The most commonplace natural flow inside one-ended inclined pipes today is wa-ter heating systems. In this study, a model was created for the estimation of the pipe outlet temperature of the fluid with the energy balance for the inside of the tank, flow rate calculation of natural circulation, and other thermal calculations in a one-ended inclined pipe. In addition, this model has been compared with the Li model in the literature, and it is easier and more successful. 100 W, 200 W, 400 W, 600 W, and 800 W thermal power was applied to a one-ended inclined pipe, and the temperature values were recorded in five minute periods in the ex-periments that lasted a total of six hours. As the average of the experiments, the estimation results for the current model and the Li model are: the average per-cent relative error rates are 6.02 and 15.1 and the coefficient of determination, R2, are 0.9865 and 0.9683, respectively.