This study mainly touches on natural gas fueled combi boiler appliances having space and water heating functions. The hot water demanded in any type of building for daily activities such as personal hygiene, cleaning, and bathing is referred to as domestic hot water (DHW). Since high efficiency targets have been already reached with respect to the space heating, most of the research and development activities are now focusing on the DHW heating function. Therefore, the subject of this study is concerned with only DHW heating function and the basic objective is making a comprehensive comparison between two mathematical models via different approaches for the same DHW circuit. In the first approach, basic heat transfer equations for DHW heating function are established by means of one-dimensional and time dependent differential equations. Later on, those transient energy equations were discretized by finite-difference method implicitly and solved simultaneously in Matlab (R). The second model has been constructed with the help of commercial software, Flowmaster (R) which is one-dimensional thermo-fluid simulation tool. Then, the numerical results have been compared with the experimental data and the validity of the models has been discussed separately on the basis of the agreement between them. The advantages of both models have been evaluated as the distinctive contributions to the literature from DHW circuit modeling viewpoint. Among the intensive research and development activities of the combi boiler manufacturers to achieve high DHW comfort scores for high selling rates in the market, decreasing the time and money spent on the laboratory tests of the appliances is a challenging issue. Number of the laboratory tests is proposed to be decreased due to the preliminary results obtained from properly working numerical models for the performance analyses of completely new appliance concepts or design modifications on the regular appliances. (C) 2016 Elsevier B.V. All rights reserved.