Cold air return vents work by allowing cool air to be suctioned into a furnace or heating/air conditioning system. The cool air is then heated or cooled, depending on the system setting, and forced back out into the home or building.
Some cold air returns draw air from outside and some from inside the structure. The major benefit of drawing inside air is that it is closer to the desired room temperature. This benefit helps use less energy to heat or cool the building.
Cold air vents are typically positioned near the floor where the coolest air is located. Because heat rises, placing a cold air return vent high on a wall leaves cold pockets of air at floor level and makes the room harder to heat. During the warmer months, vents positioned near the floor draw the coldest air to efficiently cool the building faster and with less energy.
Heat and cooling registers usually look the same as cold air return vents. If a tissue is held in front of a cold air vent, the vent pulls it because the system sucks the air into it. If the vent is a heat or cooling register, it instead pushes the tissue away.