERV 6 and MERV 8 should be sufficient. If you have allergies, or respiratory problems, or are simply looking to improve your home’s indoor air quality, a MERV 10 to MERV 12 filter should accomplish your goals.

You can also opt to install a whole-home air filtration system, which is usually equipped with a MERV 13 to MERV 15 filter. However, depending on the system, you may need to upgrade your blower to accommodate one of these units. Doing this is necessary because, unlike whole-home air purifiers, these filtration systems are typically installed inside your existing ductwork. Nonetheless, filtration systems exist that use an ion exchange process to attract and capture airborne particles without restricting airflow.

Do you want to improve your home’s air quality? Southport Home Services is here to help! We install a range of indoor air quality equipment, including air purifiers, UV lights, air cleaners, and whole-home humidifiers and dehumidifiers. We also offer a full scope of heating, cooling, and plumbing services for residential and commercial customers in the Wausau, Madison, and Caledonia areas. Contact Southport Home Services today if you have any questions or to schedule a service call.

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Thomas Suchla
Thomas Suchla


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