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The Problem with Single Duct VAV: Just Say NO to Reheat

Todd See, Flack + Kurtz Inc.
Peter Morris, Davis Langdon Adamson

This presentation builds on Mr. Morehead's presentation at the Labs21 2003 Annual Conference. The information presented at that conference will be briefly summarized and the topic addressed in more detail.

Many laboratory building HVAC systems are designed as Single Duct, Variable Air Volume Reheat (VAV Reheat) systems. The VAV Reheat system uses a single air supply to provide all cooling and ventilation needs for the laboratory and associated spaces. This approach of using a single airstream has inherent energy efficiencies that result in reheating—and overcooling—of the supply airstream, thereby contributing to the high cost of laboratory operation. Although design strategies can improve the VAV system energy efficiency, the system's fundamental flaws will cause it to always waste energy.

This presentation will review the inherent inefficiency of VAV Reheat systems and look at strategies for improvement by designing the HVAC system with separate cooling and ventilation capabilities. Several alternate systems will be presented, along with a first cost and energy cost comparison.

This presentation will reflect the combined efforts of Michael Morehead (Flack + Kurtz), Dale Sartor (Lawrence Berkeley Laboratory), Martin Dodd (EnergySoft), and Peter Morris (Davis Langdon Adamson).

Findings:

The inherent inefficiency of the VAV Reheat system is that it uses a single air supply for both ventilation and cooling. The VAV Reheat system wastes energy when the demand for ventilation air is greater than the demand for cooling air AND the outside air temperature is greater than the supply air temperature. Energy is wasted when supply air is cooled then subsequently reheated. This important concept will be explained.

Several design strategies will be presented for separating the cooling and ventilation functions in order to improve system efficiencies. Pros and cons of the different design approaches will be evaluated.

The results of a first cost comparison of the different system design approaches will be presented and evaluated.

The results of an energy consumption comparison of the different system design approaches will be presented and evaluated. Energy consumption data will be presented for various locations to test the impact of climate conditions on comparative energy consumption of each system.

Labs21 Connection:

The Variable Air Volume (VAV) Reheat system is frequently used for laboratory buildings, often without critical consideration of its energy performance. It points out that standard practice produces HVAC systems guaranteed to waste energy. Although strategies can be employed that will improve efficiency, the fundamental design flaws will present an upper limit to the efficiency that can be achieved by the VAV Reheat system. In recent years, the "efficiency bar" has been raised so that baseline for Laboratory HVAC system efficiency has become VAV systems. This presentation attempts to raise the efficiency bar higher.

Biographies:

Not available at this time.

 

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