Post-rebuild profiles of velocity, species concentration (02, CO, and CO2), and gas temperature are reported in the portnecks of a regenerative, side-port, 550-ton/day, gas-fired, flat-glass furnace. These measurements are also compared to similar ones made before the same furnace was rebuilt. Measurements were also made below one of the regenerators in the tunnel leading to the furnace stack after the rebuild. Fewer variations were observed in the exhaust profiles of most measured variables after the rebuild. Flat inlet velocity profiles were measured with a magnitude of approximately 11 m/s before and after the rebuild. The temperature of the inlet preheat air was generally speaking higher and the furnace exhaust temperature lower before the rebuild. Locations of low O2concentration in the effluent are consistent with high CO concentrations before and after the furnace rebuild. CO2concentrations are nearly uniform across the portneck height, more so after the rebuild. The measurements in the tunnel after the rebuild indicate a stratification effect in the species concentration measurements. These measurements also indicate that the combustion reactions continue inside the regenerators resulting in overall complete combustion as indicated by the very low CO levels in the tunnel. A mass balance analysis for the overall combustion reaction based on the measurements of O2and CO2and fuel flow rate in each port showed that (1) before and after the furnace rebuild the predicted CO2formed in the glass is within 15% of the value estimated by Ford personnel; and (2) the overall stoichiometry was not much different before and after the rebuild (22.5% excess air before compared to 19.2% after). The total airflow rate calculated by this analysis after the rebuild is within 8% of the plant-measured value.