Rinnai HD55i Thermal Fuse Assembly – (P55-725) The thermal fuse in a Rinnai HD55i continuous flow water heater is a fail-safe within the overheat protection circuit. If a combustion or airflow fault causes the unit to run hotter than it should, the fuse trips and cuts power, stopping the appliance before any further damage occurs rather than letting it continue running in an unsafe state. It works alongside the other sensors and switches in the HD55i's safety system, so if the fuse itself fails or becomes faulty, the water heater will simply stop operating until it's replaced and the underlying cause has been checked and resolved. This is the genuine Rinnai replacement, made to the same specification as the original, restoring that overheat protection properly rather than working around it. ⚠ Important: This is a safety-c