Herein, a fuel cell power system that works in CHP mode has been considered to provide electrical, heating, cooling and domestic hot water loads of buildings. The buildings tolerate hot summers and mild winters during their operation life. In previous research, the PEM fuel cell system was designed and number of fuel cell stacks to provide the required energy of the building was estimated. As a complementary investigation, the thermodynamic and environmental analysis of the mentioned system has been conducted in this research. Results have shown that for a 12 fuel cell stacks at a nominal capacity of 8.5 kW, the mass production of monoxide carbon, monoxide nitrogen and dioxide carbon are equal to 1272.621 (kg/year), 1609.056 (kg/year) and 26107.23 (kg/year), respectively. The mass production values are acceptable since it is within limitation ranges mentioned by many environmental protocols