To provide an inexpensive one can two water passage water heater capable of preventing a hot water supply water passage from becoming high pressure.

The present water heater passes through a hot water supply water passage to a heat exchanger, and a bath heating water passage. Herein, a downstream side through the heat exchanger in the hot water supply water passage and the bath heating water passage are communicated wiuth each other through a communication passage 38. In the cource of the communication passage 38 there is provided a pressure relief valve for relieving water on the side of the hot water supply water passage to the side of the bath heating water passage when pressure on the side of the hot water supply warter passage exceeds a predetermined pressure. Hereby, when the heat exchanger is actuated to raise internal pressure in the hot water supply water passage, the water in the hot water supply water passage can be relieved to the side of the bath heating water passage through the communication passage 38. Further, a solenoid valve 50 is provided on the side of the hot water supply water passage from the position of the pressure relief valbe 52 in the communication passage 38 for opening and closing the solenoid valve 50. Hereby, pressure is prevented from being applied to the pressure relief valve 52 and the life of the pressure relief valve 52 can be prolonged.