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Latent heat storage units are widely used in building heating systems due to its high energy storage density, whereas the practical performances of them are limited by the low thermal conductivities of phase change materials. In this paper, copper nanoparticles were added into paraffin to enhance the heat transfer rate of a latent heat storage unit using a coil heat exchanger. A three-dimensional numerical model was built to simulate the melting process of phase change material, and it was well validated against the experimental data. The simulation results showed that the nanoparticle-enhanced phase change material saved 19.6％ of the total melting time consumed by the pure phase change material. In addition, the dispersion of nanoparticles significantly alleviated the temperature non-uniformity in the unit. Moreover, for the unit using nanoparticle-enhanced phase change material, the flow rate of heat transfer fluid was not recommended higher than 0.75 m3/h. The dispersion of nanoparticles could enlarge the optimum heat transfer fluid temperature range to 60-70 °C compared with that of pure phase change material (60-65 °C). Therefore, the application of nanoparticle-enhanced phase change material in the latent heat storage unit can significantly enhance heat transfer, and the proposed optimum inlet heat transfer fluid temperature range could contribute to higher energy efficiency.