Duct Design Impacts on Energy Consumptions and Life Cycle Costs for Residential Central Heating and Cooling Systems

Abstract

In this study, a series of laboratory measurements was conducted on residential air handling units (AHUs) and air conditioners to characterize their performance at typical installed conditions. In addition, performance models of blowers and air conditioners were developed from the laboratory measurements and integrated with building energy simulations to predict the energy consumption and the life cycle cost of ductworks with respect to different climates (Chicago, IL and Austin, TX), duct materials (e.g., sheet metal and flex ducts), flow resistances, and blower types (e.g. PSC and ECM blowers).

The experimental results showed that PSC and ECM blowers have distinct airflow, power, and efficiency performance in response to increases in the external static pressure (ESP). The building energy simulation results showed that increasing the duct flow resistance from 0.3 to 0.9 in. w.g. (75 to 225 Pa) decreased airflow rates of PSC blowers and consequently decreased the annual blower electricity consumptions by 11% for the Austin home and 16% for the Chicago home. However, in systems with ECM blowers the same increase in the duct flow resistance increased the annual blower electricity consumptions by about 60% for both the Austin home and the Chicago home, primarily because ECM blowers maintained constant airflow rates over a range of pressures. For the same increase in the duct flow resistance, the electricity consumptions of condensing units in systems with PSC blowers increased by 2.7% for the Austin home and 5.5% for the Chicago home, while the electricity increase in systems with ECM blowers were less dramatic, being 1.6% for the Austin home and 1.5% for the Chicago home. Also, the simulation results indicated that although the cost-effectiveness of a specific duct design is shown to be heavily dependent on initial duct fabrication and installation costs, the use of lower flow resistance ductworks generally leads to lifetime savings in the presence of supply and return leakages of 10%. Specifically, the lifetime savings is achieved in 6 out of 8 simulated cases for the Chicago home and all of the simulated cases for the Austin home by using ductworks at lower flow resistances.