Does closing vents in unused rooms save energy?
Usually not in a typical forced-air system. Closing vents can reduce airflow and raise duct pressure instead of making the HVAC system proportionally smaller.
What you need to know
Closing supply vents in unused rooms sounds like a simple way to stop heating or cooling space you are not using. In a typical central forced-air system, DOE guidance says that is not recommended. The blower and duct system were designed around a certain airflow. Closing vents can increase pressure, reduce airflow through the air handler, stress duct connections and create comfort or air-quality problems. Purpose-built zoning is different because it is designed and controlled as a system.
Closing vents is not recommended as a DIY energy-saving strategy
It can reduce airflow and increase pressure in the duct system
Purpose-built zoning or properly designed equipment