Taco Zone Switching Relays combine clearly-labeled PC board layouts with advanced features that allow you to completely customize your system. And whether you're using basic priority switching, Taco Plug-In PowerPort Cards or add-on power controls, tailoring the system has never been easier. External indicator lights give you instant diagnostic feedback, making service calls or new installation start-ups a snap. All switching relays are UL listed, use standard "ice cube" replaceable relays and are compatible with conventional and programmable thermostats. For larger jobs, switching relays can be expanded up to 20 zones with priority. With the exclusive Taco PowerPort Plug-In Cards, you can easily add such features as post purge, priority protection and pump exercise. Any expandable switching relay can also interface with the Boiler Reset Control and Injection Mixing Add-On Power Controls. Mix-and-match as needed to customize the system.