Glad to hear this thread was worth while to post
Well since we're on the topic,how's you're evap coil looking?
The down stair's unit(s) always get dirtier than elevated one's, because their gravity fed. It doesn't matter how clean you keep you're house,all thing's fall to the ground and they get sucked into the return grills.Dog & cat fur/ dander are the biggest culprit,but rug debris ,new clothes contribute,also crap that get's carried in from pet's, people's boot's/shoes like dirt,dust ect. it's unbelievable the amount of stuff that get's in there and pack the evap coil.
If you're air media is 1"thick I will bet a bucket of eels that it is not tight to the sleeve that it is supposed to slide into, not just you'rs personally but anybody and every body's this is a common problem,mostly from bad install's people just don't take pride in what they do,so if you see a collection of dirt around the filter opening or track door (even factory doors)they are equally to blame, the foam gasket is only on one side (should be on 4 sides,but it get's worn down and dose not block what it was intended for,so they suck too!Also they'll manufacture a unique cabinet which will require a filter size that is tough to acquire so people put in the closest match that they can find.......Which equals chitty protection for the coil.
Any way,this is an added ease point for debris to get in and the coil will get packed to the point that it will restrict air flow and freeze up.
I've seen it so thick that it look's like a damp wet black piece of construction paper was placed over the inside of you're "A coil".Nasty stuff!
Every install that I do I put in a Air bear cabinet it's 4" thick gasket lined and good for 2,000 CFM. If it's just for A/C the filter is good for a year, if you using it for heating and cooling it's recommended to change the filter twice a year.