These are the deluxe air distribution systems of the HVAC industry where one unit (eg. a rooftop HVAC unit) can be controlled by multiple thermostats for multiple zones. The system will consist of 2 or more zone dampers and a bypass damper between the supply air and return air. Each zone damper is controlled by a thermostat that reports back to a central monitor that decides whether the roof unit will operate in heating, cooling or ventilation mode. Each of the zone thermostats are interconnected in a "daisy chain" so that a laptop PC with the appropriate software can plug in at any thermostat and control the system for diagnosing problems or changing the program. Zone dampers are continually moving from open to close to maintain accurate temperature control in each space. As zone dampers move to the closed position the bypass damper will open to "bypass" supply air back into the return plenum. This is to control the pressure of the air in the ductwork and modulates continually to control the pressure and therefore the noise of air movement. The priority of whether the unit operates in the heating or cooling mode is normally given to heating. The reason for this is that the heating capacity of most systems in southern Ontario is about 3 to 4 times the cooling capacity and can therefore satisfy demand very quickly. Once the heating call is satisfied the system will revert back to cooling or ventilation mode. Zones that are at "set point" of the area thermostat will close to the minimum or ventilation position while other zones are being heated or cooled. Most of these systems will incorporate energy management and each zone thermostat will have an over-ride button to condition only the required area after hours if desired. If the system is sounding a little pricey, it is, but can be retrofitted to almost any existing system and with the addition of a modem connection, can be monitored, diagnosed and adjusted from a remote location by your service contractor. If your building is large enough to warrant a full time maintenance staff the software can be added to their PC for monitoring and adjusting the system. There are many desirable features of this system outside of the fact that you can have two offices side by side with one at 74 degrees and the other at 70 degrees, but the feature that this writer likes is the fact that you can program max and min limitations on what each zone operator can adjust his or her zone to. Some of the PC graphic software packages for these systems will show the rooftop unit and symbols to show fan operation, air temperature and each zone condition. There have been several manufacturers over the years to promote their system only to become obsolete or fail to maintain parts inventory for repairs. Carrier is one system that has stood the test of time and has improved and maintained inventory over the years.