Impacts of duct leakage on central outdoor-air conditioning for commercial-building VAV systems

• Duct leakage loss model for VAV buildings is developed.
• Leakage increases electricity and natural gas use by 2.04 kWh/m2 and 0.17 scm/m2.
• Total cost savings for sealing 90% leakage is reported.
• Maximum savings (49–65%) are from reduced fan power and fan heat.
• Outdoor air-conditioning savings represent 10–20% of total savings.

The purpose of this study was to estimate the energy losses due to duct leakage during central heating and cooling of outdoor air in commercial buildings with VAV systems. This was accomplished by calculating the required cooling and pre-heating of outdoor air with and without duct leakage in three locations. Our simplified model uses hourly weather data, along with leakage level, outdoor air fraction, and fan flow profiles to estimate heating and cooling loads associated with conditioning outdoor air for a typical office building. Energy calculations are based upon the difference in outdoor air flow due to duct leakage, combined with enthalpy differences between return air and outdoor air for cooling, and between mixed air and supply air for heating. Our results indicate that 10% pre-VAV leakage at design flow, combined with 10% post-VAV leakage flow, translates to an additional 2.04 units/m2/year of electricity consumption for outside-air cooling for an office building that has a 20% outdoor air fraction in Palm Springs, CA. For the same building in Chicago with 80% heating efficiency, this leakage results in an additional 0.17 scm/m2/year of natural gas consumption for pre-heating outdoor air. Based upon local utility rate structures, annual savings estimates for sealing 90% of 19% system leakage at a 20% outdoor air fraction were developed. These savings were found to vary between $1.72 and $2.80 per m2 annually when including the impacts of duct leakage on fan power, fan heat removal, and conditioning of excess outdoor air. The outdoor air conditioning savings represents roughly between 10 to 20% of this energy cost savings.