The objective of this work is to study the feasibility of temperature moderation inside a room using a phase-change material (PCM) which is stored in storage units. A real-size room which is at temperature conditions typical in a desert region in summer is considered. The idea is to use a phase change material which could melt during the day hours, absorbing heat from the room, while at night it solidifies due to a low night temperature. The heat from the room air to a PCM unit is free or,forced-convected. The numerical model includes the transient heat conduction inside the walls/ceiling, free/forced convection of air, and radiation inside the room. The processes inside the PCM are modeled by the effective heat capacity, (EHC) method. The PCM is assumed to melt and solidify, within a certain temperature range, which represents the trite situation for most commercial-grade phase-change materials. The numerical calculations are performed for the transient temperature fields inside the three-dimensional. room, including PCM in the units, walls/ceiling, and the interior of the room. The boundary, conditions for the room are chosen according to the experimental data which were obtained in previous works. The basic conservation equations of continuity momentum, and energy, are solved numerically, using the FLUENT 6.1 software. The numerical simulations are performed for at least one full 24-h cycle. Effect of different parameters on the behavior of the system is discussed, including the mass of the PCM and radiation effects inside the room. The night cooling by free and forced convection is analyzed. It is shown that a complete 24-h cycle is feasible in a property designed configuration with a suitable PCM.