Air Distribution System Design: Good Duct Design Increases Effciency

T e c h n o l o g y F a c t S h e e t
AIR DISTRIBUTION
SYSTEM DESIGN
Good Duct Design Increases Efficiency
Buildings for the
21st Century
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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
INTRODUCTION
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.
WHY DUCT DESIGN IS IMPORTANT
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.
DUCT DESIGN OBJECTIVES
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.
BUILDING TECHNOLOGIES PROGRAM
OFFICE OF ENERGY EFFICIENCY AND RENEWABLE ENERGY • U.S. DEPARTMENT OF ENERGY
SUPPLY DUCT SYSTEMS
Supply ducts deliver air to the spaces that are to be
conditioned. The two most common supply duct systems for
residences are the trunk and branch system and the radial
system because of their versatility, performance, and economy.
The spider and perimeter loop systems are other options.
TRUNK AND BRANCH SYSTEM
In the trunk and branch system, a large main supply trunk is
connected directly to the air handler or its supply plenum and
serves as a supply plenum or an extension to the supply plenum.
Smaller branch ducts and runouts are connected to the trunk.
The trunk and branch system is adaptable to most houses, but
it has more places where leaks can occur. It provides air flows
that are easily balanced and can be easily designed to be
located inside the conditioned space of the house.
There are several variations of the trunk and branch system. An
extended plenum system uses a main supply trunk that is one
size and is the simplest and most popular design. The length of
the trunk is usually limited to about 24 feet because otherwise
the velocity of the air in the trunk gets too low and air flow into
branches and runouts close to the air handler becomes poor.
Therefore, with a centrally located air handler, this duct system
can be installed in homes up to approximately 50 feet long. A
reducing plenum system uses a trunk reduction periodically to
maintain a more uniform pressure and air velocity in the trunk,
which improves air flow in branches and runouts closer to the
air handler. Similarly, a reducing trunk system reduces the
SUPPLY DUCT CONFIGURATIONS
cross-sectional area of the trunk after every branch duct or
runout, but it is the most complex system to design.
SPIDER SYSTEM
A spider system is a more distinct variation of the trunk and
branch system. Large supply trunks (usually large-diameter
flexible ducts) connect remote mixing boxes to a small, central
supply plenum. Smaller branch ducts or runouts take air from the
remote mixing boxes to the individual supply outlets. This system
is difficult to locate within the conditioned space of the house.
RADIAL SYSTEM
In a radial system, there is no main supply trunk; branch ducts
or runouts that deliver conditioned air to individual supply
outlets are essentially connected directly to the air handler,
usually using a small supply plenum. The short, direct duct
runs maximize air flow. The radial system is most adaptable to
single-story homes. Traditionally, this system is associated
with an air handler that is centrally located so that ducts are
arranged in a radial pattern. However, symmetry is not
mandatory, and designs using parallel runouts can be
designed so that duct runs remain in the conditioned space
(e.g., installed above a dropped ceiling).
PERIMETER LOOP SYSTEM
A perimeter loop system uses a perimeter duct fed from a
central supply plenum using several feeder ducts. This system
is typically limited to houses built on slab in cold climates and
is more difficult to design and install.
Perimeter Loop
Spider
Radial
Trunk and Branch
Jumper duct
Wall
grille
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RETURN AIR TECHNIQUES
Closed interior doors create a return-air
blockage in systems with only one or two
returns. Grilles through doors or walls or
jumper ducts can reduce house pressures
and improve circulation.
Supply air
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Door
grille
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Transfer grille
Door undercut
RETURN DUCT SYSTEMS
Return ducts remove room air and deliver it back to the heating
and cooling equipment for filtering and reconditioning. Return
duct systems are generally classified as either central or
multiple-room return.
DUCT MATERIALS
Air distribution ducts are commonly constructed from sheet
metal, rigid fiberglass duct board, or flexible nonmetallic duct.
Selection of duct material is based on price, performance, and
installation requirements.
MULTIPLE-ROOM RETURN SYSTEM
A multiple-room return system is designed to return air from
each room supplied with conditioned air, especially those that
can be isolated from the rest of the house (except bathrooms
and perhaps kitchens and mechanical rooms). When properly
designed and installed, this is the ultimate return duct system
because it ensures that air flow is returned from all rooms
(even with doors closed), minimizes pressure imbalances,
improves privacy, and is quiet. However, design and
installation costs of a multi-room return system are generally
higher than costs for a central return system, and higher
friction losses can increase blower requirements.
