Abstract

The energy consumption of residential buildings in Korea has decreased by about 43% due to the widespread use of high-insulation, high-airtightness, and condensing boilers. However, further reductions are needed to achieve carbon neutrality by 2050. Condensing boilers can enhance efficiency by utilizing latent heat from flue gas condensation, but conventional High-Temperature Water supply limits this potential. This study analyzed the energy performance of a condensing boiler under different load conditions in an 84 m² experimental house in Sejong City, comparing High-Temperature Water (HTW) and Low-Temperature Water (LTW) systems. The LTW operation achieved an average efficiency of 86.9%, 8.5%p higher than HTW, due to lower return water temperatures (40.94, 41.91, 42.49°C) below the 55°C condensation point. Additionally, LTW maintained indoor temperatures close to the setpoint, while HTW caused 0.3 ~ 0.4°C overshooting, indicating unnecessary energy loss from higher supply temperatures and overheating.