How Do Return Air Vents Work?

Return air vents as part of the ventilation cycle

The typical ventilation cycle in a home starts with air entering the cold air return. A fan on the furnace sucks the air into the furnace, drawing it through a filter. The air passes through a heat chamber where it gets warm. The warmth is created either by combustion due to natural gas or propane ignition or electrical conduction. The air flows through the chamber and is pushed to the vent pipes by a blower fan.

Where are cold air returns located?

Cold air return boxes are typically located near the center of a house. Older homes (pre-1960 construction) typically have one large return on each floor. Homes built in the 1960s, 70s and 80s often have a cold air return in each room. With the advent of high-efficiency, high-capacity furnaces in the 1990s, homes needed fewer returns. The number of returns are not dependent on the number of HVAC (heating, ventilation, air conditioning) air vents.

Maintaining a cold air return

Because cold air returns draw air in to the furnace, there is slight suction. The force often picks up air particles and draws them into the HVAC system. On a clean vent, dust and other particles go straight to the in-line furnace filter. As time passes, dust collects at the vent, caking up around the entry and choking off the air intake. This causes the furnace to work harder to pull in more air. Simple dusting on a regular basis can keep a furnace working properly.