Traditionally two story homes in the Triangle have been built with a dedicated HVAC system for each floor. One unit is located in the attic and the other in the crawl space or basement. As more multistory homes are built on a slab foundation, builders are installing one unit in the attic that serves both floors. The system is “zoned” meaning conditioned air is supplied to each floor by one unit when called for by separate (independent) thermostats.
So, don’t assume that an MLS description of “dual zone” or the presence of two thermostats implies two systems!
The zones don’t have to be between floors. Larger homes are often divided within a floor, allowing temperature management by function (kitchen vs. bedroom), time of day or individual preference (moms vs. dads).
Conditioned air is directed to each zone by modulating dampers. These dampers are essentially gates within the ducts that open and close electronically to allow or inhibit the flow of air to each zone. However, the air is often forced through the system at its peak performance rate - the rate necessary when all zones call for airflow at the same time. When fewer zones call for airflow, a bypass duct relieves pressure in the closed ducts and maintains a consistent flow in the active zone.
The conditioned air in the bypass duct is directed, or essentially recycled back into the system through the return duct. Skeptics say this makes the system less efficient and more prone to evaporator coil freezing. As an inspector, I don’t have experience to judge these concerns. Refer to a HVAC contractor for more information.
More sophisticated systems don’t operate in such a binary fashion. Rather than just turn a zone on or off, they modulate airflow to keep the temperature more consistent. These systems also allow management of all zones from any control panel (thermostat) and rely less on bypass ducts by varying the fan speed.
Day-to-day operation of a simple two-zoned system differs slightly from two individual systems dedicated to each floor. All thermostats should be set to the same mode (heat or cool) and you must be careful with aftermarket thermostats with the “auto” feature. More sophisticated systems should be controlled with the panel specified by the manufacturer.
A zoned system may also require adjustment and balancing during the first year of operation. Make sure this service is included with your warranty.
The information provided here is based on extensive research, but I am an inspector - not a licensed HVAC contractor. Always consult a contractor regarding a particular installation.