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Air Filtration Basics: From MERV To HEPA

Published on: July 3, 2013

Clean air is something we all want, and proper air filtration plays a big role in keeping indoor air fresh and healthy. Selecting the best air filter for your HVAC system and keeping it squeaky clean will not only ensure the highest possible indoor air quality, it will also help your equipment to perform optimally while keeping energy costs low.

What Are MERV Ratings?

You’ve probably heard about MERV values with respect to air filters. MERV stands for Minimum Efficiency Reporting Value. This system of rating air filters was created by the American Society of Heating, Refrigerating and Air Conditioning Engineers. A MERV value can be anywhere from 1 to 20. Simply put, the higher the MERV, the more particles a filter will catch. Some of the particles trapped by air filters include dust, animal dander, mites, mold spores, pollen, viruses, allergens, skin flakes, insect droppings, carpet fibers, bacteria, airborne chemicals and cigarette smoke.

What Are HEPA Filters?

Most residential air filters have a MERV value between 1 and 16. Filters rated between 17 and 20 are High Efficiency Particulate Arresting (HEPA) filters that are used primarily in medical facilities and laboratories. To qualify for this designation, a filter must remove 99.97 percent of pollutants from the air. Although these filters can be very beneficial for those with allergies or asthma, they can also severely restrict HVAC airflow and damage your equipment. Never install this kind of filter on your HVAC system without first consulting a professional HVAC contractor with credentials and experience comparable to those contractors listed on the AC Southeast website.

Which MERV Values Mean What?

It might seem at first glance that an air filter with a MERV of 16 will always be the best choice. However, that’s not necessarily true. The MERV value of the filter that’s best for you will depend on the needs of those in your household, the kinds of contaminants that are present in your air, and the HVAC equipment you are currently using. The higher the MERV value of a filter, the smaller the particles it will collect and the better it will be at keeping indoor air clean. Filters with the lowest MERV values remove only the largest airborne particles; they are used primarily to protect HVAC equipment rather than to purify air. Filters with the highest MERVs are usually reserved for those who are especially vulnerable to pollen, allergens, viruses and bacteria. The filter with a MERV that’s right for you and your HAVC equipment will usually be somewhere in the middle range of values.

  • Many residential HVAC filters, typically the disposable panel filters, have MERVs of 4 or less. They may also come with no rating at all. These filters remove less than 20 percent of airborne pollutants from a home.
  • Filters with MERVs of from 5 to 8 will do a much better job of cleaning up your air; they can remove anywhere from 20 to 85 percent of indoor contaminants. There is a lot of variation in these mid-range filters. For example, a filter with a MERV of 5 removes 20 to 34 percent of indoor pollutants, while a filter with a MERV of 8 removes 70 to 85 percent.
  • Filters with MERVs between 9 and 16 have less variation; they all remove 85 percent or more of airborne contaminants with one exception: Filters with MERVs between 13 and 16 also clear the air of bacteria.

An air filter with a MERV of 6 or 7 will remove from 35 to 70 percent of residential airborne contaminants and provide reasonably clean indoor air. An air filter with a MERV between 8 and 12 will remove from 70 to 85 percent of airborne pollutants, keeping indoor air very clean and providing excellent dust control in most residential settings. But regardless of the rating of the filter you choose, you must still keep it scrupulously clean in order to enjoy maximum benefits. 

Types Of Air Filters

Regardless of the type of air filter you select, it should always fit snugly on your furnace or air conditioner. Be certain that the dimensions of any replacement filter match the measurements of the filter that came with your HVAC unit.

  • Washable, reusable air filters are considered by HVAC technicians to be the worst air filters on the market. They come with very low MERV values and need a lot of maintenance. Even worse, they are notorious for accumulating fungi and bacteria that can easily be blown into your home.
  • Fiberglass filters are not very effective. They are designed to protect HVAC systems, not to improve indoor air quality. Although they are replaceable and inexpensive, they are very thin and may remove less than ten percent of indoor pollution. Fiberglass filters typically have MERVs of four or less.
  • Pleated and polyester filters are similar to fiberglass filters but are considerably more effective at trapping pollutants. With MERVs between 8 and 13, they’re great dust-catchers and will remove 70 to 85 percent of residential airborne particles.
  • High efficiency filters, also known as HEPA filters, have MERV values of from 17 to 20 and will catch at least 99.97 percent of indoor air pollutants. These filters can be beneficial for allergy and asthma sufferers and those who are vulnerable to infections because they trap very small particles like mold spores, pollen and bacteria. However, if no one in your household needs extra protection from allergens or bacteria, installing a high efficiency filter is probably not worth the extra expense.

Why It’s Vital To Keep Air Filters Clean

The most essential part of maintaining a healthy indoor environment is to keep HVAC filters clean. Some filters you can clean yourself while others must be replaced. In either case, filters should be cleaned or replaced regularly. The cleaner the filter, the more contaminants it can catch, and the fresher your indoor air will be. A dirty filter not only pollutes your environment, it also forces your HVAC system to work harder. This, in turn, can shorten the lifespan of your equipment. Clogged filters also decrease HVAC efficiency, thereby raising energy costs and decreasing indoor comfort. Any HVAC contractor will tell you that operating heating or cooling units with a dirty filter is asking for trouble. Clogged air filters can harm your equipment, and many repair calls can be prevented when air filters are kept clean. Changing or cleaning your air filter regularly is the most important thing you can do to ensure that your furnace or air conditioner runs smoothly between annual preventive maintenance visits.

How Often Must I Change Or Clean My HVAC Filters?

It depends on your HVAC system and the type of air filtration system you choose. A reputable HVAC contractor can advise you on how often to change your filter, or you can find the information on the packaging that comes with the air filter you select. Most filters must be changed every month, although some can be changed every 90 days and others every 12 months. If you have pets, you may need to change the air filter more frequently than is recommended. Also, when your furnace or air conditioner is in constant use, you will need to change the filter more often.

Which Type Of Air Filtration Will Work Best For Me?

Choosing an air filter that will give you the purest air possible without compromising the airflow of your equipment is something that an AC Southeast contractor can help you with. A filter with a very high MERV might give you ultra-clean air, but it can also restrict HVAC airflow and harm your heating and cooling systems. Essentially, the higher the MERV of a filter, the cleaner your indoor air but the lower the airflow through your HVAC equipment. If no one in your household suffers from asthma, allergies or a compromised immune system, a filter with a very high MERV is not really necessary. The ideal air filter for you will impede HVAC airflow the least while delivering the cleanest possible indoor air.

As a general rule, filtration systems with a MERV rating between 5 and 7 offer good pollution control, while filters with a MERV rating between 8 and 12 offer excellent pollution control. Keep in mind also that even though air filters with MERV values from 13 to 16 will catch more pollutants on the first pass through the filter than air filters with values between 8 and 12, heating and cooling systems recirculate air through the filter. After several passes through a filter, air from which 85 percent of the airborne contaminants have already been removed will become even cleaner with each successive pass.