Check the listing for details. DuroZone 3 Zone Control Panel SMZ-AC Auto Changeover 35229. Condition: New. Listed at 379.79 USD. The SMZ-AC is a simple auto changeover zoning system incorporating individual thermostats and zone dampers to provide comfort and economy. The SMZ-AC is a first call - first served system, i.e., if a zone calls for cooling, heating calls are locked out and the system will satisfy all cooling calls. When satisfied, the system will then switch to heat and satisfy those calls. When all zones are satisfied, heating and cooling, each zone damper will be in the position set by the Damper Switch. If any zone calls, that damper will open, or remain open, and all others will close. Should another zone call, for the same equipment as the first zone to call, its corresponding damper will open. Upon satisfaction of the zone(s) all dampers will return to the position set with the Damper Switch.