According to an embodiment of the disclosure, a retrofit for a system using a fluid to transfer thermal energy in a system that utilizes a thermostat to regulate a temperature associated with a terminal is provided. The retrofit includes a return line temperature sensor configured to measure a temperature of a fluid leaving a terminal; and a circuit. The circuit is configured to: receive a signal from a thermostat, the signal commanding a modulated valve to open or close; receive a temperature from the return line temperature sensor; determine whether the temperature is either greater than a maximum set point or less than a minimum set point; and modify the signal from the thermostat if the temperature is either greater than the set point or less than set point.