I have a Lennox electric, forced air, furnance that came with the 1970
house I bought a decade or so ago.
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
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
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: