Calorimetric flow sensors are easy to fabricate and can be tailored in length if appropriate technologies are used. Such long devices can be favorably employed, e.g., in ventilation systems to judge the distribution of the mass flow. This paper presents a flow sensor based on Ag-Ni thermopiles printed on a flexible PET substrate. It reviews the basic sensor principle and its feedback controlled operation where the heating power is adjusted according to the flow velocity, which is imperative to achieve a reasonable dynamic range. Characterization data in a flow channel are compared to simulation results from a 3D FEM approach, demonstrating the feasibility of the sensor.