Assembly for controlling the temperature in a heatable boiler of a drinking water heater comprising: (a) a cold water inlet (); (b) a cold water outlet for refilling the boiler with cold water, adapted to be connected to the boiler; (c) a relief outlet for discharging hot water from the boiler; (d) a connection between the cold water inlet and the relief outlet; (e) a control valve in the range of the connection between the cold water inlet and the relief outlet, for controlling the flow of cold water to the relief outlet depending on the temperature in the boiler, wherein the control valve comprises a spring-loaded piston, which is moveably guided in an axial direction in a cylindrical space, which, on one hand, controls the hot-water flow from the boiler through the relief outlet by cooperating with a first valve seat provided between the cylindrical space and the boiler, and on the other hand controls the cold water flow by cooperating with a second valve seat, provided between the cold water inlet and the relief outlet; (f) means for venting of the boiler comprising a manually operable handle, adapted to axially move the spring-loaded piston against the spring force in the opening direction, and (g) a hot water supply passage for connecting with the boiler, wherein the hot water supply passage is aligned with the cylindrical space, and wherein (h) the hot water supply passage has a smaller diameter than the cylindrical space, and (i) the piston, the hot water supply passage and the cylindrical space are coaxially assembled, characterised in that the first valve seat is formed by a shoulder, as a result of the hot water supply passage having a smaller diameter than the cylindrical space and the handle being coaxially assembled to the piston, hot water supply passage, and cylindrical space. (a) a cold water inlet (); (b) a cold water outlet for refilling the boiler with cold water, adapted to be connected to the boiler; (c) a relief outlet for discharging hot water from the boiler; (d) a connection between the cold water inlet and the relief outlet; (e) a control valve in the range of the connection between the cold water inlet and the relief outlet, for controlling the flow of cold water to the relief outlet depending on the temperature in the boiler, wherein the control valve comprises a spring-loaded piston, which is moveably guided in an axial direction in a cylindrical space, which, on one hand, controls the hot-water flow from the boiler through the relief outlet by cooperating with a first valve seat provided between the cylindrical space and the boiler, and on the other hand controls the cold water flow by cooperating with a second valve seat, provided between the cold water inlet and the relief outlet; (f) means for venting of the boiler comprising a manually operable handle, adapted to axially move the spring-loaded piston against the spring force in the opening direction, and (g) a hot water supply passage for connecting with the boiler, wherein the hot water supply passage is aligned with the cylindrical space, and wherein (h) the hot water supply passage has a smaller diameter than the cylindrical space, and (i) the piston, the hot water supply passage and the cylindrical space are coaxially assembled, characterised in that the first valve seat is formed by a shoulder, as a result of the hot water supply passage having a smaller diameter than the cylindrical space and the handle being coaxially assembled to the piston, hot water supply passage, and cylindrical space.