The aim of presented research is to develop a numerical model and to simulate the operation of different designs of low temperature boiler prototypes with the main purpose of preventing combustion gas condensation on the heated boiler surfaces. The results of the numerical simulation performed using the computational package Fluent are compared with the measurement results. The development of prototypes and the experimental part of research have been conducted in the Laboratory for Applied Thermodynamics at the Faculty for Mechanical Engineering and Naval Architecture of the University in Zagreb. The research methodology is based on the combination of scientific and engineering methods, as a synthesis of theoretical methods and practical experiments. In the first part of this work low temperature boilers and their features are described. Expressions and calculation procedures of the heat transfer in boilers are provided. Due to the complexity of low temperature boiler models, numerical simulations of combustion gas tubes inserted with turbulators and without them are firstly performed in a wide range of parameters. In the simulations the velocity and temperature of combustion gases at tube inlet, heat transfer coefficient on the water side, the length of outer tube, the air gap thickness and heat conductance of the tube material were varied. By means of conducted numerical simulations geometry of the double walled tubes, which are used to prevent the condensation of combustion gases, is obtained. In the second part of the work, beside the description of the experimental setup, the conducted measurements and reconstructions on the low temperature boiler prototypes of nominal heat capacity 25 kW are described. The third part of the work deals with the development of low temperature boiler numerical model for numerical simulations developed with the purpose to prevent combustion gas condensation on other heated surfaces. Two models are set up: the model that comprises the water side and combustion gases in tubes and the model that comprises the burner, combustion chamber and chamber between the combustion chamber and the tubes. Following the results of numerical simulation and former conducted measurements, the low temperature boiler prototype of nominal heat capacity 25 kW is defined. The developed numerical model of a low temperature boiler enables the optimization of relevant design parameters in order to prevent combustion gases condensation, as well as the reduction of tests needed in the designing process of the boiler.