A heater includes a heater housing with a fuel cell stack assembly disposed therein. The fuel cell stack assembly includes a plurality of fuel cells which convert chemical energy from a fuel into heat and electricity through a chemical reaction with an oxidizing agent. The fuel cell stack assembly includes a fuel cell manifold for receiving the fuel within a fuel inlet and the oxidizing agent within an oxidizing agent inlet and distributing the fuel and oxidizing agent to the fuel cells. A fuel supply conduit supplies the fuel to the fuel inlet and an oxidizing agent supply conduit supplies the oxidizing agent to the oxidizing agent inlet. A sonic orifice is disposed between the fuel supply conduit and the fuel inlet or between the oxidizing agent supply conduit and the oxidizing agent inlet, thereby limiting the velocity of the fuel or the oxidizing agent through the sonic orifice.