Taco SR-503 Switching Relay w/Priority 3 Zone. Taco SR-503 3 Zone Switching Relay w/Priority. 7, Application Circulator Zoning and Boiler Control. The item "Taco SR-503 Switching Relay withPriority 3 Zone" is in sale since Wednesday, November 20, 2019. This item is in the category "Business & Industrial\HVAC & Refrigeration\HVAC & Refrigeration\ Parts & Accessories\HVAC & Refrigeration\ Controls & Circuit Boards".The seller is "htg_cooling_parts" and is located in Bethlehem. This item can be shipped to United States.