Taco Switching Relays are available with and without "Priority". When a switching relay has priority, it means that you set a zone to be the one that is most important. In other words, if changing one zone's temperature will change another slightly, a switching relay with priority will be sure to keep the designated "priority" zone at the exact temperature that you want it to be, while allowing the other zones to vary slightly if need be.
Taco heating supplies and switching relays can be found at www.supplyhouse.com
