Filter the Way to Cleaner Air

There are many sources that contaminate the indoor air we breathe. But, did you know that the HVAC system is the primary solution to help rid indoor spaces of air pollution? There are three ways HVAC systems dispose of indoor air pollution: 

  1. Filter it
  2. Dilute it with outside air, or
  3. Exhaust it to the outside

Today's post focuses on the filter types available for your HVAC system to clean the air.


Before we dig into differences, it helps to understand there is a method to rating how efficient a filter is. It is called the MERV rating. MERV stands for Minimum Efficiency Reporting Value.

MERV indicates a filter’s ability to remove airborne particles of differing sizes between 0.3 and 10 microns in diameter. A MERV rating is a numerical number assigned to the filter depending on the PSE (Particle Size Efficiency) in three different particle size ranges (0.3 to 1 micron, 1 to 3 microns, and 3 to 10 microns). A rating of 1 is least efficient, while a rating of 16 is most efficient.  Note:  ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) has developed an HVAC filter test standard to quantify the efficiency of filters. The ASHRAE 52.2-1999 standard measures the fractional PSE of an HVAC filter. 


Types of Filters 101

Fiberglass filters. This throwaway air filter is the most common type. Layered fiberglass fibers are laid over each other to form the filter media and are reinforced with a grating that supports the fiberglass to prevent failure and collapse. Fiberglass filters protect the HVAC equipment from dirt build-up, but don’t do much to clean the airstream. The typical MERV ratings for this filter are 1 to 4.  A filter that is similar is the cleanable foam filter.

Pleated filters. These filters are like fiberglass filters but have a higher resistance to airflow along with superior dust-stopping ability. Pleated filters can be made from fiberglass, polyester, paper or other synthetic materials. An antimicrobial material can be imbedded into the fiber to resist moisture and prevent mold growth. The MERV rating is between 5 and 8.

High Efficiency Particulate Air (HEPA) filters. These units filter the air passing through a very fine mesh. These filters remove 99.97 percent of all particles 0.3 microns or larger. HEPA filters remove dust, smoke, dander, mold spores, and allergens. This type of filter is used in hospitals, museums, libraries (preserving old archives), pharmaceutical and in clean rooms. In addition, pre-filters are frequently used upstream of the HEPA filter to provide gross removal of dirt so that it does not get clogged in the fine-scale filtering of the HEPA filter. The MERV rating is between 10 and 15.

Electrostatic filters. These filters use electricity to provide a positive charge to the particles in the airstream. The positive charged particles are then attracted to the negative charged plates or media within the filter. The particles get removed from the airstream but stick to the plates until the plates are cleaned.

Activated Carbon filters. This porous charcoal (high carbon content) product is used to trap significant odors. The amount of carbon activation affects the filter’s ability to absorb odors and the longevity of the filter itself. Carbon filters are used to trap fumes, gases, VOC’s, formaldehyde, ozone and particulate.

Note: The filters listed here cover most HVAC applications; however, for industrial air cleaning there are many other ways to clean air including scrubbing the air or using chemical reactions. 

If you would like an evaluation of your facility’s HVAC system or to discuss filter options, give our Mechanical Engineering Department a call at 440.953.8760.

Authored by John Milenius, PE, LEED AP BD+C. John serves as Tec’s Director of Mechanical Engineering. He is responsible for the engineering—renovation and new construction—of commercial, industrial, institutional, and federal buildings, always with an eye towards improving or maximizing energy efficiency. John has more than 29 years experience as a mechanical engineer designing HVAC, plumbing, and fire protection systems.  A runner, John enjoys the feeling of filling his lungs with clean air on a crisp, clear day.

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