Usually, there is a main duct combined with a series of branch ducts, which carry air to different rooms. Installed within these branch ducts are dampers that act like a door that opens and closes to allow or prevent the flow of air depending on which zone is calling for conditioned air.
To do this, a programmable thermostat is installed on each level of the home. Thermostats should be placed at a central location in each zone, on an interior wall. Your contractor will look for a place where lamps, sunlight, or other artificial heat can affect the thermostat.
If you adjust the thermostat for that zone to make it warmer or cooler, a signal is sent to open a damper for that particular level and shut the dampers that control other zones, which don’t need additional cooling or heating.
This allows just the right amount of conditioned air to go to that zone that needs it. Programming your thermostats to specific temperature conditions for each zone, or having your contractor do this when the system is installed, lets the controls and dampers take care of everything.
The damper system works differently than just closing a vent on a floor or wall. When you shut the vent, the heated or cooled air still must travel the length of the duct to reach the outlet. When it is denied an exit, the air usually sits in the pipe and becomes room temperature.
A damper is usually installed at the mouth of a distribution duct and either permits or allows airflow at that point. This maintains the pressure in the duct and redistributes the airflow to the rest of the structure. HVAC zone control dampers range in sizes to fit your ducts and can be round or rectangular depending on your duct style. They also can be powered by electricity or by compressed air. They also are available in "normally open" or "normally closed," depending on their intended location.