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The Problem with Single Duct VAV: Built-in Inefficiency of the Common Lab HVAC System

Michael Morehead, P.E., Flack+Kurtz Inc.


Many laboratory building HVAC systems are designed as Variable Air Volume, Single Duct Reheat (VAV Reheat) systems. The VAV Reheat system uses a single duct to supply all cooling and ventilation needs for the laboratory and office spaces. This system has inherent energy efficiencies that contribute to the high cost of laboratory operation.

The inherent inefficiency of the VAV Reheat system is that it uses a single air supply for both ventilation and cooling. If the demand for ventilation air is greater than the demand for cooling air, energy is wasted from overcooling supply air then subsequently reheating.

One example of this inefficient operating condition is when fume hoods are open and large quantities of air are supplied to the room to offset the exhaust, yet there is minimal cooling load from equipment. A second example is during un-occupied periods, when minimum ventilation rates are being maintained but there is not a cooling load.

This presentation will review the inherent inefficiency of VAV Reheat systems and look at a specific strategy for improvement by designing the HVAC system with separate cooling and ventilation capabilities. Included will be cost evaluations, design challenges, synergy with other energy efficiency strategies (such as heat recovery and thermal storage), and opportunities for future development.


Not available at this time.

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