HVAC System Sizing by Case Studies of Zero-Energy-Certified Apartments

Purpose: Current unit-area heating and cooling load criteria (KS C 9306, Korea District Heating Corporation Heat-Using Facilities Standards (KDHC)) were established decades ago and do not reflect improved building envelope performance. While thermal transmittance standards have been updated, HVAC sizing guidelines remain outdated, causing potentially oversized systems in Zero-Energy Buildings (ZEBs). This study analyzes certified ZEB apartment cases to establish unit-area HVAC loads and quantify oversizing relative to existing standards. Method: A certified ZEB Grade 5 reference model—an 84m² apartment—was modeled as a 12-story building with two units per floor, based on four certified apartments in Central Region II (Seoul, South Korea). Envelope U-values and lighting power densities from Energy-Saving Design Standards (1987, 2013, 2017), the ZEB Technology & Components guide (ZEB T&C), and Cases 1–4 were applied. Peak heating and cooling loads were calculated using RTS-SAREK and compared across standards and cases. Additionally, effects of envelope improvements on load reduction were analyzed. Result: Compared to ZEB cases, KS C 9306 showed 199% higher cooling and 696% higher heating loads, while KDHC Standards produced 46% higher heating loads. Energy-Saving Design Standards still resulted in up to 28% cooling and 159% heating oversizing. Envelope improvements produced nonlinear heating load reductions with diminishing returns, while cooling load reduction was more sensitive to internal loads. ZEB cases yielded unit loads of 52–55W/m² for cooling and 28–34 W/m² for heating, indicating the need to update unit-load criteria for ZEB-level performance.