OFFICE OF BUILDING TECHNOLOGY, STATE AND COMMUNITY PROGRAMS 

ENERGY EFFICIENCY AND RENEWABLE ENERGY • U.S. DEPARTMENT OF ENERGY
DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. 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, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government

or any agency thereof. The views and opinions of the authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Table of Contents page

Introduction . . . . . . . . . . . . . . . . . . . . . .1

Components of the Duct System . . . . .2

Energy Losses and Costs . . . . . . . . . . .2

Health Hazards . . . . . . . . . . . . . . . . . . .7

Inspection of the Duct System . . . . . . .7

Tell-Tale Signs of Problem Ducts . . . . . .9

Get a Professional! . . . . . . . . . . . . . . .10

Opportunities in New Homes . . . . . . . . .11

For Further Information . . . . . . . . . . . .12
THE DUCT SYSTEM, used in air heating and air cooling your home,

is a collection of tubes that distributes the heated or cooled air to

the various rooms. This system can make a big difference in both

the cost and the effectiveness of heating and cooling the home.

The duct system can have an important effect on health of 

the occupants through the distribution of indoor air pollution.

Changes and repairs to a duct system should always be performed

by a qualified professional.

This brochure is meant to help you understand the problems that 

can affect the duct system and how you can:

• Save money

• Improve comfort

• Protect health

Figure 1. A Common Duct System

Supply Plenum

Risers

Individual

Branch

Ducts

Trunks

Return Plenum

Riser

Panned

Individual

Branch

Ducts

Registers using

Wall Cavities as Ducts

In-line

Dampers

RETURN SYSTEM

SUPPLY SYSTEM

Registers

Furnace

Furnace

1
A duct system is a branching network of round 

or rectangular tubes — generally constructed of

sheet metal, fiberglass board, or a flexible plasticand-wire composite — located within the walls,

floors, and ceilings. Usually, you can see only the

outlet, which is a register covered with grillwork. 

Figure 1 shows a common type of duct system.

This system consists of supply ducts and return ducts.

Central heating or cooling equipment (furnace, air 

conditioner, or heat pump) contains a fan that forces

heated or cooled air into supply ducts leading to the

rooms. The fan gets its air supply through return ducts,

which in the best systems are installed in every room 

of the house. To save on installation costs, most homes

have one or two return registers located in common

areas such as hallways. (Some homes have no return

duct systems. Such design shortcuts often result in

lower efficiency and higher heating and cooling bills.)

Typical duct systems lose 25 to 40 percent of the

heating or cooling energy put out by the central

furnace, heat pump, or air conditioner. Homes

with ducts in a protected area such as a basement may

lose somewhat less than this, while some other types 

of systems (such as attic ducts in hot, humid climates)

often lose more.

Duct repairs could be the most important energy

improvement measure you can do if your ducts are in

the attic. If only one half the typical loss of uninsulated

and unsealed ducts that are in attics or crawl spaces

were saved, it would amount to $160 off the total heating and cooling bill in a typical home. This savings is

based on the national average use of natural gas and

electricity for central heating and cooling at national

average energy cost of 70 cents per them, and 8 cents

per kilowatt-hour. With these savings, the cost to seal

and insulate the ducts would most likely be paid for

after three years. These estimates apply to retrofitting 

an existing home. For new construction more of the

ductwork would be accessible to the installer and the

potential savings would be greater; and with lower cost

to install sealant and insulate, the payback would be 

less than one year.

Duct systems lose energy in two ways: by conduction 

of heat from the warm surface, and air leakage through

small cracks and seams. For simplicity, we’ll talk about

warm air for heating, but the same information applies

to cooling when the air conditioner is on.

Conduction

One way duct systems lose energy is for the warm air

inside the ducts to heat the duct walls, which in turn

heat the cold air outside the ducts. If the ducts are in 

an attic or vented crawl space that is nearly as cold as

the outdoors, this heat is completely lost. If the ducts

are in a basement, some of the heat lost from the ducts

may be recaptured by warming the basement ceiling

enough to reduce the heat lost from the house.

COMPONENTS OF THE DUCT SYSTEM

ENERGY LOSSES AND COSTS

2
Figure 2. Ideal Duct Schematic - No Leakage

living space, neutral pressure

Return duct at 

low (-) pressure

Supply duct at 

high (+) pressure

Note: buffer 

space is typically 

an attic, basement,

or crawlspace. buffer space, neutral pressure

fan

furnace

ENERGY LOSSES AND COSTS

Air Leakage

Another way that ducts lose energy is through air leakage. Sometimes this leakage is from accidental holes in

the ducts or poorly connected duct sections; but even if

the ducts are sealed, their operation can cause the house

itself to leak more air than would otherwise be the case.

