There are many attempts to use a fuel cell system as a residential power generation system. The purpose of this study is to investigate the optimal design of a water tank for a hot water system when the fuel cell co-generation system is combined with a domestic hot water supply system. The demands of hot water supply per month per home for $76-168m^2$ of acreage are investigated in Busan for a year. It showed somewhat large differences between the actual demand and the designed demand of hot water, but the actual capacity of hourly averaged hot water demands is analyzed as $60{ell}/h$ in this study based on the actual demand. The experiments are performed in the various inlet and outlet locations of nozzles in the hot water tank, and the hot water consumption rates. The experimental results showed that the optimal capacity of the water tank is $200{ell}$ when the thermal efficiency, the storing capacity of hot water and the space for installation are considered.