A Non-Intrusive Spiral Coil Heat Exchanger for Waste Heat Recovery from HVAC Units: Experimental and Thermal Performance Analysis

Heating, ventilation, and air conditioning (HVAC) systems contribute substantially to global energy consumption, while rejecting significant amounts of low-grade heat into the environment. This paper presents a non-intrusive spiral-coil heat exchanger designed to recover waste heat from the outdoor condenser of a split-type air conditioner. The system operates externally without altering the existing HVAC configuration, thereby rendering it suitable for retrofitting. Water was circulated as the working fluid at flow rates of 0.028–0.052 kg/s to assess thermal performance. Performance indicators, including the outlet water temperature, heat transfer rate, convective coefficient, and efficiency, were systematically evaluated. The system achieved a maximum outlet water temperature of 67°C and a peak thermal efficiency of 91.07% at the highest flow rate. The uncertainty analysis confirmed reliable measurements within ±3.45%. The monthly energy savings were estimated at 178.35 kWh, accompanied by a reduction in CO2 emissions of up to 187.26 kg, yielding a short payback period of 1.06 years. These results demonstrate the feasibility of spiral-coil heat exchangers as cost-effective and eco-friendly alternatives to conventional electric water heaters. The proposed approach not only enhances the overall energy utilization but also contributes to energy conservation and climate mitigation objectives.