FORCED AIR HVAC SYSTEMS
Keep Air Ducts Short, Direct, and Inside the Conditioned Space
Bird's Eye View
Forced-air systems are extremely versatile
The network of ducts that distributes air around
the house can be used for heating, air
conditioning, fresh air distribution,
dehumidification, and air filtration.
Another plus: forced-air heat is usually
cheaper than a hydronic system. Efficient
forced-air systems are well designed and
See below for:
ABOUT DUCT LAYOUT
ABOUT DUCT SEALING
ABOUT DUCT TESTING
FOR TOP EFFICIENCY, forced-air ducts should
ABOUT BALANCED SYSTEMS
be properly sized, leak-free, and located inside
a home's conditioned space.
Flex duct is not the best choice
Ducts can be made from galvanized steel, fiberglass duct board,
or a flexible, non-metallic material (flex duct). Flex duct is easy
to damage during installation, and of the available options it has
the highest resistance to airflow so its not the best choice. If it is
used, it should be installed so it’s not crushed or pinched. It
should be well supported with hangers and as straight as
possible so airflow is not impeded.
RIGID GALVANIZED STEEL
DUCT is smooth inside and
restricts air flow very little. It's
important to secure all joints
with sheet-metal screws and
Don't let square footage determine duct size
A Florida study cited by the U.S. Department of
Energy indicates an alarming number of
contractors determine duct size solely by square
footage of the house or other rules of thumb. In
other words, they are flying by the seat of their
pants. These systems will never be as efficient as
ones designed by the book, and in fact can lead
to oversized equipment that runs inefficiently and
Important duct design principles include:
Design by the book — that is, Manual D from the
Air Conditioning Contractors of America (ACCA).
Keep supply ducts short and straight.
Keep ducts in conditioned spaces.
Provide a return-air pathway from every conditioned room.
To learn more, see "About Duct Layout," below.
Put ducts in a kitchen soffit
Soffits that fill the space between kitchen upper cabinets and
the ceiling are common in both new construction and
remodels and are often used to conceal ducts.
Boxed-in soffits can be a design element.
When you can't squeeze ducts between floor joists, box in
the ductwork and finish it like a beam, like the ceiling, or
with some other treatment.
To ensure that ductwork is installed inside the conditioned
space of the home, it's important to maintain the integrity of
the home's air barrier. The drywall on the ceiling and wall
where the soffit will be installed must be hung and taped
before the duct is installed or the soffit enclosure is built. Any
penetrations through the drywall must be sealed with spray
foam or caulk. This assures a clear separation between the
walls and the ceiling which can save energy, improve IAQ,
and limit the spread of fire.
To learn more, see "More About Duct Layout," below.
Kitchen soffits are a great place to
hide ducts This keeps them inside
the conditioned space while giving
something for kitchen cabinets to
butt up to. When building soffits,
it's important to cover that
section of the wall and ceiling with
taped drywall to prevent unseen
Careful duct sizing calculations are not optional
They are required by code. For example, section M1601.1 of the International Residential Code
(IRC) requires that "Duct systems serving heating, cooling and ventilation equipment shall be
fabricated in accordance with the provisions of this section and ACCA Manual D or other approved
More duct system requirements can be found in Chapter 16 of the IRC.
Duct sealing requirements
The IRC requires all duct seams to be sealed against leakage. These requirements can be found in
section N1103.2.2 ("Ducts, air handlers, filter boxes and building cavities used as ducts shall be
sealed") and section M1601.3.1 ("Joints of duct systems shall be made substantially airtight by
means of tapes, mastics, gasketing or other approved closure systems").
According to section N1103.2.3, "Building framing cavities shall not be used as supply ducts."
Florida was the first state in the country to require new homes with forced-air HVAC systems to
meet pressure-balancing requirements between rooms. Section 601.4 of the Florida Building Code,
which took effect on March 1, 2002, specifies that the pressure difference between the common
central space and any room with a closed door except bathrooms and laundry rooms shall be nor
more than 2.5 Pascals.
Planning has a ripple effect
Early planning for the location of heating and air conditioning ducts can involve more than a
single trade. A relatively simple change in floor or wall framing, for example, may make the
installation of ductwork easier and less expensive, and help keep ducts out of the way.
Collaboration is to everyone's benefit.
