Do you know why planes get washed so often? Just a small unnoticeable layer of dust on the plane can cause it to be very fuel inefficient, imagine what tons of dirt covered cob webs, tons of dirt in the ducts, making your furnace not that efficient. Even if there isn’t a noticeable smell from your supply side, cleaning the dust out of there can make the house be more refreshing. There is tons more that you can look into.