Whole-House Air Purifiers: Efficiency Report (cont.)

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HEY, MERV. Three years ago, we told you that the top minimum efficiency reporting value (MERV) had increased dramatically for media filter cabinets. (Electronic air cleaners aren’t rated in MERV because of different testing criteria; they use a MERV-equivalent rating.) That rise apparently hit a ceiling, because the top rating for a whole-house air purifier remains MERV 16, which means that the filter can capture 95 percent of particles down to 0.3 microns in size.

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Of course, models that have high-efficiency particulate air (HEPA) filters that capture 99.97 percent of particles and essentially are rated as high as MERV 20 continue to exist. The important factor to keep in mind, though, is that you likely aren’t getting the air-cleaning performance that you’re led to believe that you’ll get by the MERV rating that’s listed by manufacturers.

Testing methodology that American Society of Heating, Refrigerating and Air-Conditioning Engineers uses to determine MERV is a problem, says Kirk Sullivan of manufacturer IQAir. Because the tests are carried out in glass chambers that are the size of a small bathroom and not the entire square footage of a home, it doesn’t reflect real-world conditions, he says. The tests don’t consider leakage through poorly fitted seams, which can send more particles into the airstream if the purifier isn’t installed properly in the ductwork of your heating and cooling, or HVAC, system.

Another flaw of the MERV-rating test is that a manufacturer can specify the speed at which it would like its air purifier tested, which will tweak the results for a better rating, says Glory Dolphin, who is the CEO of IQAir North America. A typical HVAC system runs at a speed of 2,000 cubic feet per minute (cfm). A manufacturer can opt to have its whole-house air purifier tested at, say, 400 cfm, which would produce a higher MERV rating, Dolphin says. 

Although whole-house air purifiers are tested in a simulated HVAC system in a lab, technicians test the purifier on a ductless system. Ducts differ in every home, so what works well in a lab might not work so well when it comes to your HVAC system.

The three independent experts with whom we spoke agree. Consequently, they say, whole-house air purifiers that claim an efficiency of, say, 90 percent to 95 percent in capturing particles really are closer to a capture efficiency of 80 percent to 85 percent.

We believe that when you shop for a whole-house air purifier, you should knock off two points of MERV from the stated amount to get a more realistic idea of a model’s effectiveness. In other words, if a whole-house air purifier is rated, say, MERV 13, you should consider it to be MERV 11 to make a healthier assessment before you buy. 

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