Mechanism of Mixing Water Regulation and Overheating Retrofit Strategies for Underfloor Heating Terminals in Systems with Coexisting Radiators

In district heating systems where radiators and floor radiant heating terminals coexist, the high-temperature supply water must be cooled by mixing with return water before delivery to underfloor heating terminals. However, overheating frequently occurs at these terminals in practice. Taking the mixing water system for underfloor heating terminals as the research object, this study develops a required mixing ratio model and a coupled hydraulic–thermal model to investigate the underlying overheating mechanism. The results show that the supply control valve has reached its lower regulation limit, and the actual mixing ratio remains below the target value—this mismatch is identified as the primary cause of terminal overheating. Quantitative analysis reveals that the original system exhibits temperature deviations of 3.5–6.0 °C under proximal high-pressure conditions and 1.6–2.1 °C under distal low-pressure conditions. To resolve this issue, three graded retrofit schemes are proposed: supply valve parameter enhancement (Scheme A), three-way valve scheme (Scheme B), and dual-valve scheme (Scheme C). Simulation results indicate that Scheme B reduces the temperature deviation to within ±0.5 °C under distal conditions, while Scheme C achieves the same under proximal conditions. Economic analysis indicates that, under the precondition that temperature control requirements are satisfied, the recommended schemes yield static payback periods of 3.8 years for mild overheating with Scheme A, 3.9 years for moderate overheating with Scheme B, and 4.1 years for severe overheating with Scheme C. A field case study of Scheme A demonstrates that the supply water temperature deviation is reduced from approximately 2.0 °C to within 0.5 °C under low-load conditions, confirming the effectiveness of the valve enhancement strategy. Due to current field constraints, the experimental validation is limited to Scheme A, while Schemes B and C are evaluated through the validated simulation model. The findings offer a quantitative reference for overheating diagnosis and retrofit selection in existing mixing water systems for underfloor heating terminals.