I think you may be right that with that many years of use there is plenty of accumulation of deposits which may be the root cause of the odors. Since the heater is in a very place, If I have to do the labor the remove & re-install it I will seriously think about replacing with a new one. I wonder if there is a way to flush it out while leaving it in place by connecting a garden hose up to it or it there is a drain for flushing besides the tiny drain plug (does not appear to be )