If the furnace always has a filter...then only when the house is built as gypsum pittsburgh duct cleaning
gets into the ducting. The builder cleans it out before selling it, so never? It is a money scam. If you never have a filter in your furnace then the dust is in all the ducting and needs to be cleaned out yearly. Cold air vents in the middle wall of the house sucks the cold floor air down to the furnace and through the filter and clean hot air is pumped out. So the hot air vents remain clean. All the dust is caught in the filter which you change yearly.