Reducing Duct Leakage in Large Commercial & Institutional Buildings
Josh Quinnell, Ph.D., Ben Schoenbauer, Dave Bohac, P.E.
Air leakage from distribution ductwork wastes energy by increasing fan power and discarding conditioned air. In Minnesota commercial and institutional (C&I) buildings, HVAC fans consume about 2,800 GWh of electricity per year (EIA 2008). Assuming about 5% duct leakage, approximately 380 GWh of fan power are lost on duct leakage per year. Duct leakage also results in significant heating and cooling energy penalties when conditioned air leakage is discarded from the envelope in exhaust or relief air systems. The recent development of a novel, patented sealing process (Aeroseal) makes it possible to tightly seal ductwork in retrofit applications as it requires significantly less access than traditional methods.
This project characterized duct leakage in several types of Minnesota C&I buildings, completed retrofit duct sealing on a subset of C&I duct systems, and estimated the energy savings and cost effectiveness of retrofit sealing measures. The project then analyzed the results to develop screening criteria that displace cost-prohibitive leakage measurements and tested the criteria in a short pilot program to identify cost-effective duct sealing opportunities.
Duct leakage for C&I ductwork systems was one-half to two-thirds less than anticipated, between 0% and 29% of measured flow rates
Retrofit duct sealing was successful in 75% of systems using both traditional and Aeroseal methods, and an average of 81% leakage was sealed and the median sealing rate was 86%. The Aeroseal method was especially effective, often reducing leakage effectively to zero in a variety of scenarios.
Energy & Cost Savings from Duct Sealing
In most sealed systems, 64% of energy saved was from heating (natural gas), 29% was from fan energy (electrical), and 6% was from cooling (electrical). The largest portion of cost savings come from reduced fan energy due to the higher cost of electricity. For a typical system, 66% to 75% of cost savings are from reduced electricity, and 25% to 33% of cost savings are from heating (natural gas). Simple payback periods range from 5 years to 142 years, with an average payback of 31 years and a median payback of 17 years.
Simple Screening Criteria
The project team identified four criteria that can be used to eliminate systems with poor payback and identify systems that are good candidates for cost-effective retrofit duct sealing: system type, operating pressure, design flow, and apparent tightness.
Screening potential systems according to simple criteria in lieu of measuring duct leakage reduced the average payback from 31 years to 15 years for the original 20 systems, and by 7 years when used as the basis of system selection in the pilot program.
Recommendations for CIP
Duct leakage in existing buildings has emerged as a new opportunity. Project results suggest that about 10% to 15% of C&I buildings have leakage rates high enough to justify retrofit duct sealing work with moderate to good payback of 7 years or less. The following are potential utility program opportunities:
Read the Executive Summary
- Incorporate retrofit duct sealing into existing commercial auditing, recommissioning, and turn-key savings programs.
- Conduct outreach to inform and educate vendors and trade allies about the benefits of retrofit duct sealing measures.
- Integrate Aeroseal duct sealing in to new construction practices.
This project supported in part by a grant from the Minnesota Department of Commerce, Division of Energy Resources through the Conservation Applied Research and Development (CARD) program. And with co-funding by CEE in support of its nonprofit mission to advance research, knowledge dissemination, and program design in the field of energy efficiency.