[HayZee518]

a furnace like hot air needs make up air to replace what's blown into a room. this is accomplished by a cold air return from the room(s) as you know heat rises and pushes cold air down near floor level. a register near the floor takes this cold air and returns it to the furnace either through a duct or one hollow stud space in one wall. the remainder of the cold air return can be ajoist space in the basement covered with sheet metal. near the furnace a duct connects to the furnace or is left open to enter where an air filter is in the blower compartment.