Keeping It Neutral
A state of neutral air pressure in a room is the optimum condition for cooling. The interior of a home contains a finite volume of air. Supply ducts distribute air into rooms under positive pressure at a velocity and volume determined by the output of the blower in the air handler, the sizing of the supply ductwork and other factors relating to duct layout. Air is pulled out of the rooms into the returns and circulated back to the air handler at slightly negative pressure created by the blower suction. In a typical residential system, the returns convey about 400 cubic feet per minute of air back to the air handler. When the supply influx and return air volume are properly balanced, a state of neutral air pressure in the room results, and cooling is optimized.
Pressure imbalances resulting from defective returns diminish cooling performance. Because returns are under negative pressure, leaks in a return duct pull outside air into the system, usually from an attic or crawl space where the returns are typically routed. This additional air volume unbalances the system, tipping the balance of room pressure from neutral to positive. A room under positive air pressure leaks cooled, conditioned air out of the house through every crack and gap in the structure. The comfort level in the room declines as cool air is pushed out and the temperature rises. Utility costs also increase as the air conditioner runs longer “on” cycles to meet thermostat settings.
It's The Humidity, Too
Humidity reduction is another reason cool comfort depends on proper recirculation by the returns. Extracting humidity from interior air is an important function of central air conditioning. Dry air cools more efficiently. Humid air, conversely, holds more heat than dry air, and humid rooms feel warmer. Interior water vapor originates from household activities, including cooking, bathing and simply human respiration. If the returns are not adequate to circulate household air back to the air handler where humidity is condensed by the evaporator coil, water vapor will quickly accumulate in living spaces, and cooling will suffer.
Starving For Air
Proper return air flow is at risk in homes where, instead of dedicated return vents in each room, a single central return serves multiple rooms. Anytime the door to a room with a supply outlet but no return vent is closed, the air path from the supply vent in that room to the central return is obstructed. This blocks the recirculation of air and starves the air handler of adequate air volume, resulting in reduced airflow to other rooms and impacting cool comfort throughout the entire house.