In this undergraduate thesis the calculation of two types of heat exchanger, parallel flow and counter-flow heat exchanger, was performed. The aim of the thesis is to size different types of heat exchangers. The fluids in the calculation of the heat exchanger are water and flue gases created by the combustion of pellets. Heat exchangers have the task of ensuring, by transferring heat from the flow of flue gases to water, a heat flow for heating a family house in the amount of 35 kW. In addition to the calculation of the heat exchanger designs, this thesis also includes the calculation of the combustion of pellets with the aim of calculating the air excess ratio required for complete combustion and the required mass flow of pellets that would ensure the required amount of heat flow. Calculation of different designs of heat exchangers with different temperatures of flue gases at the entrance yielded values based on which the comparison of heat exchangers was made. During the calculation, various assumptions were used, which makes the calculation predominantly theoretical in nature. By varying the inlet temperature of the flue gases into the heat exchanger, different necessary surfaces for heat exchange were obtained. By increasing the flue gas inlet temperature and using counter-flow instead of parallel-flow heat exchangers, the required surface area for heat exchange was smaller.