A water heater is proposed with a reservoir vessel (4) with connections in the floor of the reservoir vessel (4) for the forward flow (14) and return flow (15) of a solar collector system and for the forward flow (16) and return flow (18) of a hot water space heating system. The combustion chamber (52) of a gas- or oil-fired system (50) is fitted in the top part of the reservoir vessel. Inside the reservoir vessel (4), a device (22) is provided to ensure the temperature-dependent stratification of the forward flow water of the solar collector system and a device which ensures the temperature-dependent stratification of the return flow water of the space heating system. The inlet (33) of the forward flow (16) of the space heating system is situated in the region of the upper floor (34) of the reservoir vessel. A vertical heat exchanger (36) for the waste water is enclosed in a vertical guide pipe (42, 43) which in its upper and lower sections is connected to the reservoir vessel in the region of the latter's upper (34) and lower (10) floors.
