Flow-through drinking water heater - with a first heat exchanger , - with a second heat exchanger , - with a primary line arrangement for connecting the primary sides of the heat exchangers to a heat generator , - with a secondary line arrangement for connecting the secondary sides of the heat exchangers to a hot water circulation line and to a cold water line , - wherein the primary line arrangement connects the primary sides of the heat exchangers in series, - wherein the secondary line arrangement connects the secondary sides of the heat exchangers in series and connects the cold water line through the second heat exchanger to the secondary-side inlet of the first heat exchanger , - with a controllable first pump for supplying hot water to the primary side of the first heat exchanger , - with a controllable second pump for supplying hot water to the primary side of the second heat exchanger , - with a first temperature sensor arranged in the secondary line arrangement for detecting the return temperature in the secondary-side return of the first heat exchanger and - with a control device for recording the measured values of the temperature sensor and for controlling the first pump and the second pump , characterised in, - that the control device is set up to control the pumping capacity of the first pump to a predetermined secondary-side return temperature of the first heat exchanger , - that the control device is set up to activate the pumping capacity of the second pump if the required total heat transfer capacity exceeds a predetermined threshold value heat transfer capacity of the first heat exchanger , and - that the control device is set up to deactivate the pumping capacity of the second pump if the required total heat transfer capacity falls below the predetermined threshold heat transfer capacity of the first heat exchanger . - with a first heat exchanger , - with a second heat exchanger , - with a primary line arrangement for connecting the primary sides of the heat exchangers to a heat generator , - with a secondary line arrangement for connecting the secondary sides of the heat exchangers to a hot water circulation line and to a cold water line , - wherein the primary line arrangement connects the primary sides of the heat exchangers in series, - wherein the secondary line arrangement connects the secondary sides of the heat exchangers in series and connects the cold water line through the second heat exchanger to the secondary-side inlet of the first heat exchanger , - with a controllable first pump for supplying hot water to the primary side of the first heat exchanger , - with a controllable second pump for supplying hot water to the primary side of the second heat exchanger , - with a first temperature sensor arranged in the secondary line arrangement for detecting the return temperature in the secondary-side return of the first heat exchanger and - with a control device for recording the measured values of the temperature sensor and for controlling the first pump and the second pump , characterised in, - that the control device is set up to control the pumping capacity of the first pump to a predetermined secondary-side return temperature of the first heat exchanger , - that the control device is set up to activate the pumping capacity of the second pump if the required total heat transfer capacity exceeds a predetermined threshold value heat transfer capacity of the first heat exchanger , and - that the control device is set up to deactivate the pumping capacity of the second pump if the required total heat transfer capacity falls below the predetermined threshold heat transfer capacity of the first heat exchanger . <EOS>