AIR DISTRIBUTION
SYSTEM DESIGN
Good Duct Design Increases Efficiency
BUILDING TECHNOLOGIES PROGRAM
OFFICE OF ENERGY EFFICIENCY AND RENEWABLE ENERGY · U.S. DEPARTMENT OF ENERGY
Buildings for the
21st Century
Buildings that are more
energy efficient, comfortable,
and affordable... that's the
goal of DOE's Building
Technologies Program.
To accelerate the development
and wide application of energy
efficiency measures, the
Building Technologies Program:
· Conducts R&D on technologies
and concepts for energy
efficiency, working closely with
the building industry and with
manufacturers of materials,
equipment, and appliances
· Promotes energy/money
saving opportunities to both
builders and buyers of homes
and commercial buildings
· Works with state and local
regulatory groups to improve
building codes, appliance
standards, and guidelines for
efficient energy use
T e c h n o l o g y F a c t S h e e t
I N T R O D U C T I O N
Central heating and cooling systems use an air
distribution or duct system to circulate heated
and/or cooled air to all the conditioned rooms
in a house. Properly designed duct systems
can maintain uniform temperatures
throughout the house, efficiently and quietly.
W H Y D U C T D E S I G N I S I M P O R T A N T
The efficiency of air distribution systems has
been found to be 60-75% or less in many
houses because of insufficient and/or poorly
installed duct insulation and leaks in the duct
system. Properly designed and installed duct
systems can have efficiencies of 80% or more
for little or no additional cost, potentially
saving a homeowner $50-200 or more per year
in heating and cooling costs. Moreover,
efficient duct system designs can reduce
equipment size, further saving money for new
or replacement equipment.
Duct systems that leak and/or do not
distribute air properly throughout the house
may make some rooms too hot and others too
cold. Leaky and unbalanced duct systems may
force conditioned air outside and
unconditioned air into the house. This
increases heating and cooling costs and may
also draw humidity, dust, mold spores, and
other contaminants into a home from the attic,
crawlspace, or garage and radon gas from the
soil. In extreme cases, poorly designed and
installed duct systems can induce
backdrafting--spillage of flue gases from
combustion appliances (e.g., furnace, water
heater, fireplace) into the living space--
primarily when atmospheric or natural-draft
flues are used rather than powered
combustion systems.
Duct systems that are undersized, are pinched,
or have numerous bends and turns may lead
to low air flow rates and high air velocities.
Low air flow rates cause the heating and
cooling equipment to operate inefficiently.
High air velocities increase noise.
D U C T D E S I G N O B J E C T I V E S
The objectives of good duct design are
occupant comfort, proper air distribution,
economical heating and cooling system
operation, and economical duct installation.
The outcome of the duct design process will
be a duct system (supply and return plenums,
ducts, fittings, boots, grilles, and registers)
that
·
Provides conditioned air to meet all room
heating and cooling loads.
·
Is properly sized so that the pressure drop
across the air handler is within manufacturer
and design specifications.
·
Is sealed to provide proper air flow and to
prevent air from entering the house or duct
system from polluted zones.
·
Has balanced supply and return air flows to
maintain a neutral pressure in the house.
·
Minimizes duct air temperature gains or
losses between the air handler and supply
outlets, and between the return register and
air handler.