Perhaps you've seen the random flyers in the mail. They often have headlines similar to "Whole House Air Duct Cleaning" and a special price ranging from $40 to $80. The specials include seemingly many things: unlimited vents, one return duct, and one main duct.
I was quite certain we needed our vents cleaned. Waiting too long to change out our central air unit's filter yields a disgusting, solid black film. Clearly these ducts had never been cleaned since the townhome's construction in 1987.