The invention provides a method for operating a line circuit (4) for waste heat utilization of an internal combustion engine (2), and a control unit (1). A feed pump (6), at least one heat exchanger (8), an expansion machine (10), a condenser (12) and a control unit (1) are situated in the line circuit (4). The control unit (1) calculates a time period which is required until individual components or all the components of the line circuit (4) have reached a predefined temperature, in order to start up the line circuit (4) at least partially.