In a fluid heating apparatus, a heat protect temperature sensor indirectly detects
temperature of fluid heated by an electric heater adjacent to the electric heater.
Also, a heated fluid sensor detects the temperature of the fluid adjacent to a
fluid inlet of a heat exchanger. When it is determined that the temperature difference
between the detected temperatures exceeds a predetermined level, electric power
supply to the electric heater is discontinued. Thus, boil-dry is restricted early
with a simple means.
