Although common sense tells you that shutting the door and closing the vents at an unused room reduces the heating/cooling cost for your home, it doesn’t actually work out that way. Here’s five reasons why: 

1) Your home’s HVAC system only functions properly when the amount of air returning back to the furnace or air handler equals the volume delivered through the supply vents (registers) in each room. In many older homes, the undercut gap beneath an interior door is the only way that air can return from a room with a closed door, and it is not sufficient to equal the capacity of the supply vent. 

    So a room with a closed door becomes pressurized and the air flow to the room is reduced, but the air pressure pushes conditioned air through small openings at places like the base of the wall behind the baseboard, around electric receptacle boxes in the wall, and around windows and doors. So, essentially, the room starts leaking air.

2) When the supply vent is closed, the duct run also becomes pressurized and begins to leak conditioned air into the attic over time.

3) Replacement for the shortage of returning air gets sucked in through the chimney, gas water heater or furnace flues, along with any small openings to the exterior in the other rooms that become depressurized by the imbalance of air flow. 

4) Closed doors and supply vents create back-pressure on the blower in the furnace or air handler, making it work harder and shortening its lifespan. 

5) The combination of conditioned air lost in pressurized rooms and the unconditioned, outside air pulled into the home at depressurized rooms eliminates any possible energy savings and can even increase your utility bill. 

    So it is best for your comfort, the life of the HVAC system, and keeping your utility bills down to leave the vents and doors open. This no-closed-door policy does not apply to rooms with return air vents, jump ducts, or transfer grilles, all of which provide a return route for air when a door is closed.

    Also, see the blog posts Is it alright to close the air conditioning vents in unused rooms and Frequently Asked Questions (FAQ) about Air Conditioning Troubleshooting.

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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?  

   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.