While it would seem sensible to close the air vents (supply registers) in unused rooms to save a little on your heating/cooling cost each month, it doesn’t work out that way. Here’s four reasons why:

1) Let’s start with the fact that a well-designed HVAC system has ducts that are sized—going from a larger to smaller cross-section as they get further from away from the air handler—to get just the right amount of air flow to each room, without too much or too little at either end of the duct runs. Then the air vents they connect to are also calibrated to match the rooms they serve. A system that is correctly designed and installed is called “balanced.” Closing any of the air vents fouls up the equilibrium of the design, causing changes in the amount of air delivered to the vents that remain open, often in an unpredictable way. 

2) Closing an air duct increases the back-pressure on the blower, making it slow down and work harder, which shortens the life of the motor—an expensive component to replace.

3) The additional pressure in the ducts increases leakage at the connections of duct components. It’s estimated that as much as 20% of the conditioned air flow in the average home’s duct system leaks out into the attic anyway, but closed ducts will increase the leakage and possibly force open more seams.

4) The back-pressure of a closed air vent at the end of a long duct run often forces cold air leakage just behind the air vent during the summer, which causes condensation wetness around the leak in a hot, humid attic. The result is sometimes mold growth at and behind the air vent. Here’s an infrared photo of a closed and leaking vent.

    So, closing the vents in unused rooms causes problems that are not offset by the minor cost savings created, and we recommend that you don’t do it.

    Also see our blog posts Frequently Asked Questions (FAQ) about Air Conditioning Troubleshooting and Will closing doors reduce my heating and cooling costs? and Why is my air conditioner not cooling enough?

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  To learn more about heating and air conditioning systems, see these other blog posts:

• How can I find out the SEER of my air conditioner? 

• My air conditioner won't turn on. What's wrong? 

• How can I find out the size of my air conditioner? 

• How can I tell whether the condenser (outdoor unit) is an air conditioner or heat pump? 

• Where is the air filter for my central air conditioner and furnace? I can’t find it? 

• Does an old air conditioner use more electricity as it ages? 

• How did homes stay cool in Florida before air conditioning? 

• What is wrong with an air conditioner when the air flow out of the vents is low?

• Why has the thermostat screen gone blank? 

• Why does it take so long to cool a house when an air conditioner has been off for a while? 

• Why is my air conditioner not cooling enough? 

• What are the most common problems with wall/window air conditioners?  

• Will closing doors reduce my heating and cooling costs? 

   Visit our HEATING AND AIR CONDITIONING page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.