Effect of gradient phase-change temperatures on the thermal performance of floor heating systems with encased phase-change materials

As phase-change materials (PCMs) are increasingly used in radiant floor heating systems (RFHS), their ability to reduce temperature fluctuations and improve energy efficiency has been widely demonstrated. However, existing studies have primarily focused on PCMs with a single phase-change temperature, whereas gradient PCMs in encased floor systems remain insufficiently investigated. This study proposes an optimized design that integrates gradient PCMs into a single–layer encased RFHS. A computational fluid dynamics (CFD) model was used to systematically investigate the effects of the phase-change temperature ranges of the inner and outer PCM layers on the thermal behavior of the RFHS. The results reveal a distinct optimization mechanism between the two PCM layers: the outer PCM layer dominates the floor surface response speed and temperature uniformity by regulating heat transfer near the floor surface, whereas the inner PCM layer governs the heat release rate and high-temperature duration by controlling latent heat storage near the hot water pipe. By optimizing the phase-change temperature gradients of the inner and outer PCM layers, the optimal configuration was identified (outer layer: 32–37 ℃; inner layer: 38–43 ℃). Compared with the conventional singletemperature PCM design, the optimized system achieved a 9.89% increase in phase-change rate, a 24.32% extension in high-temperature duration, a 1.8% rise in average temperature, and a 5.41% reduction in temperature fluctuation. This study provides theoretical guidance and technical support for improving the thermal efficiency of encased RFHS and promoting their application in green buildings.