PURPOSE: To prevent the reverse flow of drain by a method wherein a closed drain break device which is full of air and has a lower end through which a communicating pipe to a drain pipe runs out is provided, and by inserting the drain conduit of a water boiler in an upright manner through the lower end of the closed drain break device, and the upper end thereof is opened.

CONSTITUTION: A heater 4 is incorporated in a water boiler 1, a water feed pipe 6 in which a stop valve 5 is located is run in the water boiler, and a hot water feed pipe 7 to a hot water feed faucet is run out therethrough. A drain conduit 9 for expansion water in the water boiler is connected to a branch pipe 7' of the hot water feed pipe 7 through a safety valve 8, a standing-upright part 9' at its tip is contained in a closed drain break container 3, and an opening part 10 is formed in the peripheral surface of an upper end or an upper end surface. The closed drain break container 3 is formed such that a plastic elongated clear cap 3c is screwed in a manifold 3a through a coupling pipe 3b. A vacuum breaker 14 and a reverse flow preventing valve 11 are mounted to a lower end discharge port 3d of an L-cylinder 3a, and a communicating pipe 13 in which a vacuum breaker 14 and a reverse flow preventing valve 11 are mounted and which has a trap 12 is run out from the lower end discharge port, and the tip thereof is connected to a drain pipe 2. This constitution enables the feed of drain from the water boiler to the drain pipe without a reverse flow through simple constitution.