In 2011, Katipamula and his PNNL colleagues set out to adapt the controls already found in air handling units for use in packaged rooftop HVAC units. The goal was to enable packaged units to automatically adjust their operations based on conditions inside and outside a building. Using sensors and variable-speed motors, the controls decide when and how fast ventilation fans should run, and if the units can use naturally cold air from the outside instead of mechanically cooling indoor air.

While the PNNL team was evaluating how these controls could work, they learned a few companies were simultaneously and independently in the process of developing such advanced controls. During the summer of 2012, the team installed one of the commercially available control kits on 66 rooftop HVAC units at eight volunteer commercial buildings in California, Ohio, Pennsylvania, and Washington. The buildings included shopping malls, grocery stores, big-box stores, and a medical clinic. The installed devices, manufactured by Transformative Wave of Kent, Washington, were chosen because they most closely resembled the advanced controls PNNL had envisioned.

Katipamula and his colleagues found that, compared to standard operations, the HVAC units using advanced controls cut their energy use from 20 to 90 percent. The average energy reduction was 57 percent. Larger buildings such as malls, which need bigger HVAC units, saved more energy than smaller buildings. And buildings that ran ventilation fans more, such as stores open long hours, tended to save more energy.

Translating the energy savings into dollars saved depended on local power costs. Nationwide, energy costs an average of 10 cents per kilowatt-hour, though areas with abundant and inexpensive power supplies often pay less and large cities with greater energy needs generally pay more. When using the national average, researchers found all the field-tested HVAC units would have saved an average of $1,489 annually per unit. The team calculated it would take a building owner three years to recoup the cost of buying and installing advanced controls with that average cost savings. Commercial buildings often have multiple rooftop HVAC units, so actual savings per building would depend on the number of units used.

But the exact payback period depends on several factors. To help building owners weigh the costs, the research team developed a table that lays out which specific combinations of an HVAC unit’s size, the number of hours its fan runs daily, and the local energy rate would result in a three-year or less payback period. The team concluded installing advanced controls in smaller units with a capacity of 15 tons or less could achieve a three-year payback in areas where energy costs 12 cents per kilowatt-hour or more, or where sufficient utility incentives were available.

“I’m proud to see the advanced controls my colleagues and I evaluated not only work in the real world, but produce significant energy savings,” Katipamula said. “We hope commercial building owners will be inspired by these tangible savings and install advanced controls in their rooftop HVAC units.”

Building owners interested in upgrading or replacing their rooftop units can learn more from the Advanced RTU Campaign, which was formed by DOE’s Better Building Alliance, ASHRAE, and the Retail Industry Leaders Association (RILA).

For more information, visit www.pnnl.gov.

Publication date: 6/2/2014

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