A method of controlling a hydronic system is provided. The hydronic system includes a plurality of fluid heat exchange units for feeding a load, and a bypass valve for bypassing the load. The method includes operating at least a first and a second fluid heat exchange unit in the hydronic system to heat or cool a fluid. An output fluid flow of each of the operating fluid heat exchange units is monitored. The monitored output fluid flow of each fluid heat exchange unit is compared to a predetermined fluid flow setpoint. The output fluid flow of each of the operating fluid heat exchange units is adjusted towards the predetermined fluid flow setpoint if the monitored output flow is different from the predetermined fluid flow setpoint by at least a predetermined margin. A combined output fluid flow of the operating fluid heat exchange units is also monitored. The bypass valve is at least partially opened if the combined output fluid flow is below a predetermined minimum combined output fluid flow. A previously idle fluid heat exchange unit is operated if the combined output fluid flow is greater than a predetermined maximum combined output fluid flow for the number of operating fluid heat exchange units.