PURPOSE: To enable to dispose a hot-water supply pipe on either side of a tank and to facilitate installation and modification of a water heater, by providing overflow ports and cisterns on both sides of the tank.

CONSTITUTION: A pair of overflow tanks 2a, 2b with plugs are formed on both sides of a tank 1 of a solar heat collector at its upper portion, and swan portions of a pair of cisterns 3a, 3b with plugs are connected to lower portions of the tank 1 on both sides thereof. Further, the tank 1 is formed with lot-water supply ports 4a, 4b with plugs, and a hot-water supply pipe 6 is connected to either one of the overflow ports 2a, 2b in a freely detachable manner to suit the location where the water heater is to be installed. A constant-temperature mixing valve 7 is provided at an intermediate portion of the hot-water supply pipe 6. The mixing valve 7 is connected, in a freely detachable manner, to either one of the cisterns 3a, 3b, preferably one located on the side of the hot-water supply pipe 6, via a connecting pipe 8.