Reducing Duct Leakage in Large Commercial & Institutional Buildings
Josh Quinnell, Ph.D., Ben Schoenbauer, Dave Bohac, P.E. — Jun 2013
The project team conducted a pilot to determine if it was possible to identify cost-effective retrofit duct sealing opportunities. After identifying four successful criteria (system type, operating pressure, design flow, and apparent tightness), average payback was reduced from 31 years to 7 years. About 10% to 15% of Minnesota commercial and institutional buildings likely have appropriate operational characteristics and leakage rates that are high enough to achieve this payback.
Full project update
Why this research is needed
In large buildings, duct leakage can cause significant HVAC system energy penalties, even when the ducts are entirely within the building enclosure. Duct leakage in supply and exhaust systems often results in extra heating or cooling energy due to increased amounts of makeup air. Energy used by supply, return and exhaust fans to move air through leakage paths is also wasted. Fan energy accounts for over 12% of C&I sector electricity use (EIA 2008) and the heating and cooling loads are often the largest energy loads in a building. Even small rates of duct leakage can have big impacts. For example, 10% of duct leakage needlessly increases fan energy by as much as 27%. For a typical supply system 10% of duct leakage can increase heating and cooling energy requirements by 3% each.
- Leaks from supply ducts into return plenums (typically above suspended ceilings) allow some of the supply air to bypass the conditioned space and return to the air handler without delivering heating or cooling to the space.
- Leaks into exhaust ducts from interstitial spaces result in wasted fan energy and diminish design exhaust flow rates; exhaust leaks in conditioned spaces waste both fan and thermal energy
- Duct leaks can also impact outdoor airflow rates and infiltration, increasing heating and cooling energy use.
Project process and expected outcomes
The project team has completed all field work and submitted the final report to the Minnesota Department of Commerce, Division of Energy Resources, for review. This final report includes lessons learned to clarify conclusions that will be useful to utility program staff, contractors, and future researchers.
Initial findings indicate that Aeroseal, a patented sealing method, makes it possible to tightly seal duct work through retrofits. Thanks in part to this method, the overall cost was much lower than anticipated. Staff are currently developing plans to expand the project’s scope to apply guidelines and expected savings to a new set of buildings to better understand both how to make retrofit duct sealing cost-effective and how Aeroseal would work on larger, leakier duct systems than were initially tested.
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.