© 2022 Elsevier LtdIn this study, a boiler heating unit of a building is used as a heat source for an economizer heat exchanger which is connected to a PCM (phase change material) latent heat storage unit-based heat exchanger used for additional heating system. The system is simulated and analyzed for 24 min. It is found that during the melting process of the RT55 PCM, the heat flux increases up to the 250th second and conduction-dominant heat transfer occurs. The flow velocities around the serpentine tube heat exchangers are higher (between 0.01 and 0.03 m/s) between the 200th and the 800th second where transmission-dominant heat transfer is dominant. At the end of the process, the average temperature of the PCM and the liquid fraction reach 339 K and 86%, respectively, with the heat transferred from the flue gas to the heat carrier fluid at 802 K. The economizer heat exchanger efficiency rises to 51% due to the temperature difference between the heat transfer fluids. In addition, when the temperature is close to the PCM melting temperature, between the 450th and 610th second, the efficiency of the PCM heat exchanger rises to 13% as a maximum rate.