The paper presents the concept and measurement results of a solar thermal pilot plant for decentralized feed-in to a 2^nd generation district heating network. Ambitious conditions for both target temperature level and pressure difference are fulfilled with a return line to supply line feed-in (RL/SL) design. The use of water as solar liquid requires an active frost protection using return line district heating water. The stagnation state is investigated by the application of distributed temperature sensing (DTS). The measurement results include one frost protection and almost one feed-in season.