Media post-filter provides enhanced filtration on select models. Solid state power supply is self-regulating and maintains peak efficiency during a wide range of cell dirt loading conditions. Low-profile door with test button to check system operation. Helps filter efficiency-robbing "gunk" before it coats critical system parts. Very low pressure drop. Exclusive 10-year Clean Coil Guarantee. Rugged zinc-coated, roll-formed cabinet provides superior strength and corrosion protection. Efficiency Standard: Efficiency ratings are based on American Society of Heating, Refrigerating and Air-Conditioning Engineers Standard 52.2-1999.,Efficiency ranges are defined for small particles, E1=0.3 to 1.0 microns; medium particles, E2=1.0 to 3.0 microns; and large part. Fractional Efficiency: E1=73%, E2=88%, E3=95%.