New burner design technologies are using the momentum of fuel gas injection to entrain flue gas for the purpose of diluting the combustible mixture in order to reduce NOX emissions. Using an eductor, these designs entrain flue gas from above the convection section of a furnace stack and mix with the fuel downstream of the eductor. This diluted fluegas/fuel mixture results in lower local adiabatic flame temperatures providing a reduction in NOX emissions. Test results show that NOX reduction performance is strongly dependent on the mass ratio of flue-gas to fuel. An entrainment mass ratio of flue-gas/fuel typically ranges between 2 to 3 pound flue-gas per pound fuel, leading to a NOX reduction of approximately 50 to 70 % for boiler burner applications. The flue gas entrainment performance is effected by pressure drop through the upstream and downstream FIR piping system. This paper describes optimization of an existing FIR system not meeting initial design performance. To improve the entrainment ratio several tools were used to re-design and optimize the FIR system. These tools included Computational Fluid Dynamics (CFD), semi-empirical modeling and cold flow test results. A detailed description and discussion of the results are presented.