A significant increase in the efficiency of furnaces burning natural gas is achieved by increasing the radiation heat transfer from the flame. In this study, the effect of cylindrically shaped additional radiation surface called, filling material (FM), on emissions and temperature distribution in the combustion chamber (CC) was investigated experimentally. Experiments were carried out on a fire tube water heater described in the standard EN 676 for the firing rates of 58, 87 and 116 kW. Two diameters of the filling materials, 25 and 30 cm, and two lengths of 20 and 40 cm were considered. The flame temperature and nitrogen oxide (NO) emission were measured for different positions and geometries of the filling material in the combustion chamber. The flame temperature and the temperature drop of the flame in the back flow increased with the increasing diameter and length of the filling materials compared to the case without filling material. It was observed that the filling material with the larger diameter increased the heat transfer rate in the back flow compared to the case without filling material. The in-furnace measured NO concentration was in good agreement with NO concentration in the well-mixed flue gas. The 20 cm long filling materials decreased the NO emission. Increased length of the filling material resulted in increased the NO emission. (c) 2009 Elsevier Ltd. All rights reserved.