One place I know with Russel coils that had this issue also had high voltage. Wondering how much hotter a heater might run with high voltage now.
Either way a KE2 made the problem go away
EBM EC fans will not burn themselves out as they use magnets inside to keep them turning ,, when the stator can't pass these magnets it just stops.
The primary function of the design engineer is to make things difficult for the fabricator and impossible for the serviceman.