Effective air filtration provides the primary defense for building occupants and HVAC equipment against pollutants generated within a building as well as pollutants from air drawn into a building from the HVAC system. That is why selecting the right HVAC filter is so critical. With today’s higher standards in filtration, it is possible to produce cleaner, purer air and reduce indoor air quality (IAQ) problems.

The Filtration Efficiency Equation

The first step in determining the best type of HVAC filter needed is to identify the types and sizes of particular pollutants in the building. Figure 1 shows the sizes of various particles that may cause IAQ problems — from submicron smokes and fumes to larger dust particles.

Removal of all airborne contaminants is simply not practical in most facilities, so once problematic pollutants are identified, it is time to look at filter efficiency. Filtration efficiency defines how well the filter cleans indoor air by removing airborne particles.

Low-efficiency filters (in the range of 25 percent efficiency on 3 – 10 micron particles) are typically used to keep lint and dust from clogging the heating and cooling coils of an HVAC system. Medium- and high-efficiency filters (up to 95 percent efficiency on 3-10 micron particles) are typically used to remove bacteria, pollen, soot and other small particulates.

ASHRAE HVAC Standard

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) has developed an HVAC filter test standard to quantify the efficiency of filters. The ASHRAE 52.2-2007 standard measures the fractional particle size efficiency (PSE) of an HVAC filter. This indicates the filter’s ability to remove airborne particles of differing sizes between 0.3 and 10 microns in diameter. A Minimum Efficiency Reporting Value (MERV) is assigned to the filter depending on the PSE in three different particle size ranges (0.3 to 1 micron, 1 to 3 microns, and 3 to 10 microns). The MERV is a numerical system based on minimum PSE. A MERV of 5 is least efficient, while a MERV of 16 is most efficient. Recent studies suggest that an appropriate minimum efficiency is MERV 8 to provide good HVAC system cleanliness and efficient operation.

In addition to the performance factors measured under the ASHRAE 52.2 Test Standard, consider these additional variables when selecting a filter:

• Energy efficiency – how the filter’s resistance to airflow affects the energy consumed by the HVAC system. Energy expenditures can account for about 81 percent of the annual operating costs of an air filtration system.
• Moisture resistance — how high humidity and moisture affect the filter.
• Temperature limitations — how the filter performs at application temperatures.
• Flammability — how the filter performs in flammability tests. Check to see if UL Class I or Class II rated filters are needed to conform to local building codes.

Tip: To make the right filter choice, do not simply focus on MERV, as the MERV scale is non-linear. To make the best choice, review the Fractional Particle Size vs. Particle Diameter curve that is included with the test report. The curve will tell you the efficiency of the filter for the specific particle size you are interested in. Refer to Figure 1 to determine the size of the particles you are interested in capturing.

Filter Technology

There are many types of HVAC filters on the market today. In most buildings, the best filter choice is a medium-efficiency pleated filter (MERV 7-8), which has a large filter media area. Keep in mind that large filter media areas tend to be more cost-effective than smaller ones. Large filter media areas mean lower resistance to airflow and greater contaminant-holding capacity. Lower resistance to airflow reduces fan energy requirements, and greater contaminant-holding capacity may mean fewer filter changes.

In some buildings, it may be appropriate to use both a medium-efficiency pre-filter and a high-efficiency extended surface filter. The major advantage of using a pre-filter is to extend the life of the high-efficiency filter. In the absence of pre-filters, high-efficiency filters may load quickly with large particles, which causes the airflow resistance to increase, thus increasing the load on the system fans, and potentially causing filter bypass due to leakage at joints in the HVAC system.

Controlling gaseous pollutants requires specialized air filtration products. Traditional particulate air filters — even HEPA filters — are not effective at removing gaseous contaminants. Most gas phase air filters are made with activated carbon. Of all the adsorbents known, activated carbon is one of the strongest physical adsorbents, so it is an excellent material for creating a fresh and clean-smelling environment by removing odors and other gaseous contaminants from the air.

Advanced Synthetic Filter Media

Pleated air filters used in HVAC systems are made with a wide range of filter media, including fiberglass, polyester, paper and synthetic nonwoven materials. Recent advances in nonwoven technologies have allowed for improvement in both performance and value of synthetic filter media over the standard filter media used for years in HVAC filters.

Unlike traditional media, the synthetic filter media in more modern filters can be made of thermally bonded, continuous, hydrophobic (moisture-repelling), polyolefin fibers that resist shedding and do not absorb moisture. This is important in resisting bacterial growth, and it keeps shed fibers from getting into the HVAC coils or into the air space of the building. Moreover, synthetic filter media can be manufactured without the use of chemical binders, meaning that humidity will not affect the structure of the filter. Unlike traditional filter media, which are made with a surface-loading structure, synthetic filter media can be made with a gradient density structure that provides a solid mechanical foundation to maintain high efficiency over the useful life of the filter.

For more information on indoor air quality and air filtration issues, visit the Resource Center at www.kcfiltration.com.