Lennox Corporation X6667 Filter 21x26x4 MERV 10 Use in Lennox PureAir PCO-20C Air Cleaner MERV 10 Efficiency to capture particles as small as 0.3 microns Actual Size: 20-3/4" x 25-3/4" x 3-3/4" Replace every 6 months in normal use Genuine Lennox OEM Filter