The use of underground air-conditioning tunnel for Tehran’s climate has been investigated. This study is conducted over a 24-h period from 0 to 24 h. Simulation is performed using computational fluid dynamics. The continuity, Navier–Stokes, and energy equations are solved for the air inside the underground pipe. Also, the energy equation is solved for the surrounding soil. The numerical solution is performed by the Fluent software by adding a user-defined function. The simulation results for this system indicate that the inlet air temperature has a significant effect on the outlet temperature. The results show that for a short period in the middle of the day, the maximum output temperature of the system reaches to 27 °C. So, except for a short period of the day, this exchanger creates a comfortable condition for the city of Tehran. Also, due to the thermal saturation of the soil, this system is not suitable for continuous work. It is only suitable for industrial environments that operate in a short period of time during the day.