WHAT ABOUT DUCTS IN FLOORS AND CEILINGS?
Hiding ducts in chases and soffits
Sometimes you can't reconcile the direction of floor joists with the direction of duct runs.
Box them in and finish like the ceiling. The structural part of the chase can be made from 2x
material, but Rhode Island builders Mike Guertin and Rick Arnold save space by making them
from scraps of ½-in. sheathing and 1x3 cleats. This approach is especially useful in a basement
renovation where every inch of headroom counts.
Don't forget about this
A common oversight when building a duct chase is to forget the air barrier. The interior of the
box will not be in a conditioned space unless the sides of the box which border an insulated
ceiling or wall are sealed to prevent air leaks — with taped drywall, for example. Any
penetrations in this air barrier must be repaired before the ductwork is installed.
Sealing ducts from the inside
In an existing home, where most duct seams are inaccessible, sealing duct leaks can be tricky.
One option is a proprietary process called Aeroseal. Aeroseal contractors seal duct leaks by
spraying an aerosolized mastic into the duct system. The sticky particles find the leaks and plug
them. The process has its limitations — large leaks are particularly hard to seal — but it can be
valuable in certain circumstances.
WHEN INSTALLING DUCTS
Four rules of thumb
Seal all duct joints with mastic.
Keep ducts inside conditioned space.
Use ductwork, not framing.
DUCT DETAILS ARE WORTH THE TROUBLE
The goal is a balanced system
In a well designed HVAC system, supply and return ducts deliver an equal amount of air. The
system inhales as much as it exhales. In a poorly designed duct system, the air imbalance is
made up through your walls, floor, and roof.
Leaky supply ducts waste energy. Suppose the supply side of the system is leaky but the
return ducts are well sealed. One obvious outcome is that some of the heated or cooled air isn’t
getting where it’s supposed to go. That’s an energy loss. Another consequence is negative
pressure inside the living space. The imbalance can draw air into the house from the attic, crawl
space or around doors or windows. This replacement air could contain mold, dust, or other
Leaky return ducts lower air quality. When there are leaks in the return side of the system,
another kind of problem develops. The furnace fan or air handler can draw in air from a
basement or crawl space, with the potential of introducing cold air (an energy loss), radon, dust,
flue gases or other pollutants.
WHAT'S WRONG WITH THIS PICTURE?
a) The air handler and duct work are in the hottest and coldest part of the house.
b) The wall insulation isn't working.
c) There's more insulation on the wall than on the ductwork.
d) All of the above.
LEED-H Under EA5 (Energy & Atomosphere), duct design and tightness prerequisites; up to 3
points for superior distribution efficiency.
NGBS Under Ch. 7 — Energy Efficiency: mandatory to seal all duct distribution components per
Energy Star thermal bypass checklist (701.4.3); 5 pts. for duct systems designed and installed
per ACCA Manual D (704.4.1); up to 23 pts. for HVAC installation testing/commissioning (704.5
ABOUT FORCED AIR
An affordable choice
Air can be warmed in a variety of ways — with a hydronic coil from a geothermal heat pump or water
heater, by electric resistance coils, by an air-to-air heat pump, or by a furnace that burns natural
gas, propane, fuel oil, or firewood. This versatility, along with lower installation costs, makes
warm-air furnaces the most common type of heating system in the U.S.
Some homeowners avoid forced-air systems, fearing they will be noisy, create uncomfortable drafts,
or spread dust throughout the house. But a well designed and properly installed system should do
none of those things.
Ductwork should be part of home design
The key to an efficient system, and a comfortable house, is careful duct design and installation. The
two most common problems are routing ducts through an unconditioned space, such as an unheated
attic or crawl space, and failing to provide a return-air pathway from each conditioned room to the
furnace or air handler. Poorly designed ductwork can result in pressure imbalances within the
building envelope, making the house uncomfortable and contributing to moisture problems and high
energy bills. Sealing ductwork to prevent air leaks is essential.
ABOUT DUCT LAYOUT
Efficient duct systems are compact
Ducts that run in unconditioned spaces significantly erode performance and waste money. Duct sizes
should be the result of careful calculations, not guesswork or rules of thumb. Some basics of good
Design by the book. Guides published by the Air Conditioning Contractors of America (ACCA)
should be the foundation for sizing ducts. No contractor should specify a system without using the
ACCA’s Manual J, which calculates heating and cooling loads, and Manual D, which tells the
contractor how to size the ducts. (For more information on this topic, see Saving Energy With Manual
J and Manual D.)
