In the ventilation system "Main lines" are the primary ducts that distribute conditioned air from the central unit to different areas of your home or business. Identify the main lines as the large ducts branching out from the furnace or central unit.

In the ventilation system, "return vents" are the openings or grilles through which air is drawn back into the system for recirculation. They are typically located on walls, ceilings, or floors.

In the ventilation System, "supply vents" are the openings in walls, floors, or ceilings through which conditioned air is delivered to the rooms.