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

Technology Fact Sheet

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.
SUPPLY DUCT CONFIGURATIONS

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

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 L OOP 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.

Spider

Radial Trunk and Branch

Perimeter Loop
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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.

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.

S HEET 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.

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.

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.

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.

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.

Transfer grille

Jumper duct

Door undercut

Wall 

grille

Door 

grille Supply air

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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.

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

DUCTS INSIDE CONDITIONED SPACE
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.
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

Written and prepared for the

U.S. Department of Energy by:

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

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

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.

AIR DISTRIBUTION SYSTEM DESIGN

Printed with a renewable-source ink on paper containing at least

50% wastepaper, including 20% postconsumer waste.

March 2003 DOE/GO102002-0782

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:

• 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.

• 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).

• 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.

• 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

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.