Airfoilmod hit the nail on the head in post 2261. The FOHE isn't a fuel heater it is an oil cooler. The fuel system itself isn't a fail safe design.
Having established that the current design traps ice RR and Boeing have to go to the 737 design of the fuel system where there is a bypass with a fuel heater. Those of us who flew the 732 remember well cycling the fuel heater and watching the oil temperature rise as an indication of it working.
It might also be prudent to put the bypasses back on all aircraft in particular ETOPS twins that are flying longer sectors than envisaged when they were designed.