Closing air vents in the home to save energy is a longstanding energy-saving theory. As spring turns toward summer, in some households shutting air vents in rooms that won’t be occupied during the season to prevent wasted energy is a yearly tradition. And why not? It makes perfect sense not to route expensive conditioned air into empty rooms. That’s what most people thought until scientists at the Lawrence Berkeley National Laboratories decided to settle the matter in 2003. What was once common knowledge now turns out to be something closer to an urban legend. Research shows that closing air vents not only doesn’t save energy, it may actually increase operating costs. Another energy-saving myth bites the dust.
The misconception about closing air vents probably got started back in the day when homes were uninsulated and leaked air like a hay barn. Today, however, residences typically have several inches of insulation inside walls and a foot or more in the attic. In addition, homes are built to high standards of airtightness. The heating and cooling system that keeps these homes comfortable is designed to deliver a precisely calibrated volume of air to meet the structure’s heating or cooling load requirement. System capacity and ductwork design calculations are made under the assumption that air passageways to each room will remain unobstructed. When one or more supply vents are closed, a Pandora’s box of potential dysfunction opens.
Here’s a rundown on some of the reasons why closing air vents to save energy is an idea whose time has come — and gone:
The output of the blower fan and the size of your supply ducts are matched to push a certain amount of air to all rooms in the home. Meanwhile, the return ducts are designed to pull an equal volume of air out of the rooms and convey it back to the unit. This creates a condition of neutral air pressure in each room. When the air supply vents in a room are closed, the return vents — which can’t be closed — continue to pull air out of the room, inducing negative pressure. A depressurized room draws warm or cold outdoor air in through the multiplicity of tiny cracks and gaps present in any home, lowering the room temperature substantially. This enclosed zone of acute warm or cold air and negative pressure functions as a cold or heat sink that draws cool air or warmth out of adjoining rooms by conduction through walls and ceilings. Your unit runs longer “on” cycles to compensate for the lost cold air or heat energy in the living spaces and cooling or heating costs climb.
Your ducts leak heated and cooled air. The only question is, how much? The Department of Energy estimates that residential ductwork in the typical home leaks at least 20 percent of the conditioned air it conveys. Homes built more than 10 years ago, when inexpensive, less durable ductwork was common, tend to leak even more. However, the amount of leakage varies according to the static pressure inside the ductwork. Closing even a single supply outlet increases air pressure in the system and all leaks spill even greater amounts of cooled or heated air into unconditioned areas like the attic or crawl space. As conditioned air is forced out through leaks, rooms take longer to reach thermostat settings and the unit works overtime. Berkeley National Laboratory researchers determined that whatever energy gains may be realized by diverting conditioned air from an unoccupied room are more than offset by losses from increased leakage.
In an HVAC unit, unrestricted system airflow not only assures optimum energy-efficiency and performance, it protects internal components. When airflow is obstructed by closed vents, the blower fan is pushing against increased static pressure and may overheat and/or wear out prematurely. In addition, reduced airflow can overheat the unit, tripping high-limit switches that shut down the system or even damaging the heat exchanger, a critical safety component, as well as the single most expensive part in the system.