Designs that use the house structure or building framing (e.g.,
building cavities, closets, raised-floor air handler plenums,
platform returns, wall stud spaces, panned floor joists) as
supply or return ducts can be relatively inexpensive to install.
However, they should be avoided because they are difficult to
seal and cannot always be insulated. In addition, because such
systems tend to be rough and have many twists and turns, it is
difficult to design them so as to ensure good air distribution.
Even return plenums built under a stairway or in a closet, for
example, should be avoided if a completely ducted system is
possible.
CENTRAL RETURN SYSTEM
A central return system consists of one or more large grilles
located in central areas of the house (e.g., hallway, under
stairway) and often close to the air handler. In multi-story
houses, a central return is often located on each floor. To
ensure proper air flow from all rooms, especially when doors
are closed, transfer grilles or jumper ducts must be installed in
each room (undercutting interior doors to provide 1 inch of
clearance to the floor is usually not sufficient by itself).
Transfer grilles are through-the-wall vents that are often
located above the interior door frames, although they can be
installed in a full wall cavity to reduce noise transmission. The
wall cavity must be well sealed to prevent air leakage. Jumper
ducts are short ducts routed through the ceiling to minimize
noise transfer.
SHEET METAL
Sheet metal is the most common duct material and can be used
on most all supply and return duct applications (for plenums,
trunks, branches, and runouts). Sheet metal ducts have a
smooth interior surface that offers the least resistance to air
flow. When located in an unconditioned space, they must be
insulated with either an interior duct liner or exterior insulation.
They must also be carefully and completely sealed during
construction/installation, using approved tapes or preferably
mastic, because each connection, joint, and seam has potential
leakage. Screws should be used to mechanically fasten all
joints.
AIR DISTRIBUTION SYSTEM DESIGN
FIBERGLASS DUCT BOARD
Fiberglass duct board is insulated and sealed as part of its
construction. It is usually used to form rectangular supply and
return trunks, branches, and plenums, although it can be used
for runouts as well. Connections should be mechanically
fastened using shiplap or V-groove joints and stapling and
sealed with pressure-activated tapes and mastic. Fiberglass duct
board provides excellent sound attenuation, but its longevity is
highly dependent on its closure and fastening systems.
FLEXIBLE NONMETALLIC DUCT
Flexible nonmetallic duct (or flex duct) consists of a duct inner
liner supported on the inside by a helix wire coil and covered
by blanket insulation with a flexible vapor-barrier jacket on the
outside. Flex duct is often used for runouts, with metal collars
used to connect the flexible duct to supply plenums, trunks,
and branches constructed from sheet metal or duct board. Flex
duct is also commonly used as a return duct. Flex duct is
factory-insulated and has fewer duct connections and joints.
However, these connections and joints must be mechanically
fastened using straps and sealed using mastic. Flex duct is
easily torn, crushed, pinched, or damaged during installation. It
has the highest resistance to air flow. Consequently, if used, it
must be properly specified and installed.
DUCT AND REGISTER LOCATIONS
Locating the air handler unit and air distribution system inside
the conditioned space of the house is the best way to improve
duct system efficiency and is highly recommended. With this
design, any duct leakage will be to the inside of the house. It
will not significantly affect the energy efficiency of the heating
and cooling system because the conditioned air remains inside
the house, although air distribution may suffer. Also, ducts
located inside the conditioned space need minimal insulation
(in hot and humid climates), if any at all. The cost of moving
ducts into the conditioned space can be offset by smaller
heating and cooling equipment, smaller and less duct work,
reduced duct insulation, and lower operating costs.
DUCTS INSIDE CONDITIONED SPACE
There are several methods for locating ducts inside the
conditioned space.
•
Place the ducts in a furred-down chase below the ceiling (e.g.,
dropped ceiling in a hallway), a chase furred-up in the attic, or
other such chases. These chases must be specially
constructed, air-sealed, and insulated to ensure they are not
connected to unconditioned spaces.
•
Locate ducts between the floors of a multi-story home (run
through the floor trusses or joists). The exterior walls of these
floor cavities must be insulated and sealed to ensure they are
within the conditioned space. Holes in the cavity for wiring,
plumbing, etc., must be sealed to prevent air exchange with
unconditioned spaces.
