The dog days of summer provide an opportune time to review cold air returns and in the process provide a rarely discussed tip for improving air conditioner performance. Sealing and insulating ductwork, closing window shades, weatherproofing windows and doors, and installing a programmable thermostat are all effective tips that any expert will recommend to make your air conditioner more efficient. However, removing obstructions from cold air return grilles is one tip that often gets forgotten by even the experts.
Before discussing how eliminating obstructions from cold air returns can improve air conditioner performance, it is important to understand how modern forced air heating and cooling systems function. A fan circulates air through your home by blowing conditioned air through supply vents and sucking stale air back through return vents. That’s right. Your air conditioner sucks and that is a good thing. The system recycles air by sucking it back through air returns. The recycled air is filtered, dehumidified, chilled (or heated, depending on the season), and then forced back through registers to cool (or warm) your home.
Traditionally, air return vent covers were called "cold air returns" and registers were called "heat registers". Homes were heated long before they were cooled with air conditioning. In the early days, cold air passed through air returns and warmed air passed through heat registers. The introduction of economical residential central air conditioning brought about another dimension to air returns and registers. In the winter, cold air still passes through air returns and warm air passes through registers but in the summer, their roles reverse. Air returns still suck and registers still blow regardless of the season but in the summer time, it is warm air that passes through air returns and cold air that passes through registers. Although the terms "cold air return" and "heat register" are now a bit antiquated, they are still used frequently.
With science and history out of the way, we wrap things up with some home economics. To reduce summer cooling expense, improve the efficiency of your air conditioner by removing obstacles from air return grilles. If the air return is blocked, it causes the blower to work harder to recirculate air. If you've ever ran over a throw rug with a vacuum cleaner you've seen and heard first hand how much harder the vacuum cleaner works to create suction with the throw rug sucked into its port. A similar reaction occurs when your air conditioner fan is forced to suck air through blocked air return covers. For a number of reasons, registers are often used in place of grilles on air returns. In most cases this is not a problem but generally the register's dampers should remain open if the register is covering an air return. If the same air return drop leads to vents in multiple rooms, you might adjust the dampers on registers that are close to the blower to create greater suction from air returns that are further in proximity from the blower. If the ductwork is properly implemented, this should not be necessary but since little engineering gets applied to residential ductwork, adjusting dampers on air returns can be used to balance air flow. As a rule of thumb, however, leave cold air returns open to improve the efficiency of your air conditioner and reduce your summer cooling bill.