To provide a hot water heater which can effectively make use of radiation from feed header and return header and a space defined under a stair of a plural-floor housing.

A feed header 6 and a return header 8 of a piping of a hot water heater are installed in a space 9 defined under a stair of a two-storied housing. Accordingly, radiation from the feed header 6 and the return header 8 which is wasted conventionally is utilized, and heating of the stair 10 and a hall 11 on the second floor is performed, thus enabling an effective utilization of radiation and a dead space. Furthermore, the inspection of piping is facilitated, the distance between respective radiators is shortened, and the capacity of a circulation pump can be minimized, thus enabling an inexpensive and economical hot water heating.