My pier and beam home is 1900sqft single story built in '52. I moved into it about 10 years ago, and the indoor unit was set up to suck the return air directly out of the bottom thru the floor from the crawl space. Every exterior wall inside the house has a return air vent on the floor. Also the dirt floor in the crawl space was all covered in clear plastic. There are also exterior crawl space vents with wire mesh surrounding the house, but these were and are not sealed up from the outside air, only a metal screen. I became concerned early on with my house that my HVAC filters would hardly ever get dirty, even after leaving them in for months. Then I became really concerned after I went thru and replaced/upgraded all my wiring devices thru the house (switches & outlets), every switch and outlet box were caked in nasty dust (the kind of dust I should have been seeing in my hvac filter). I naturally assumed all my return air vents must be clogged, but they were only a little dirty which I thorough cleaned. I am confused to why my unit wants to use my walls as an hvac filter instead of thru the return air vents or even the outside air via the unsealed exterior crawl space vents. Is my house too large for such a return air set-up? I actually would rather have the return air set-up changed to come straight from the house so I can better filter my interior air. AND I don't trust the air coming from my crawl space anyways cause we're talking 60 years of pest control dust, mold left over from a leaks before I got there, and the occassional decomposing rat carcus. Sorry for the long post. I greatly appreciate your time and suggestions.