The Northwest Service Center, Dallas, has approximately
16,000 square feet of office space for the streets and sanitation departments.
The site, however, is primarily industrial space: 42,000 square feet for fleet
vehicle maintenance; 10,000 square feet for vehicle fueling and washing; and
25,000 square feet for covered storage.
In 2003, the Dallas City Council mandated that all new
facilities owned and operated by the city achieve Leadership in Energy and
Environmental Design (LEED) Silver certification as a fundamental program
requirement. Located on a 17-acre brownfield redevelopment site, the Northwest
Service Center is Dallas’ new maintenance and operations facility that services
the city’s fleet of over 1,200 service vehicles. The new center is also
designed to meet city-mandated green requirements.
The facility’s designers had to develop an HVAC system that
provides safe and adequate ventilation for a vehicle maintenance and operation
facility while also meeting LEED Silver guidelines established largely for
The solution was found in a careful balance of seasonal
heating and cooling loads, and in a ventilation system designed around McQuay
Vision™ air-handling units, energy recovery wheels, and variable air volume
The LEED Green Building Rating System® is a voluntary
standard. It was established by the U.S. Green Building Council in 1999 and is
widely recognized as a third-party verification system and guideline for
measuring what constitutes a green building.
A LEED-certified building means it has achieved at least a
minimum standard as judged in six categories: sustainable sites; water
efficiency; energy and atmosphere; materials and resources; indoor
environmental quality; and innovation and design process. Points are awarded in
each category, depending on how the facility meets each category’s
requirements. A building must receive a minimum of 26 points for LEED
certification; up to 69 points are possible. There are four levels of LEED
certification: Certified (26-32 points); Silver (33-38 points); Gold (39-51
points); and Platinum (52-69 points).
The energy and atmosphere category, one-third of potential
LEED points, directly addresses the HVAC system and its effect on the
environment. This includes the amount of energy the HVAC system consumes, the
environmental implications of generating that energy, and the ozone depletion
potential of the refrigerant used in the equipment.
Once a building has met the three prerequisites -
fundamental building systems commissioning, minimum energy performance, and
fundamental refrigerant management - the LEED-New Construction (NC) provides up to 10 points for the percentage
of energy performance increase beyond the minimum required in the prerequisite.
The facility’s designers developed an HVAC system designed
around McQuay Vision™ air-handling units, energy recovery wheels, and variable
air volume control.
The Northwest Service Center has approximately 16,000 square
feet of office space for the streets and sanitation departments, the site,
however, is primarily industrial space: 42,000 square feet for fleet vehicle
maintenance; 10,000 square feet for vehicle fueling and washing; and 25,000
square feet for covered storage.
LEED-NC certification is structured on gains in energy cost
savings compared to a typical office building as defined by ASHRAE 90.1
standards. At present, LEED certification programs do not take into
consideration the unique requirements of industrial applications. For example,
while air conditioning is the largest energy consumer in a typical commercial
office building, the Northwest Service Center has a relatively small cooling
requirement: only the 16,000 square foot offices are cooled.
“Industrial buildings typically don’t have a huge air
conditioning load like an office building, so it’s harder to figure out ways to
get efficiencies when you don’t have those big loads to drop down from,” said
Charlie Aldredge, vice president and project manager, Huitt-Zollars Engineering
Inc., Fort Worth, and a member of the Northwest Service Center project design
In the center, the largest source of energy consumption
comes from a requirement for 100 percent outdoor makeup air exchangers in the
vehicle service area in order to ensure adequate and safe air exchange for
occupants. This is where the engineers had to gain the efficiencies necessary
to meet the LEED requirement. Also, since the area is not air conditioned,
those efficiencies had to be gained from winter heating loads rather than
To meet the 100 percent outdoor air exchange requirement in
the vehicle service area, three large McQuay Vision air-handling units were
installed. These pull in outside air at a total exchange rate of approximately
40,000 cfm (two units at 15,000 cfm and one unit at 10,000 cfm). In comparison,
a typical office building pulls in 10-20 percent outside air.
To achieve the required energy efficiencies in the service
area, Sergey Aleksanyan, P.E., at Huitt-Zollars and the mechanical engineer of
record, specified energy recovery wheels for each Vision unit. These capture
energy from the heated exhaust airstream and transfer it to the cool outside
air being pulled into the space. Up to 75 percent of the energy from the
exhaust air stream can be recovered.
“Our solution for meeting the energy requirement was to gain
winter efficiencies to offset the lack of a cooling load,” said Aleksanyan.
