The Lennox X6673 MERV 11 Furnace filter is a 5-inch deep pleated filter. The 20″x25″x5 version we tested contained 14 pleats. Its excellent permeability score stands out among other pleated filters we have tested, however, its filtration efficiency is far from impressive.

At 1,280 cfm, the pressure drop came at a quarter that of the worst performers in the department, the permeability department. That’s a good thing if your furnace tends to overheat or if your AC tends to freeze. It’s also great for your energy bills and your HVAC system’s general operating cost. Since it’s a 5-inch deep filter, it tends to hold more dust than the standard 1-inch filter.

As for filtration performance, the X6673 gets performed poorly at the sub-micron range (24.5% @ 0.3-1µm) offers mediocre performance at the 1-3µm level (57.1%). It does an OK 3-10µm level (81.3%). Please note that Fltest.org does not incorporate this category (3-10µm) into our total score weight.