•
Locate ducts in a specially-constructed sealed and insulated
crawlspace (where the walls of the crawlspace are insulated
rather than the ceiling).
Ducts should not be run in exterior walls as a means of moving
them into the conditioned space because this reduces the
amount of insulation that can be applied to the duct and the
wall itself.
A supply outlet is positioned to mix conditioned air with room air
and is responsible for most of the air movement within a room.
Occupant comfort requires that supply register locations be
carefully selected for each room. In cold climates, perimeter floor
outlets that blanket portions of the exterior wall (usually
windows) with supply air are generally preferred. However, in
today’s better insulated homes, the need to locate outlets near
AIR DISTRIBUTION SYSTEM DESIGN
the perimeter where heat loss occurs is becoming less
important. In hot climates, ceiling diffusers or high wall outlets
that discharge air parallel to the ceiling are typically installed. In
moderate climates, outlet location is less critical. Outlet locations
near interior walls can significantly reduce duct lengths
(decreasing costs), thermal losses (if ducts are located outside
the conditioned space), and blower requirements. To prevent
supply air from being swept directly up by kitchen, bathroom, or
other exhaust fans, the distance between supply registers and
exhaust vents should be kept as large as possible.
The location of the return register has only a secondary effect
on room air motion. However, returns can help defeat
stratification and improve mixing of room air if they are placed
high when cooling is the dominant space-conditioning need
and low when heating is dominant. In multi-story homes with
both heating and cooling, upper-level returns should be placed
high and lower-level returns should be placed low. Otherwise,
the location of the return register can be determined by what
will minimize duct runs, improve air circulation and mixing of
supply air, and impact other considerations such as aesthetics.
DUCT DESIGN METHOD
The air distribution system should be designed at the same time
the house plans are being developed, following the procedures
in the Air Conditioning Contractors of America’s (ACCA’s)
Manual D: Residential Duct Systems. Planning locations for
ductwork, structural framing, plumbing, and electrical wiring
simultaneously avoids conflicts between these systems.
The following eight steps should be followed in the design of
an air distribution system to ensure efficiency and comfort:
1. Select the general type of heating and cooling equipment
(e.g., furnace, heat pump, air conditioner). The heating and
cooling equipment should be selected based on occupant
preferences, availability of different fuels (e.g., natural gas,
electricity), installation costs, and operating costs.
2. Select the general type of air distribution system (supply and
return duct systems). The general designs and duct materials
for the supply and return duct systems should be selected
after considering the type of equipment selected and its
location, the local climate, the architectural and structural
features of the house, zoning requirements, and installation
and operating costs. ACCA’s Manual G: Selection of
Distribution Systems and Manual RS: Comfort, Air Quality, and
Efficiency by Design can assist in this selection.
3. Calculate the design heating and cooling loads of each room
of the house and the loads that are associated with the entire
house using ACCA’s Manual J: Residential Load Calculation
(eighth edition). Room loads are used to determine the air flow
needed for each room, and the house loads are used to size
and select specific heating and cooling equipment models.
4. Size and select the specific models of the heating and cooling
equipment using ACCA’s Manual S: Residential Equipment
Selection. This precedes the duct sizing calculations because,
in residential applications, the blower (fan) data of the selected
equipment establish the duct design criteria. In addition,
identify any component or device (e.g., filter, humidifier,
electric resistance heater, cooling coil) that was not included
when the blower data and their associated pressure drops
were developed.
5. Develop a scale drawing or rough sketch of the air distribution
system showing the location of the air handling equipment,
supply outlets, return openings, loads and air flow rates
associated with each supply and return register, location of
duct runs, lengths of straight duct runs, fitting types, and
equivalent lengths of the fittings. Be sure to account for the
direction of joists, roof hips, and other potential obstructions
such as two-story foyers or rooms.
6. Determine the size of all the ducts based on the room loads,
blower data, pressure drops of additional components or
devices, and equivalent duct lengths following the procedures
in ACCA’s Manual D: Residential Duct Systems. Several duct
layouts may need to be examined before reaching a final design.
7. Select and size the air distribution system devices (return
grilles and supply air diffusers, grilles, and registers) using
ACCA’s Manual T: Air Distribution Basics for Residential and
Small Commercial Buildings. These must be selected to
maintain air velocities below values that will cause noise but,
in the case of supply outlets, sufficiently high so that air is
disbursed to exterior walls or ceilings as desired.
