Taco SR501-845RP is a Universal Replacement Relay that connects a thermostat to the "T T" terminals on the switching relay.
This next generation Relay boasts unique and sturdy design ensures both reliability and functionality. When the thermostat calls for heat, the relay energizes and power is given to the circulator. The Indicator lights show full functionality of the switching relay. The green light should always be on, indicating that power is connected. When the thermostat calls for heat, both the appropriate circulator and red indicating light is energized. Universal Replacement Relay provides the ultimate performance and unparalleled functionality.