A method of operating a steam generator, the steam generator comprising: - a water container, - two heating devices (18, 20) on the water container, which are located in different height regions separated from each other in vertical direction, one heating device being an upper heating device and the other heating device being a lower heating device, - a flat first temperature detection device (16, 26) covering a temperature detection area, which temperature detection area includes at least the area covered by the heating devices (18, 20), - a second temperature detection device in the form of a spot-like temperature sensor (22a, 22b) being located on the water container in a region of an upper border of the upper heating device, - a control device for evaluating the first temperature detection device (16, 26) and the second temperature detection device (22a, 22b) and for activating and deactivating the two heating devices (18, 20), with the steps of: - filling the water container with water for a predefined first filling duration D 1, - after the first filling duration D 1 has elapsed, the lower heating device is activated, wherein the water container is continuously filled further with water and wherein the upper heating device is not activated, - in case the first temperature detection device (16, 26) detects a temperature exceeding a first temperature threshold T thr, the lower heating device is deactivated, wherein the water container is filled further with water, - as soon as the first temperature detection device (16, 26) does not detect any temperature above the first temperature threshold T thr, but only a temperature below the first temperature threshold T thr, the lower heating device is activated again, - as soon as the first temperature detection device (16, 26) does not detect any temperature above the first temperature threshold T thr for a duration of at least 5 sec, the control device defines the quantity of water in the water container to be as high as 60% to 90% of a height of an upper rim of the lower heating device, - as soon as the second temperature detection device (22a, 22b) detects a temperature of at least 95°C, the upper heating device and the lower heating device are both activated for heating operation. - a water container, - two heating devices (18, 20) on the water container, which are located in different height regions separated from each other in vertical direction, one heating device being an upper heating device and the other heating device being a lower heating device, - a flat first temperature detection device (16, 26) covering a temperature detection area, which temperature detection area includes at least the area covered by the heating devices (18, 20), - a second temperature detection device in the form of a spot-like temperature sensor (22a, 22b) being located on the water container in a region of an upper border of the upper heating device, - a control device for evaluating the first temperature detection device (16, 26) and the second temperature detection device (22a, 22b) and for activating and deactivating the two heating devices (18, 20), - filling the water container with water for a predefined first filling duration D 1, - after the first filling duration D 1 has elapsed, the lower heating device is activated, wherein the water container is continuously filled further with water and wherein the upper heating device is not activated, - in case the first temperature detection device (16, 26) detects a temperature exceeding a first temperature threshold T thr, the lower heating device is deactivated, wherein the water container is filled further with water, - as soon as the first temperature detection device (16, 26) does not detect any temperature above the first temperature threshold T thr, but only a temperature below the first temperature threshold T thr, the lower heating device is activated again, - as soon as the first temperature detection device (16, 26) does not detect any temperature above the first temperature threshold T thr for a duration of at least 5 sec, the control device defines the quantity of water in the water container to be as high as 60% to 90% of a height of an upper rim of the lower heating device, - as soon as the second temperature detection device (22a, 22b) detects a temperature of at least 95°C, the upper heating device and the lower heating device are both activated for heating operation.