Abstract: A control system and method provide a multiple option select function in which an option select resistor acts as a selector. The control system determines the resistance value of the resistor and maps the resistance value to a data entry in an option select table. The index of the data entry is determined and used to reference option parameters in at least one option parameter table. The index allows the control system to select an option parameter from many possible option parameters in the option parameter table. Furthermore, additional option parameter tables may also be referenced to select multiple option parameters using a single select resistor. The control system is disclosed as controlling a water heater system, although such control systems may be used to control other systems, such as furnaces and hydronic heating and water systems. A calibrating circuit is used to measure a transient response of an RC circuit.