8. Select the insulation levels for the duct system in accordance
with the 2000 International Energy Conservation Code.
AIR DISTRIBUTION SYSTEM DESIGN
For more information, contact:
Energy Efficiency and
Renewable Energy
Clearinghouse (EREC)
1-800-DOE-3732
www.eere.energy.gov
Or visit the Building Technologies
Program Web site at
www.buildings.gov
Or refer to A Builder’s Guide to
Residential HVAC Systems
NAHB Research Center
800-638-8556
www.nahbrc.org
Or refer to the Residential Duct
Design: A Practical Handbook
(Report CU-7391)
Electric Power Research Institute
800-313-3774 press 2
www.epri.com
DESIGN RECOMMENDATIONS AND
KEY DESIGN ELEMENTS
In designing the air distribution using ACCA’s
Manual D: Residential Duct Systems, consider
the following recommendations before
finalizing the design:
•
•
•
Southface Energy Institute
404-872-3549
www.southface.org
U.S. Department of Energy’s
Oak Ridge National Laboratory
Buildings Technology Center
865-574-5206, www.ornl.gov/btc
The International Energy
Conservation Code can be
obtained from the International
Code Council, 703-931-4533
www.intlcode.org
NOTICE: Neither the United States
government nor any agency
thereof, nor any of their employees,
makes any warranty, express or
implied, or assumes any legal liability or responsibility for the accuracy, completeness,
or usefulness of any information,
apparatus, product, or process disclosed. The views and opinions of
authors expressed herein do not
necessarily state or reflect those of
the United States government or
any agency thereof.
When using a central return system, include
(a) a return on each level of a multi-story
house, (b) a specification to install transfer
grilles or jumper ducts in each room with a
door (undercutting interior doors to allow
1 inch of clearance to the floor is usually not
sufficient), and (c) if at all possible, a return
in all rooms with doors that require two or
more supply ducts.
•
Specify higher duct insulation levels in ducts
located outside the conditioned space than
those specified by the 2000 International
Energy Conservation Code, especially when
variable-speed air handling equipment is
The entire air distribution system should be
“hard” ducted, including returns
(i.e., building cavities, closets, raised-floor air
handler plenums, platform returns, wall stud
spaces, panned floor joists, etc., should not
be used).
Written and prepared for the
U.S. Department of Energy by:
Manuals D, G, J, RS, S, and T
can be obtained from the
Air Conditioning Contractors
of America
1712 New Hampshire Ave., NW,
Washington, DC 20009
202-483-9370, www.acca.org
Design the air distribution system to be
located inside the conditioned space of the
house to the greatest extent possible. Do not
locate ducts in exterior walls.
•
In two-story and very large houses, consider
using two or more separate heating and
cooling systems, each with its own duct
system. In two-story homes, for example,
upper stories tend to gain more heat in
summer and lose more heat in winter, so the
best comfort and performance is often
achieved by using separate systems for the
upper and lower stories.
•
Consider supply outlet locations near interior
walls to reduce duct lengths.
•
Locate supply outlets as far away from
exhaust vents as possible in bathrooms and
kitchens to prevent supply air from being
swept directly up by the exhaust fans.
•
Consider installing volume dampers located
at the takeoff end of the duct rather than at
the supply register to facilitate manual
balancing of the system after installation.
Volume dampers should have a means of
fixing the position of the damper after the air
distribution system is balanced.
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March 2003
DOE/GO102002-0782
being used. Lower air flows provided by
variable-speed heating and cooling systems
to improve operating efficiency increase the
resident time of air within the air distribution
system, which in turn increases thermal
losses in the winter and thermal gains in the
summer. Attic insulation placed over ducts
helps where it is possible.
•
Specify that all duct joints must be
mechanically fastened and sealed prior to
insulation to prevent air leakage, preferably
with mastic and fiberglass mesh. Consider
testing of ducts using a duct blower to
ensure that the air distribution system is
tight, especially if ducts are unavoidably
located in an unconditioned space. A typical
requirement is that duct leakage (measured
using a duct blower in units of cubic feet per
minute when the ducts are pressurized to
25 Pascals) should not exceed 5% of the
system air flow rate.
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