Thermally primed fuel processing assemblies and hydrogen-producing fuel cell systems that include the same. The thermally primed fuel processing assemblies include at least one hydrogen-producing region housed within an internal compartment of a heated containment structure. In some embodiments, the heated containment structure is an oven. In some embodiments, the compartment also contains a purification region and/or heating assembly. In some embodiments, the containment structure is adapted to heat and maintain the internal compartment at or above a threshold temperature, which may correspond to a suitable hydrogen-producing temperature. In some embodiments, the containment structure is adapted to maintain this temperature during periods in which the fuel cell system is not producing power and/or not producing power to satisfy an applied load to the system. In some embodiments, the fuel cell system is adapted to provide backup power to a power source, which may be adapted to power the containment structure.