Rinnai HD55i Thermal Fuse Assembly – The Genuine Article The Rinnai HD55i Thermal Fuse Assembly is a top of the line safety component designed specifically for use in Rinnai HD55i continuous flow gas water heaters. This thermal fuse is a crucial part of the overheat safety circuit - basically, if your water heater starts getting too hot, it'll trip a circuit breaker to prevent any further damage from being done. A thermal fuse is basically a fail-safe in the heater's safety system. If there are any problems with the combustion or airflow then the fuse will kick in and automatically cut power to stop the water heater getting trashed or shutting down till it can be properly sorted. In Rinnai systems, the thermal fuse is part of a network of safety sensors & switches which work together to