A method of determining an operating water temperature for a boiling water heater, the method including the following steps: (a) adding water to a tank to a predetermined level; (b) energising a heating means within the tank to heat the water in the tank to an initial predetermined temperature; (c) de-energising the heating means when the initial predetermined temperature is reached and measuring an initial overshoot temperature differential above the initial predetermined temperature; (d) calculating a further predetermined temperature being the sum of the initial predetermined temperature and a predetermined amount of the initial overshoot temperature differential; (e) waiting until the temperature of the water in the tank has reduced by a predetermined drop from the initial predetermined temperature; (f) energising the heating means within the tank to heat the water in the tank to the further predetermined temperature as calculated in (d) above; (g) de-energising the heating means when the further predetermined temperature is reached and measuring a further overshoot temperature differential above the further predetermined temperature; (h) re-calculating the further predetermined temperature being the sum of the current further predetermined temperature and a predetermined amount of the current further overshoot temperature differential; (i) waiting until the temperature of the water in the tank has reduced by a predetermined drop from the further predetermined temperature of step (h); (j) continuing steps (f) to (i) until the further overshoot temperature substantially equals the further predetermined temperature, thereby becoming a boiling calibration temperature; and (k) substracting an offset temperature from the boiling calibration temperature of step (h) to thereby determine the operating water temperature. (a) adding water to a tank to a predetermined level; (b) energising a heating means within the tank to heat the water in the tank to an initial predetermined temperature; (c) de-energising the heating means when the initial predetermined temperature is reached and measuring an initial overshoot temperature differential above the initial predetermined temperature; (d) calculating a further predetermined temperature being the sum of the initial predetermined temperature and a predetermined amount of the initial overshoot temperature differential; (e) waiting until the temperature of the water in the tank has reduced by a predetermined drop from the initial predetermined temperature; (f) energising the heating means within the tank to heat the water in the tank to the further predetermined temperature as calculated in (d) above; (g) de-energising the heating means when the further predetermined temperature is reached and measuring a further overshoot temperature differential above the further predetermined temperature; (h) re-calculating the further predetermined temperature being the sum of the current further predetermined temperature and a predetermined amount of the current further overshoot temperature differential; (i) waiting until the temperature of the water in the tank has reduced by a predetermined drop from the further predetermined temperature of step (h); (j) continuing steps (f) to (i) until the further overshoot temperature substantially equals the further predetermined temperature, thereby becoming a boiling calibration temperature; and (k) substracting an offset temperature from the boiling calibration temperature of step (h) to thereby determine the operating water temperature.