An understanding of pressure differences in the duct

system helps to better understand air leakage in the

home. Air moves from high pressure to low pressure. 

To get air to move from the supply duct into the room it

serves, the air in the duct has to be at a higher pressure

than the air in the room. Similarly, to move air from the

room into the return duct, the air in that duct has to be at

a lower pressure than the air in the room. The registers 

are the openings through which this air is intended to

move. The duct walls provide the barriers that prevent

air from moving where we don’t want it to go.

The fan of the central furnace creates these pressure 

differences. When the fan stops, these pressures quickly

equalize and the flow of air through the duct stops, too.

Figure 2 shows a duct system that does not leak. The

furnace fan produces a high pressure in the supply ducts

and a low pressure in the return ducts. The high pressure

forces warm air from the supply ducts to flow into the

rooms, and low pressure draws room air back into the

return ducts.

3
4

ENERGY LOSSES AND COSTS

Leaky Supply Ducts

Figure 3 shows perhaps the simplest example of duct

leakage. Here the supply ducts leak, but the return ducts

are air tight. Even though half the duct system is good,

two bad things still happen. First, some of the air that

has just been warmed by the furnace is lost. Second,

this air has to be replaced. If it isn’t, the house would

soon be pumped down to a vacuum, and we know that

doesn’t happen. What does happen is that cold air from

the outside is drawn into the house through cracks and 

small holes in the outside walls. Usually these occur

around doors and windows. Some houses have more of

these than others, but no house is air tight. So we’ve

lost some of the hottest air in the house (air that just

came from the furnace), and replaced it with the coldest

air around (air from the outside). In other words, a leaking supply duct is an energy loser in two ways: the

energy loss that does not go to the rooms, and the extra

energy needed to heat cold air that leaked into the house.

Figure 3. Duct Systems With Supply Side Leaks

living space at negative pressure

Return duct,

sealed Supply duct 

leakage

Supply duct leakage 

is made from outside 

air leaking into house Air leakage from outside

into living space

buffer space, neutral pressure

fan

furnace
5

Figure 4. Duct System with Return Side Leaks

living space at positive pressure

Return duct,

(low pressure)

drawing air from

buffer space

Possibility of

drawing

unwanted

gasses from

buffer space

into ducts

Supply duct 

sealed

Supply duct leakage 

is made from outside 

air leaking into house Excess air leakage from

the living space

buffer space, neutral pressure

fan

furnace

ENERGY LOSSES AND COSTS

Leaky Return Ducts

Suppose the supply ducts are tight but the returns 

leak, as shown in Figure 4. The return duct is at a low

pressure— lower than the house or the outside — so

cold air from the outside is pulled into this duct. This

cold air is heated in the furnace (along with air that

came from the house through the return registers). 

The amount of air delivered to the house by the supply

registers is greater than what the return ducts took from 

the house (the difference being the cold air that leaked

into the return ducts). To equalize the flows, heated

room air leaks out of the house through the same holes

and cracks that, in the previous example, allowed air to

leak in. So cold air is pulled in and warm air leaks out.

In addition to creating energy losses, leaky return ducts

can create health problems (see below).
6

ENERGY LOSSES AND COSTS

Zone Pressurization

Ducts can cause air leakage in the house even if neither

the supply nor the return ducts leak themselves. Figure

5 shows how this can happen. Imagine that a home has

a return register in one room but no supply (the room on

the left in Figure 5), and a supply register in another

room but no return. Now close the door between these

rooms. The room with the supply duct (the room on the

right in Figure 5) will have relatively high pressure. The

supply duct will be trying to blow this room up like a

balloon. Similarly, the room with the return will have

relatively low pressure. So inside air will leak out from

the room on the right, and outside air will leak into the

room on the left. This places an added load on the heating equipment. The situation described here is somewhat simplified to show the basic idea, but variations of 

it are common in real homes. Most new homes built today

do not have duct returns in each room. The problem can

be avoided in rooms with no return register and doors

that are often closed by installing an opening covered

by a louvered grill in the door or in the adjoining wall.

Energy Losses When the Fan Is Off

So far, we’ve been talking about what happens when the

central furnace fan is running. But even when it’s off

(which is most of the time) the leaks in ductwork add to

the air leaks in the rest of the house. The cracks in ductwork typically have an area that is 10 to 20 percent of

the leakage area of the house. Over the course of a heating season, the energy losses from ducts when the fan is

off can be nearly as great as when the fan is on!

