Duct System Performance and Energy Losses in Large Commercial Building

This paper presents the findings of recent work characterizing thermal distribution systems in four large commercial buildings by the Lawrence Berkeley National Laboratory (LBNL). The buildings are multi-story large-office buildings or retails buildings with floor spaces of more than 2,000 m². They typically contain single-duct or dual-duct constant air volume (CAV) systems, or variable air volume (VAV) systems. Thermal distribution losses in the duct systems studied are categorized into air leakage and conduction, with the latter including convective and radiative losses. This paper reports air leakage results in terms of effective leakage area (ELA) and ASHRAE-defined duct leakage class, and compares with those found in previous studies. The new data of leakage classes indicate that air leakage in large commercial duct systems varies significantly from system to system, and from system section to system section. Most of the duct systems measured are much leakier than the ductwork specified as "unsealed ducts" by ASHRAE. Energy losses due to air leakage and heat conduction through duct systems are significant.