A method is provided for operating a heater including a heater housing extending along a heater axis; a plurality of fuel cell stack assemblies disposed within the heater housing along the heater axis and having a plurality of fuel cells which convert chemical energy from a fuel cell fuel into heat and electricity through a chemical reaction with a fuel cell oxidizing agent; and a plurality of combustors disposed within the heater housing along the heater axis. The method includes supplying a combustor fuel to the plurality of combustors, combusting the combustor fuel to produce a heated combustor exhaust when the fuel cell stack assemblies are substantially electrochemically inactive, and using the heated combustor exhaust to elevate the temperature of the fuel cell stack assemblies to be electrochemically active.