In this study, characteristics of three different combi-heaters and heat exchangers were experimentally investigated. All combi-heaters tested have similar technical specifications and have almost the same heat load. Main difference among the combi-heaters is the type of heat exchanger used. Tests were performed for combi heaters containing a combined heat exchanger (K1), a flat plate heat exchanger (K2), double heat exchanger (K3). The objective of this study is to determine the performance of all combi heaters and to determine the benefits and advantages of K1. Performance of all combi heaters were investigated for the heating cycle, domestic water cycle with and without the heating cycle in use, and similarly heating cycle with domestic water in use and finally for different mass flow rates. According to the results of different and parametric experiments for performance evaluation, for the combi heater with combined heat exchanger (K1), advantages in heat transfer characteristics have been detected compared to the other two combi-heaters tested.