Lennox X6667 Healthy Climate PureAir PCO-20C MERV 10 replacement filter - 21" x 26" x 4" - Fits the larger PCO-20C PureAir unit. The Lennox Healthy Climate PureAir replacement furnace filter uses a MERV 10 pleated filter to capture particles down to 0.3 micron. In most home environments, this filter only needs to be replaced every 6 months. Replace every 6 months in standard home environments.