The present disclosure is directed to systems and methods for automatically detecting a leak of a closed loop hydronic boiler system having a makeup water inlet line. The system may include a fluid flow sensor configured to sense fluid flow through the makeup water inlet line, and optionally a pressure sensor and/or a water chemistry sensor. In addition, the system may include a processor configured to detect positive movement of makeup water through the makeup water inlet line indicative of a leak of the hydronic boiler system based on the measured fluid flow, and cause a display to display information indicative of the leak upon detection of positive movement of makeup water through the makeup water inlet line. In some embodiments, the processor further may automatically adjust operations of the hydronic boiler system responsive to detection of the leak.