Figure 5. Excess Air Leakage in the House 

Caused by Isolation of Zones

Mild low (-) 

pressure

Mild high (+) 

pressure

Return duct at 

low (-) pressure

Strong high (+)

pressure

Air leakage 

from living space 

to outside

Air leakage 

from outside to 

living space 

buffer space, neutral pressure

fan

furnace
7

HEALTH HAZARDS

INSPECTION OF THE DUCT SYSTEM

Leakage in the duct system can be hazardous to

your health. Especially problematic are leaky returns

in an enclosed space such as a basement or garage that

also contains the furnace. This was the situation shown

in Figure 4. If the return ducts leak, their low pressure

can pull down the pressure in the basement or garage as

well, and this can suck flue gases from the furnace and

radon gas from the soil surrounding the home. The flue

gases can be hazardous to health if they contain carbon

monoxide. Exposure to radon gas from the ground is the

second leading cause of lung cancer (after smoking).

Although experts disagree about how common these

hazards are, by upgrading the energy efficiency of the

duct system you have an opportunity to avoid these

potential problems in your home.

You probably wonder how you can know if your

system is losing large amounts of energy. Although

it is often difficult to be sure without testing, some

tell-tale signs, if present in your duct system, should

make you have it checked by a professional.

It will help to make a simple diagram of the system.

This can be a rough sketch. There is no need for 

blueprint quality here.

The first thing you need to do is find the central heating

unit. That should be no problem if it is located in a basement. It is probably something you pass by almost every

day. However, it may be located in an attic or crawl space.

Safety Tips

To get near the ducts, you may have to look into spaces

you have never been in before, especially if the duct

system is located in an attic or crawl space. Therefore,

be sure to follow common-sense safety rules:

• Guard against falls, cuts, and other personal injuries.

• Do not open up or probe into any electrical

devices, wires, or connections.

• Wear an approved mask if you go into an area

with fiberglass or loose fill insulation.

• Before you touch any uninsulated duct, hold your

hand about an inch from it to check if it is hot.

This is especially important in furnaces fueled

with gas or oil, because the vent pipe that carries

the flue gases to the chimney looks like a small

duct yet may have a temperature of several hundred degrees Fahrenheit. Generally the vent will

be a round sheet-metal pipe about six inches in

diameter that leads upward from the furnace 

cabinet. (If the furnace is one of the new high 

efficiency models, it might be vented horizontally.)

Filters

While you are at it, you might want to locate the filter,

which is usually within the central fan unit or at the

return register. The filter removes dust and other small

particles that otherwise could interfere with the operation

of the blower and the furnace heat exchanger. When it fills

up with dust, it cannot do its job and needs to be changed.
8

INSPECTION OF THE DUCT SYSTEM

Experts recommend that the filter be changed several

times during each heating season. At about a dollar

each, frequent filter changes are a low-cost way to save

energy, protect your heating equipment, and remove

some of the dust that otherwise would be delivered to

the living space.

How to Distinguish Between Supply 

and Return Ducts

Once you have found your central heating equipment,

you will notice ducts leading away from it. There is a

supply duct and a return duct, but which is which? In

one common type of installation, the return duct leads

down from the basement ceiling to enter the furnace

near the floor. The supply duct runs out from the top of

the furnace. This kind of system is shown in Figure 1.

There are many types of installations and it is not

always easy to tell which is the supply and which is the

return just by looking at the ducts, particularly if you

are not familiar with them.

You can wait until the furnace comes on, or the thermostat

can be pushed up for a short while to force the furnace

to come on. If the ducts are made of sheet metal, you

should be able to feel the supply ducts getting warm.

The returns will not change much in temperature.

Another possibility is to follow what you think are the

supply ducts to one of the registers. If this is in fact 

the supply side, you should be able to feel air coming

out of the register.

How to Identify Trunks and Branch Ducts

After you have identified the supply and return sides 

of the duct system, you can follow the supply side.

Most often, you will see something like what is shown

in Figure 1— a long, boxlike structure called a trunk

which sprouts smaller branch ducts that lead out toward

the rooms of the home. Another common installation

has all the supply ducts branching directly from the 

furnace like the arms of an octopus.

Building Spaces Used as Ducts

So far we have assumed that the duct system is completely

separate from the other components of the home. Often

this is not so. To save money, builders sometimes use

the building structure itself as part of the duct system.

One common tactic is to use the spaces between basement

or ceiling joists as ducts. (Joists are the horizontal-running

boards—generally 2” x 10” or 2” x 12”—that support

the floor above.)

