Method for controlling a boiler of the single-tube boiler type, which can be integrated in a combined cycle power plant with a steam turbine and a gas turbine, wherein the boiler comprises an HP economizer, an HP evaporator and at least one superheater, wherein opening of a valve for regulating the flow volume in a feedwater supply line into the high-pressure evaporator is controlled, characterized in that opening of the valve for regulating the flow volume in a feedwater supply line into the high-pressure evaporator is controlled in dependence upon a temperature set-point value for the steam for the steam turbine and also upon a measured steam temperature at the outlet of the at least one superheater and by means of a plurality of control steps (PID 1 - 4) in series, wherein the measured steam temperature at the outlet of the superheater, a degree of superheating at the outlet of the high-pressure evaporator and a degree of subcooling of the feedwater at the inlet into the high-pressure evaporator are integrated in the control steps (PID 1 -4), wherein the method comprises the steps: - measuring the steam temperature at the outlet of the superheater, - in a first control step, comparing the steam temperature with a set-point value for the turbine steam temperature according to an operating state of the steam turbine, and generating a set-point value for the degree of superheating at the outlet of the high-pressure evaporator, based on the difference between the measured value and set-point value of the steam temperature at the outlet of the superheater, - measuring the current degree of superheating at the outlet of the high-pressure evaporator, - in a second control step, generating a set-point value for the degree of subcooling of the feedwater at the inlet of the high-pressure evaporator, based on the difference between the set-point value and current measured value of the degree of superheating at the outlet of the high-pressure evaporator, - measuring the current degree of subcooling of the feedwater at the inlet into the high-pressure evaporator, - in a third control step, generating a set-point value for the flow volume for the feedwater upstream of the high-pressure evaporator, based on the difference between the set-point value and the current measured value for the degree of subcooling of the feedwater, - based on the set-point value for the flow volume of the feedwater, generating a control signal for the opening of a valve in the feedwater supply line and transmitting the control signal to the valve. - measuring the steam temperature at the outlet of the superheater, - in a first control step, comparing the steam temperature with a set-point value for the turbine steam temperature according to an operating state of the steam turbine, and generating a set-point value for the degree of superheating at the outlet of the high-pressure evaporator, based on the difference between the measured value and set-point value of the steam temperature at the outlet of the superheater, - measuring the current degree of superheating at the outlet of the high-pressure evaporator, - in a second control step, generating a set-point value for the degree of subcooling of the feedwater at the inlet of the high-pressure evaporator, based on the difference between the set-point value and current measured value of the degree of superheating at the outlet of the high-pressure evaporator, - measuring the current degree of subcooling of the feedwater at the inlet into the high-pressure evaporator, - in a third control step, generating a set-point value for the flow volume for the feedwater upstream of the high-pressure evaporator, based on the difference between the set-point value and the current measured value for the degree of subcooling of the feedwater, - based on the set-point value for the flow volume of the feedwater, generating a control signal for the opening of a valve in the feedwater supply line and transmitting the control signal to the valve.