The Problem with Single Duct VAV: Just Say NO to
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 reheatingand overcoolingof 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).
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.
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.
Not available at this time.
Back to the Agenda