by Stephanie Earley, the marketing manager of filtration products for the Kimberly-Clark Corporation, Roswell, GA. 

Good indoor air quality (IAQ) depends on a number of factors, including effective filtration, which provides the primary defense for building occupants and HVAC equipment against particular pollutants.

Today’s higher standards in filtration, coupled with rigorous attention to filter selection and maintenance, helps to produce cleaner, purer air and reduce IAQ problems.

Which HVAC filter is right for you?

Various particle sizes will cause different types of problems: Facility managers should work to identify the types and sizes of particular pollutants in their buildings to determine the best type of HVAC filter for their needs.

Selecting HVAC filters based on the needs of the facility instead of simply their initial costs will lead to a review of filter efficiency as a determining factor. Filtration efficiency defines how well the filter will remove contaminants.

• Low-efficiency filters are typically used to keep lint and dust from clogging the heating and cooling coils of an HVAC system.
• Medium- and high-efficiency filters are typically used to remove bacteria, pollen, soot and other small particulates.

Initial and sustained efficiency are the primary performance indicators for HVAC filters.

• Initial efficiency refers to the filter’s efficiency “out-of-the-box.”
• Sustained efficiency refers to efficiency levels maintained throughout the service life of the filter.

Some filters have lower initial efficiency and do not achieve high efficiency until a “dirt cake” has built up on the filter – typically after 30 days. Other filters offer both high initial as well as sustained efficiency, meaning they achieve an ideal performance level early and maintain that performance level.

ASHRAE Standards

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) developed two HVAC industry standards that address the efficiency issue: ASHRAE 52.1 and ASHRAE 52.2.

ASHRAE 52.1 measures:

• Pressure drop – how the filter affects air flow and energy costs. A low pressure drop typically translates into higher energy efficiency. A high pressure drop means reduced air flow to the HVAC unit, requiring more energy to operate the unit.
• Arrestance – the amount of synthetic dust a filter is able to capture.
• Dust spot efficiency – a measure of the ability of the filter to remove atmospheric dust from the test air.
• Dust holding capacity – the amount of dust a filter can hold until a specified pressure drop is reached. Higher capacity means a longer filter life. (When evaluating dust holding capacity, it’s important to compare dust holding capacities between filters at the same final pressure drops to make accurate comparisons of projected filter life.)

The ASHRAE 52.2 Standard measures the fractional particle size efficiency (PSE) of an HVAC filter, which indicates the filter’s ability to remove airborne particles between 0.3 and 10 microns in diameter.

MERV ratings

A Minimum Efficiency Reporting Value (MERV) rating is assigned to the filter media depending on the PSE in three different particle size ranges (0.3 to 1 micrometer, 1 to 3 micrometers, and 3 to 10 micrometers).

MERV is a numerical system of rating filters based on a minimum particle size efficiency. A rating of 1 is least efficient, while a rating of 16 is most efficient. Lower ratings typically reflect a more cost-conscious choice.

Other considerations

In addition to the performance factors measured under ASHRAE 52.1 and 52.2, consider these additional variables when selecting a filter:

• Moisture resistance – how high humidity and moisture affect the filter.
• Temperature limitations – how the filter performs at application temperature.
• 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.