A heater assembly includes a plurality of fuel cell stack assemblies which each have a plurality of fuel cells, a fuel inlet, and an air inlet; a fuel supply conduit which communicates fuel to the fuel inlets; and an air supply conduit which communicates air to the air inlets. An orifice is disposed between the fuel supply conduit and the fuel inlet or between the air supply conduit and the air inlet of each fuel cell stack assembly. The plurality of fuel cell stack assemblies are arranged in fuel cell stack assembly groups such that the orifices of each of the fuel cell stack assembly groups are configured to provide a magnitude of restriction that is unique to their respective the fuel cell stack assembly group, thereby providing uniformity of flow of the fuel or the air to the plurality of fuel cell stack assemblies.