ASHRAE Journal - March 2012 - 92

emerging technologies
energy savings and market Factors
Use of a VAV system is more efficient than use of a constant air volume (CV) system. VAV systems use about 60% of the airflow of CV systems. This corresponds to fan horsepower reductions between 40% and 60%, providing significant system power reductions. Airflow and pressure reductions capable with a VAV system translate into energy savings due to decreased fan power and fan runtime. Three key economic factors drive retrofit feasibility: implementation cost, annual energy cost savings, and the owner’s minimum attractive rate of return. The determining factor is the implementation cost, which can vary by more than a factor of 10, whereas the energy cost savings may only vary by a factor of two.4 Savings can also be incurred from terminal box modifications and component choices. For example, by replacing 25% of the pneumatic VAV terminal controllers in a San Diego commercial office building with digital controllers the building operator was able to use static pressure and SAT reset control schemes. Data from a two-year measurement period shows a 73% reduction in fan energy; about $120,000 was saved in yearly energy costs, or 800,000 kWh per year. A high-performance VAV system using 48°F (8.9°C) supply air and a combination of the strategies discussed earlier can consume 10% to 20% less energy than a conventional CV system depending on climate. Choice of VAV terminal affects system performance. Series fan-powered units are very inefficient; efficiency can be as low as 15%. Parallel fan-powered units help to eliminate reheat, and all wasted heat from the fan is converted to additional space heating as the fan is predominantly run during heating mode. Regardless, VAV reheat terminal units using dual maximum control outperform dual duct and fan-powered systems in most cases.2 The adoption of a VAV system is one step towards an energy efficient HVAC design. However, proper control strategies and VAV terminals must be chosen to ensure that the possible energy savings are realized in practice.

references
1. DOE. 2010. Buildings Energy Databook. Table 3.1.5. http:// tinyurl.com/DOE3-1-5. 2. PIER. 2005. Advanced Variable Air Volume System Design Guide. http://tinyurl.com/PIER-Guide. 3. Murphy, J. 2011. “High-performance VAV systems.” ASHRAE Journal 53(10):18 – 28. 4. EPA. 2008. ENERGY STAR Building Upgrade Manual. Chapter 8. http://tinyurl.com/EPA-Manual.

John Dieckmann is a director and Alissa Cooperman is a technologist in the Mechanical Systems Group of TIAX LLC, Lexington, Mass. James Brodrick, Ph.D., is a project manager with the Building Technologies Program, U.S. Department of Energy, Washington, D.C.

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