For a heating system's cold air returns to function effectively, a house needs them in every room, open and uncovered, ensuring the even distribution of heat. Depending on the size and configuration of the house, cold air return registers should be low on the floor or near the ceiling.
Rather than an external fresh air vent, a cold air return register draws cooling air from inside the house, bringing it back to the furnace to be heated again. Insufficient cold air returns cause air stagnancy, stuffiness and cold spots. Multi-story houses with floor heating vents require cold air returns placed high to draw air up, while they function better near in the floors in smaller houses.