The recirculation pump on the Rinnai RL75 is no longer functioning. During the diagnosis found that the pump and clock are functional. Contacted Rinnai and talked with a technical specialist, checked the dip switches, and narrowed the problem down to either the fuse for the recirculation on the board, or the PCB Board itself. The fuse was tested and even replaced and the recirculation did not function. At this point the operation of the pump is likely a bad PCB Board according to the Rinnai Specialist, and should be replaced. Replaced existing, leaking, expansion tank with new expansion tank. RINNAI PCB Board part# 104000233