Will a filter perform in your HVAC system the same way
it performed in a test laboratory?

This is actually a very important question and one that is not asked or addressed frequently enough. Logic tells us that the answer is no. Test laboratories are controlled environments and ‘real world’ HVAC systems can be anything but controlled. All filter testing standards are accelerated tests and utilize a ‘synthetic’ test dust to speed up the testing.

Atmospheric Dust magnification 5000 times ASHRAE Test Dust magnified 1000 times

The test dust is significantly different than ‘typical’ atmospheric dust (see photo above) and this can lead to distorted test results. These results can be even more distorted when you consider higher efficiency filters typically have prefiltration and are not exposed to large particulate like that present in the test dust.

Another common difference is bypass of unfiltered air. During filter testing all bypass is eliminated by taping the filter into the test duct (see photo below) unlike what happens in the real world. Studies have shown that the bypass of unfiltered air can dramatically lower the effective efficiency of high efficiency filters.

Filter being ‘taped’ into a test duct to eliminate by-pass.

ASHRAE 52.2, the current test standard, only utilizes the lowest efficiency numbers from the test to determine the efficiency rating – these numbers are almost always the initial efficiency numbers - this tends to under-represent what the efficiency will be after a dirt cake is formed during the filters life.

Another significant issue is efficiency degradation – certain medias that filters are manufactured with have electrostatic properties (sometimes additionally enhanced during manufacturing). These medias suffer a quick and dramatic efficiency loss when put into service and are unable to reach their rated efficiencies again.  

This  phenomenon has been documented by numerous scientific studies. When these filters are tested under the current test methods the degradation in efficiency does not occur - so the minimum efficiency of the filter is dramatically overstated from what occurs in real world applications.

All of the above points can cause a filter not to perform as stated on a test report. So the conclusion is to be aware of these factors and eliminate as many of them as possible (i.e. – utilize filters that do not suffer efficiency degradation, eliminate bypass in HVAC system and use gasketing on filters or filters that self-gasket).

It is important to confirm the ‘System’ efficiency by utilizing particle counters to take particle counts upstream and downstream of the filter bank to validate the filtration systems performance. ASHRAE is in the process of issuing guidelines to perform this validation.

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