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Combi boiler type heating appliances are used nearly in every residential building for both space and domestic hot water (DHW) heating functions. There are challenging targets regarding both of these functions. Since there is a huge laboratory testing procedure behind each appliance design and the structural or operational changes on the regular designs, simulation models could be established for the initial evaluations of the appliance testing. Therefore, due to the estimations from the preliminary results of the simulations, the number of the laboratory tests could be decreased with cost, time, and energy savings. In this study, the main objective is modelling DHW heating circuit of a combi boiler appliance with the help of Transient System Simulation Tool (TRNSYS 18) to calculate DHW outlet temperature under various operating conditions. The TRNSYS model is validated experimentally and compared with the previous works of the authors. A good agreement is achieved in both transient and steady-state regions and the TRNSYS model is found superior when compared to the previously established one-dimensional model of the authors. DHW circuit model is validated only for economic (eco) working mode simulations in this study. Mean absolute error (MAE), mean square error (MSE), and root mean square error (RMSE) values are compared for the outcomes of the previously constructed one-dimensional model and currently established TRNSYS model with reference to the experimental data. TRNSYS model decreases all of these errors calculated according to the overall temperature profiles including the transient region for the central heating water at the heat cell inlet, central heating water at the heat cell outlet, and the inlet and outlet temperature difference of DHW at the experimentally investigated DHW flow rates of 5 l/min, 7 l/min, and 8.7 l/min.