This is what Willis is talking about. If coolant leaks out, and refer is still trying to operate, the boiler area can be superheated and present a hazard. The picture above is a unit that failed in this manner. When I arrived on the job site, the refer was still trying to function on AC and the discolored area you see was glowing red-hot. This is because there is no longer any coolant in the refer to dissipate heat, and since the refer can't cool, the board will continue to send power to the element. The thermal fuse will not activate in time to prevent a potential hydrogen fire, it is there to minimize the on-going over-heating hazard after such a failure. Tim
