Zone duct systems can help save energy by appropriately closing off certain air-conditioning vents. The system uses a single heating and air conditioning system to control by multiple zones, with multiple thermostats. The home’s needs determine how many dampers may be opened, providing conditioned air to its zone. When the HVAC system is running, but fewer than the total number of zone dampers are open, the bypass duct is called upon to relieve extra pressure, and keep good air flow in the ducts. Experts agree these bypass ducts are problematic. They make the evaporator coil less efficient, steal air from open dampers, and encourage the coil to freeze up.
Read the full article here: The Achilles’ Heel of Zoned Duct Systems | GreenBuildingAdvisor.com