It has some odd-sized air filter, the number escapes me, but that never mattered much as it also came with two washable foam filters.
Well, as I have gotten older and the climate has warmed each winter due to global warming (NOT), I find myself less willing to wander outside in the snow, take off the form faucet cover, uncoil the frozen hose and run ice cold water over my hands while trying to get dish detergent to foam so I can wash the X%#@* foam filter each month.
So, I got the bright idea of buying the disposable filters in 20 X 20 X 1 size at my local hardware store for 79 cents and cutting them to size.
Works fine, but as I cut the darn thing, I find I lose track of the "flow direction" -- on the side of these filters they have an arrow with the word flow, which I assume means the filter is only supposed to be installed with one side UP. As I write this, I just realized I could probably use a sharpie to put red "X" on the up side and that would solve my problem....but since I am already so far along on this message.....
Does it REALLY matter which side goes up on these things? Each side look identical to me. For those who care, here is a pix from the filter maker's website: http://www.aafintl.com/upload/product%20media/images/air%20filter/stratadensity_panel.jpg