I dont know how dirty you call dirty ,the point is they loose
effectiveness quickly When they are really dirty , they have lost
cleaning ability long ago, where paper gets better. Its just better to
clean them often. Check your blower and furnace to see if you have
buildup , that will help determine if you wash it enough. Also I
beleive they pull around 100 watts For me that would add 85.00 a year
running 12 hrs a day. My large paper filter costs 20.00 and lasts a
year. But true It doesnt run 12 hr , unless its real hot or cold out.
Either way , there is a trade off.