CH-09-022 — Laboratory Measurements to Quantify the Effect of Bypass on Filtration Efficiency

The issue of filter bypass has long been a topic of much interest in the HVAC industry, but to date, there has been limited work that quantifies the effect of bypass on filtration efficiency. In this research, an ASHRAE 52.2 compliant test loop was used to study bypass. A matrix consisting of filters ranging from MERV 2 to MERV 14, different gap geometries, and gap sizes was completed. The results show that the efficiency degradation effects of filter bypass are more pronounced with increasing filter efficiency, as well as increasing gap size. For the first set of tests, the pressure drop across the test filter was held constant with and without bypass. It is recognized that this assumption is not realistic in all field installations. As a result, a second set of tests was developed and completed where fan speed was held constant with and without filter bypass. The results suggest that filter bypass can cause significant and predictable degradation of filter efficiency and an assessment of bypass is essential for high-performance filtration.