Abstract: A hydronic heating system including a single self-contained hydronic control unit. The hydronic control unit allows for zoned heating operation in which a series of individual room thermostats and zone valves control the flow of heated water into each of the heating zones. The hydronic control unit operates an oil or gas fired boiler to maintain boiler water temperature at a selected value. The hydronic control unit includes a priority terminal which can be connected to a priority heating zone, such as an indirect fired water heater. Upon receiving a demand for heat from the priority heating zone, the hydronic control unit diverts the flow of heated water from the boiler to the priority zone exclusively. The hydronic control unit further includes a terminal for connection of an outdoor air sensor. In cooperation with the outdoor air sensor, the hydronic control unit can operate in a boiler reset operating mode such that the boiler temperature is related to the outside air temperature.