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

Michael Morehead, Flack + Kurtz Inc.

Objectives:

This presentation builds on Mr. Morehead's presentation at the Labs21 2002 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 contribute to the high cost of laboratory operation. Although design strategies can improve the 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.

Findings:

  • The inherent inefficiency of the VAV Reheat system is that it uses a single air supply for both ventilation and cooling.
  • The 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 concept will be explained.
  • Climate data will be presented that shows how often the first condition above occurs. Data will be presented for different climates.
  • The likelihood of the second condition above occurring will be evaluated for different types of spaces within laboratory buildings.
  • Several strategies will be presented for separating the cooling and ventilation functions in order to improve system efficiencies. Pros and cons of the different approaches will be evaluated.
  • In response to questions raised at the Labs21 2002 Annual Conference, humidity control of the ventilation system will be addressed.

Labs21 Connection:

This presentation addresses energy efficiency strategies by optimizing the cooling and ventilation systems of the laboratory. 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. In recent years, the "efficiency bar" has been raised so that baseline for laboratory HVAC system efficiency has become variable air volume systems. This presentation attempts to raise the efficiency bar higher.

Biography:

Michael Morehead has over fourteen years experience in the design of mechanical systems for laboratory, institutional and commercial buildings. He is a Senior Associate with the San Francisco office of Flack + Kurtz Inc., a global provider of mechanical, electrical, plumbing, fire protection, and information technology engineering services. Mr. Morehead has been the Project Manager and Lead Mechanical Engineer for numerous major building design projects encompassing over three million square feet of built area, including over 800,000 square feet of laboratory facilities. His designs focus on maximizing the energy efficiency of mechanical systems that meet the project objectives while keeping within the constraints of the project budget. Mr. Morehead holds a degree in Architectural Engineering from Penn State University. Previous speaking engagements include the Labs21 2002 Annual Conference.

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