Although this type of construction can be made to 

operate efficiently, it often leads to significant energy

losses. One reason is that joist-space ducts are likely 

to be uninsulated. Another problem is that they may

have unintended leakage paths to the outside, typically

through the end of the joist cavity.

With returns, it is even more common to see portions 

of the building structure used as part of the duct system.

Some homes have no return at all; the furnace simply

has an intake grille through which basement air is

drawn in to be warmed and distributed to the home.
9

TELL-TALE SIGNS OF PROBLEM DUCTS

Now that you know here each branch duct leads,

you are in a better position to ask whether your

system is likely to be a big energy loser. Here are

the things to look for.

Uninsulated Ducts in Unconditioned Spaces

Heat transfer through duct walls can contribute significantly

to energy losses. Conductive heat losses are typically 

at least as great as the energy losses due to air leakage.

If the duct system runs through an attic or vented crawlspace and is not insulated, you can be sure that much

energy is being wasted. If the ducts are in a basement,

you will have to weigh the fact that insulating the ducts

will cause the basement to get colder. If both the ducts and

the basement walls are uninsulated, you should consider

insulating the basement walls instead of the ducts.

Disconnected, Torn, or Damaged Ducts

A thorough inspection of the duct system should be

made to look for holes large enough to see. Some 

sections of duct that are supposed to be joined together

may have fallen away from each other, leaving a gap

through which large quantities or air can leak. Flexible

duct sections may have been torn during installation 

or afterward. Fiberglass ductboard sections are subject

to damage if weight is placed on them. Whatever the

cause, visible holes in ductwork are a clear indication

that the system needs fixing.

Blind-Alley Ducts

Occasionally found in duct systems that use joist spaces

or other parts of the building structure to channel air flow,

blind-alley ducts occur as a result of mistakes made during

installation. A blind-alley duct leads nowhere (except

possibly to the outside), while the register it was supposed

to serve has no source of heat. The room containing this

register will then be too cold. If it is an important room,

the thermostat setting may be raised in an attempt to get

enough heat to this room. If a room always seem too

cold or a register doesn’t seem to have any air flowing

out of it, it may be worth investigating.

Inadequate Return-Side Ductwork

As we’ve noted, it is common to find building spaces

pressed into service as part of the duct system. These

tend to be leaky, especially on the return side. Even

worse, some homes are designed without any return

ductwork at all. In that case, unless the furnace is in 

the conditioned space, it will be surrounded by cold

basement or crawl-space air and will have to use more

energy to warm this cold air for delivery to the home

than it would have if warmer air from the living space

were available from return ducts. A system without

return ductwork can also depressurize the furnace room,

giving rise to the health hazards we’ve already discussed.
10

TELL-TALE SIGNS OF PROBLEM DUCTS

GET A PROFESSIONAL!

Other Evidence of Supply-and 

Return-Side Leakage

In any kind of duct system, the joints between duct sections should be sealed against leakage. If duct tape was

used for this purpose, it often loses adhesiveness after a

few years. In such cases you can see it falling off the

ducts or you can easily pull it away. Duct sealing should

be done using a silicone caulking or a special cement

(mastic) with an embedded fabric. If your return ducts

are insulated, you may see accumulations of soot or

other dark material on the insulation where it covers

loose duct joints. This dark area is a coating of dust

which over time has accumulated on the surface as 

the air is being pulled through the insulation.

Another fairly common type of energy-wasting air 

leakage is found in systems where ducts, water pipes,

or vent pipes lead between the basement and the attic. 

If there are openings around these pipes that allow 

heated air to flow out or cold air to flow in, then the

pressure difference between the basement and the attic

is likely to increase air infiltration into the basement. 

It is usually a good idea to seal this flow path.

These are all signs that serious duct leakage may be

occurring, leakage that could, with reasonable effort,

be eliminated.

Because of the possible effects that changing the

duct leakage pattern can have on indoor air pollution, you should not attempt to repair duct leaks.

Suppose, for example, that you find several disconnected duct joints in your supply system. Wouldn’t it make

sense to hook them back up? Probably it would, but if

the return system has leaks you can’t fix, you might end

up with an unbalanced system like the one in Figure 4.

There are many variations on this theme, but this illustration shows that safe duct repairs require a licensed

heating, ventilating, and air-conditioning contractor to

repair ducts. Try to find one that has sent their technicians

to a good duct-repair school. Such courses typically

involve at least a week of intensive training.

One of the special tools that would be used by a technician is a blower door— a temporary barrier set up to

cover an open outside doorway, in which is mounted a

fan that is used to apply a small pressure to the house.

The main use of the blower door is to test for air leakage

in the house, but it is also used in duct leakage tests.

