Indoor air quality (IAQ) is critical for organizations of all kinds. IAQ is a compliance issue in industries such as healthcare and manufacturing, but it is also important for the health and safety of people in all facilities.

The air filters in a building’s HVAC system are one of the most important factors affecting IAQ. These filters are commonly chosen based on their Minimum Efficiency Rating Value (MERV). While this is a useful metric, it is important for building engineers and maintenance teams to understand what this value really means, and how other factors affect filtration efficiency and overall air quality.

The Basics: What Is MERV and Why Is it Important?


Joe W. Fly Co. CEO, Joe Fly Jr., explains how MERV actually works in practice—and how other factors affect indoor air quality.

MERV is a common system used to classify different filters according to their ability to remove particles from indoor air. A filter’s MERV is calculated using tests defined by ANSI/ASHRAE Standard 52.2—“Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size.”.

The key word to understand in MERV is “Minimum”—the standard tells the user what the filter’s efficiency is at its worst. The previous standard measured average efficiency, which provided a much less specific rating of a filter’s performance.

MERV ratings range from 1 to 16. In most commercial buildings, you’ll find filters with a MERV rating around 8. In critical application—for example, a hospital’s surgery center or a semiconductor plant—you’ll find MERVs closer to 14 or 15.

How Does a Filter’s MERV Value Affect a Building’s Air Quality?

In theory, the higher the MERV, the cleaner the air—but in practice, there are many other factors that affect IAQ. Using a filter with higher MERV should help improve the air quality in a building—but only if all of the filters are properly installed and sealed so there is no unfiltered air getting through. Air bypass can significantly reduce the effectiveness of your HVAC filters.

Are Two Filters with the Same MERV Always Equal?

Filters can differ in cost and efficiency even if they have the same MERV. Filters can have different pressure drops—affecting energy use. Dust-holding capacity is another important factor, as it affects how often the filter needs to be replaced.

It’s also important to understand that MERV is a range, so two different filters rated MERV 8 aren’t necessarily 100% equal in terms of filtration efficiency. One filter might be at the lower end of the MERV 8 range, closer to MERV 7, while another filter might be at the higher end of the range, closer to MERV 9.

Is There a Better Way to Evaluate Filter Efficiency?

The best way to understand the MERV of a filter is to look at an independent ASHRAE 52.2 test. Examining the complete result of the ASHRAE 52.2 test provides a more accurate picture of how effective a particular filter is. An independent test can be supplied from any manufacturer.

What Happens if You Mix and Match Different MERV Filters?

Because air takes the path of least resistance, more air will be forced through the filter with the lower MERV—reducing air quality toward the level of the “weakest MERV in the chain.” Using filters that don’t fit together properly also reduces IAQ, because unfiltered air will flow through the gaps.

How Do Gaps Between Filters and Frames Affect IAQ?

Air Quality is severely impacted by gaps in a filter bank. Air always takes the path of least resistance, so unfiltered air will flow through any gaps between filters and/or holding frames. Although the individual filters might be MERV 14, if you tested the entire filter bank, it could be performing as low as a MERV 8 because of air bypass.

How Can You Identify Efficiency Issues in Your System?

Because so many different factors affect IAQ and filter performance, it helps to have your equipment examined by air filtration experts. Joe W. Fly Co. provides free on-site system survey and evaluation, and turn-key filter installation services for organizations across all market sectors in Texas and surrounding state.

 

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