In this study, effects of the addition of solid surface on carbon monoxide (CO) emission reduction were investigated in a combustion chamber of a three pass fire tube water heater. The solid surface called as the filling material (FM) was placed coaxially in the center of the combustion chamber. The experiments were carried out on a 116 kW fire tube water heater as described in the European Standard EN 676. A natural gas burner was equipped to this heater. The geometry of the FM and excess air ratio (EAR) were the parameters for the experimental study. The CO and hydrogen (H(2)) concentrations along with the flame temperature were measured in the combustion chamber. It was found that the FM decreased the CO and H2 concentrations considerably. But the flame temperature increased with increasing diameter and length of the FM. For the stochiometric air-fuel ratio, CO emission decreased dramatically compared to the no-FM-case. In the case of without FM, the flame diffused radially in the combustion chamber and the CO concentration increased for the stochiometric air-fuel ratio. It is apparent that the FM diameter is more effective to reduce the emission of the incomplete products. (C) 2010 Elsevier Ltd. All rights reserved.