One test for duct leakage involves simultaneously 

pressurizing the house with the blower door and the

ducts with a smaller version of this device. Another test

uses the blower-door result plus some simple pressure

measurements to estimate duct leakage.
11

GET A PROFESSIONAL!

OPPORTUNITIES IN NEW HOMES

A contractor’s representative or service technician

should be able to answer the following questions to

your satisfaction:

• How do faulty duct systems lose energy? (They

should know at least as much about the subject 

as you do now.)

• How would you test my duct system to see if it

needs repair?

• What would you do to fix it if you find that 

work is needed?

Our discussion of health issues should not deter you

from doing something about duct leakage, which could

be contributing to a potentially serious health problem

right now and you might not know it. When a qualified

professional repairs your duct system, it is quite likely

that the quality of your indoor air will improve. A 

qualified practitioner will perform the tests necessary 

to make sure that no problems are created where they

didn’t exist before.

We have discussed the ductwork in existing homes.

If you are building a new home, you’re in luck!

You have the opportunity to make sure that the duct 

system will deliver top-notch comfort and efficiency 

by specifying to the builder that you want a leak-free,

insulated duct system.

Even better, discuss with your builder the option of

locating the ducts within the conditioned space and hiding

them so that they don’t show. It is possible to box in

ductwork installed near the intersection of a wall and

the ceiling, or to use other builders’ tricks so that the

raw duct materials will not be visible. It will be easier

to do this than might at first appear because an energyefficient duct system in an energy-efficient home can be

less bulky than a standard duct system. This is because

the amount of heating and air conditioning needed will be

much less than in a standard installation. This will permit

the use of a smaller furnace and air conditioner, which

require a smaller amount of air flow in the duct system.

When it is possible to reduce the size of the duct system

and the central unit, you save on equipment, materials,

and installation costs, possibly enough to pay for the cost

to hide ducts that are located within the conditioned

space. In that case, energy savings will start to flow

immediately. Even if some additional cost is involved,

a duct system properly installed inside the conditioned

space is energy loss-free and will likely be one of your

best investments.
FOR FURTHER INFORMATION

The first two references below are readable guides

requiring no specialized knowledge, although the

professional can pick up quite a bit from them,

too. The others are really intended for the professional, but could be a next step for the homeowner

who becomes deeply interested in the subject.

Home Energy Magazine, Special Issue-Ducts

Rediscovered.September/ October 1993. 2124 Kittredge

Street, No. 95, Berkeley, CA 94704.

This Old House; Heating, Ventilation, and Air

Conditioning. Trethwey, Richard. Little, Brown. 1994

Duct Design for Residential Winter and Summer Air

Conditioning and Equipment Selection (Manual D). Air

Conditioning Contractors of America, 1513 16th Street,

N.W., Washington, DC 20036.

Flexible Duct Performance and Installation Standards.

Air Diffusion Council, One Illinois Center, Suite 200,

111 East Wacker Drive, Chicago, IL. 60602-5398.

A Guide to Insulated Air Duct Systems. North American

Insulation Manufacturers Association, 44 Canal Center

Plaza, Suite 310, Alexandria, Va 22314.

Installation Standards for Residential Heating and Air

Conditioning Systems. Sheet Metal and Air

Conditioning Contractors’ National Association, Inc.,

4201 Lafayette Center Drive, Chantilly, VA 22021.

Energy-Efficient Design of New Low-Rise Residential

Buildings. Standard 90.2-1993. American Society of

Heating, Refrigerating, and Air Conditioning Engineers,

1791 Tullie Circle, N.E., Atlanta, GA 30329.

12
Buildings for the 21st Century

Buildings that are more energy-efficient,

comfortable, and affordable...that’s the goal 

of DOE’s Office of Building Technology, State

and Community Programs, (BTS). To accelerate

the development and wide application of energy

efficiency measures, BTS:

• 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

• Provides support and grants to States and 

communities for deployment of energy-efficient

technologies and practices

Printed with a renewable-source ink on paper containing 

at least 50% wastepaper, including 20% postconsumer waste.

DOE/EE0109

Revised November 1999

Prepared by:

Brookhaven National Laboratory

Associated Universities, Incorporated

Department of Applied Science

Upton, New York 11973

Prepared for:

U.S. DEPARTMENT OF ENERGY

Assistant Secretary for Energy Efficiency

and Renewable Energy

Office of Building Technology,

State and Community Programs

Washington, D.C. 20585

To Obtain Free Copies Write to:

Energy Efficiency and Renewable Energy Clearinghouse

P.O. Box 3048

Merrifield, VA 22116

or phone toll free: 1-800-DOE-EREC (363-3732)