An electronic temperature control system for controlling current flow to a heating element in a tankless, liquid water heater, said system comprising:
- a temperature sensing device in fluid communication with a liquid to be temperature controlled, said temperature sensing device being operable to enable real-time sensing of a temperature level of said liquid and to provide a first output signal representative of said sensed temperature level;
- an operator controllable temperature selection device for enabling operator selection of a desired temperature level of said liquid and providing a second output signal representative of said selected temperature level;
- a differential amplifier responsive to said first and second output signals for generating an error signal representative of a difference between said first and second outputs;
- error range comparison circuitry for providing an upper reference signal limit and a lower reference signal limit, said limits defining a predetermined, maximum desirable error range which corresponds to a maximum desirable temperature level error range, said error range comparison circuitry further including an upper limit comparator responsive to said upper limit signal and said error signal and a lower limit comparator responsive to said lower limit signal and said error signal, said upper limit comparator being operable to provide an upper limit output indicative of when said error signal is greater or less than said upper limit signal, and said lower limit comparator being operable to provide a lower limit output signal indicative of whether said error signal is greater or less than said lower limit reference signal, said upper and lower limit outputs operating cooperatively to provide a high gain selection signal when said error signal is within said predetermined error range and a low gain selection signal when said error signal is outside of said predetermined error range;
- reference signal means for generating a system reference signal;
- first and second gain elements responsive to said system reference signal, said first gain element providing a first gain signal, said second gain element providing a second gain signal, said first gain signal being greater than said second gain signal;
- digital switch means responsive to said first and second gain signals and said upper and lower gain selection signals, for selecting either said first or second gain signals;
- first amplifier means responsive to said digital switch means and said error signal for generating a filtered, amplified correction signal having either said first or said second gain;
- second amplifier means responsive to said system reference signal and said filtered, amplified correction signal for generating a conditioned correction signal, said digital switch means being responsive to said conditioned correction signal;
- ramp generator means for generating a ramp signal of predetermined magnitude and frequency;
- comparison means responsive to said ramp signal and said conditioned correction signal for comparing said ramp and conditioned correction signals and generating a switching signal indicative of whether said ramp signal is greater or less than said conditioned correction signal;
- and
- second switch means responsive to said switching signal for selectively enabling a current flow from a current source to flow through said second switch means and through a heating element to thereby heat said heating element, thereby controllably heating said liquid in fluid contact with said heating element.