A plurality of heaters are disposed end to end within a bore hole of a formation where the bore hole extends from an upper end to a lower end such that a lower heater of the plurality of heaters is proximal to the lower end of the bore hole while every other of the plurality of heaters is distal from the lower end of the bore hole. Each of the plurality of heaters includes a fuel cell stack assembly having a plurality of fuel cells which convert chemical energy from a fuel into heat and electricity through a chemical reaction with an oxidizing agent. Each of the plurality of heaters has a thermal output that is less than or equal to a predetermined value except the lower heater of the plurality of heaters which has a thermal output that is greater than the predetermined value.