Thermal characteristics of a thermal storage tank containing phase change material (PCM) and installed in an air distribution system were investigated. PCM in the form of a mixture of trimethylolethane and urea with a melting temperature of 13℃ was used in flat rectangular containers. Experiments were conducted in order to examine temperature responses in a charging (freezing) process and a discharging (melting) process. Due to sub-cooling, the charging temperature was found to necessarily be below 5℃. For the discharging process, temperature difference between the inlet and outlet was maintained at 2 - 4℃. On the basis of the experimental results, a proposed simulation model based on the enthalpy method was used to successfully simulate the sub-cooling phenomena.