WATER HEATER

PROBLEM TO BE SOLVED: To provide a water heater using a water heating and retaining system for achieving the stable function of instantly heating water by enabling heating up to a required heat retaining temperature while preventing excess inner pressure rise even in the cold condition of a heat exchanger. SOLUTION: For a water heater provided with a heating circuit having an expansion tank open to the atmospheric air, a combustion burner burns in the state of a hot water supply circuit being closed during non-use of a hot water for heating a heat exchanger. When the temperature of a canned body is not higher than a set temperature A, the combustion burner burns (YES in S11, S12). When the temperature of the canned body before burning rises up to a MAX temperature after burning (YES in S13), the inner pressure of the hot water supply circuit is determined to rise a preset amount and a water resupply solenoid valve opens a minimum time TSV for releasing a hot water equivalent to thermal expansion to the expansion tank to make the inner pressure open (S14). Then, S11-S14 are repeated for heating up to the set temperature of the canned body.