A large-scale hot water production system using a natural circulation loop was developed. A computer simulation program was developed to design and evaluate thermal performance of the natural circulation system for hot water production. An experimental apparatus was set up and was tested against various conditions to exhibit a stable operating region of the natural circulation loop. When the system was a stable state for heating rate (695 ㎾) and feed water was circulated at 0.3 $ell$/s constantly. A unstable state was checked by experiment and that time flow rate was oscillated with 0.4∼0.6 $ell$/s. The result showed that the program can predict the thermal performance of the large-scale hot water system using the natural circulation loop and can be utilized to design the system.