The behavior of heat transfer and emissions in a semi-industrial 300 kW(th) natural gas fired furnace with three pairs of regenerative flameless oxidation burners was studied. The furnace offers unique possibilities for varying burner positions and firing modes (parallel and staggered). The operational behavior of two burner configurations have been compared regarding emissions (NO, CO) and temperature uniformity, for both parallel and staggered firing mode. Additionally, the flue gas O-2 percentage (excess air ratio) and the cycle time have been varied. Parallel firing mode results in a higher temperature uniformity ratio in the furnace and in a lower NO emission. CO emission did not vary much between parallel and staggered mode. (C) 2011 Elsevier Ltd. All rights reserved.