Typical results over 12 months   Click on graphics for larger images. The airflow restriction on an airfilter is measured by presuure drop in inches of water (w.g.). According to the Modern Refrigeration and Air Conditioning manual the pressure drop of a dirty filter should not exceed .5" w.g. (see excerpt below) the pressure drop on the average 12 month usage of the Enviro Max® filter is far less than the accepted level. New pressure drop = .12" w.g. 4 months pressure drop = .16" w.g.   8 months pressure drop = .22" w.g. 12 months pressure drop = .24" w.g. Excerpt from Modern Refrigeration and Air Conditionaing © 2000.