All the systems have return and supply sides to recirculate the air in your house, they require alternate cleaning. All supply vents that blow hot or cold air in the house come from supply main duct (line). Some systems may have relatively the same return side construction (multiple return vents). However some systems do not have return duct system, they have either single return vent or in very rare cases do not have one (mobile homes). The biggest part of any system is the main line (supply and return) main lines contain most of dust and debris.