“The exhaust air energy is used for preheating the makeup air. The rotary
wheels recover energy from the exhaust air and return it to the supply air.” In
addition to achieving LEED points for energy efficiency, this solution earned
the center an additional LEED point for innovation because energy recovery
wheels are not considered standard design.
To achieve additional efficiencies, the office area is
conditioned by two McQuay Vision air-handling units with variable air volume
(VAV) controls, energy recovery wheels, and hot water heating. The VAV controls
provide energy efficiency by matching airflow volume to load requirements; the
motor runs at a lower amp draw under part load, thereby using energy only as
needed to meet conditions. The energy recovery wheels lower heating and cooling
costs by pre-treating the outside air using building exhaust air. A
high-efficiency McQuay Model AGZ air-cooled, scroll compressor chiller (130
tons) provides chilled water for cooling. The fact that the chiller operates
using an HCFC-free refrigerant (Râ€'407C) gained the project an additional LEED
“Energy model data shows that energy costs for the total
project will be 67 percent of those for a typical LEED-NC building,” said
Rachael Green, LEED director for Mitchell Enterprises LD, the building
project’s general contractor. “That’s a 33 percent savings, and enough to get
four LEED points for optimizing energy. The McQuay high-efficiency chiller, VAV
control, and innovative energy wheels - along with other factors such as
high-efficiency boilers, domestic hot water demand reduction, good insulation
and windows, and lighting efficiencies, - all contributed to the total energy
These plans and solutions put the Northwest Service Center
on track to exceed the Silver requirement with an expected 44 LEED points,
enough to achieve LEED Gold certification later this year.
“Silver, I thought, would be very tough to get when we first
started because of the largely industrial nature of these buildings,” said
Bobby Williams, chief estimator for RMF Contractors who installed the air
conditioning and plumbing systems. “If we get a LEED Gold rating, then you know
everyone has done exceptional work.”
“From roof color to glass selection - it’s not just
equipment - it all has to work in harmony to get the maximum energy reduction,”
said Williams. “We could put the most efficient equipment in the world in the
building, but if they put a black roof on a building in Texas, or use
uninsulated glass or inefficient light fixtures, you’re not going to see the
benefit of that efficient equipment.”
MORE THAN HVAC
Other LEED accomplishments on this project included a 68 percent wastewater generation reduction, 56 percent recycled content in materials, 99 percent construction and demolition materials recycled, and over 90 percent locally-produced materials. IAQ requirements were achieved through installation of IAQ monitoring systems and control of indoor chemical and pollutant sources, including low VOC materials such as mastics, insulation, and sealants.
“Designing the facility to LEED requirements was a tough challenge,” said Jack Ireland, director of equipment and building services, city of Dallas, commenting on the rigorous process changes required to meet LEED certification.
“But in the long run the city will see the savings in energy consumption, in water consumption, and hopefully in less employee illness and absenteeism from better IAQ - not only for the shop workers, but for the office workers as well.” For more information, visit www.mcquay.com.
Sidebar: How Energy Recovery Wheels Work
Energy recovery wheels reduce energy costs by transferring heat and moisture from one airstream to another. In the winter, they capture heat and humidity in the exhaust air stream and transfer it to the incoming fresh air. This saves much of the cost of heating and humidifying the incoming cold, dry air. Energy recovery wheels can recover 75 percent of the energy from an exhaust airstream and can cut winter humidification energy costs by up to 60 percent. (See Figure 1.)
In the summer, energy recovery wheels work in reverse: They capture heat and humidity in the incoming fresh airstream and transfer it to the exhaust airstream. This saves much of the cost of cooling and dehumidifying the incoming airstream. Energy recovery wheels can cool outdoor air to about 67°F, significantly reducing the load on conventional cooling equipment. (See Figure 2.)
Heat and moisture are captured in the energy wheel’s corrugated desiccant material. The wheel rotates between the incoming and exhaust airstreams where variances in pressure and temperature cause heat and moisture to move from one airstream to the other. Only heat and moisture transfer through the energy recovery wheel. Air is purged from the wheel before entering the other airstream. (See Figure 3.)
For example, the Vision air handlers in the Northwest Service Center vehicle maintenance area exhaust 100 percent of the air in the facility. In the winter, their energy recovery wheels capture heat in the exhaust airstream before it leaves the building and transfers it to the incoming airstream. Contaminants carried in the airstream are exhausted outside. Publication Date: