While vacuuming today we decided to take off the floor registers. Whoah. Scary stuff. One of the registers had about two inches of dirt in the ductwork underneath. I also found rubber bands, cigar wrappers, washers, sticks, a piece of drywall, and a Lego spear.
Then we vacuumed under the fridge. Even scarier. The coils were enrobed in probably an inch or two of dust.
Did the previous owners just never clean anything?