A Florida study cited by the U.S. Department of Energy indicates an alarming number of contractors
determine duct size solely by square footage of the house or other rules of thumb. In other words,
they are flying by the seat of their pants. These systems will never be as efficient as ones designed
by the book, and in fact can lead to oversized equipment that runs inefficiently and wastes energy.
Design early. Duct layout depends on floor and wall framing, the location of drop ceilings, the type
of insulation installed in an attic or crawl space, and a variety of other design and construction
details. The most efficient and most economical systems are likely to be those incorporated into
overall house plans early in the game, when there’s still time for the builder, architect, and HVAC
contractor to collaborate.
Keep supply ducts short and straight. Centrally located air handlers allow supply and returns to
be as short as possible, minimizing energy losses and reducing the number of joints that are
potential leaks. Ducts with lots of twists and turns slow down airflow, increasing energy losses and
lowering the performance of the HVAC system. Tight, well insulated building envelopes with tripleglazed windows allow registers to be located on interior walls with no loss of comfort. Locating
registers on interior walls can dramatically shorten duct lengths.
Keep ducts in conditioned spaces. It makes little sense to install ductwork with R-6 insulation in
an attic. During the summer, when the air conditioner is struggling to cool the house, the attic
temperature will usually be significantly higher than the outdoor temperature, making the system
work that much harder. Leaky ductwork makes the problem worse and contributes to a variety of air
Ducts and air handlers are best located in areas that are heated and cooled just like the rest of the
house. Don't put ducts in exterior walls. That reduces the amount of insulation that can be placed in
the wall cavity, and increases the risk of condensation.
Provide a return-air pathway from every conditioned room. The best forced-air systems
include a return-air grille connected to return-air ductwork in every conditioned room. If the budget
is too tight to allow this option, every bedroom will need a transfer grille or jumper duct connecting
the bedroom with a room containing a return-air grille.
Use ducts, not building cavities.
Some builders have used joist bays or other building cavities as supplies or returns.
Since these areas are very difficult to seal properly, the use of panned joist bays in
supply air systems is no longer allowed by building codes.
ABOUT DUCT SEALING
Leaky ducts do more than waste energy
Supply and return ducts are assembled on site from many individual pieces, and each connection is a
potential air leak. Leaky ducts create a number of potentially serious problems in addition to wasting
energy dollars. Unhealthy air from an attic or crawl space can be sucked into the system and
distributed around the house. Leaks also can contribute to the growth of mold and mildew.
Some HVAC contractors argue it's impossible to seal ducts completely, but that attitude arises out of
habit rather than logic. After all, plumbers have been installing leak-free pipes for decades. Duct
manufacturers are beginning to address duct leakage by marketing new products with sealed seams.
Examples include GreenSeam duct from Ductmate (East Monongahela, PA), Green Label duct fittings
from Seal-Tite (Hillsboro, OH), and SpirAmir duct from SpirAmir (Dublin, CA).
For detailed duct-sealing instructions, see “Duct Tape and Mastic.”
Mastic. The best material to use for sealing ducts is mastic. Mastic is a gooey,
non-hardening material with the consistency of smooth peanut butter. It is
spread over duct seams with a paintbrush, trowel, or gloved fingers. Gaps in
ductwork or plenums that are over 1/8 in. wide can be sealed with mastic as
long as the gap is first reinforced with fiberglass mesh. Sections of galvanized
duct should always be joined with sheetmetal screws before seams are sealed with mastic.
Better duct tapes. Although it's widely agreed that the best product for sealing duct seams is
mastic, some HVAC installers find it convenient to seal some duct seams with tape. By now,
everyone knows that gray hardware-store duct tape fails quickly and should never be installed on
ducts. There are three categories of tapes worth considering:
Oriented polypropylene (OPP) tape is a smooth film-backed tape that resembles packing tape. Some
brands have a shiny "metallized" plastic finish. Duct joints sealed with OPP should also be clamped.
Three brands of OPP duct tape are Intertape Polymer Group's AC698 tape, Shurtape Technologies'
DC-181 tape, and Venture Tape's 1599B tape.
Aluminum foil duct tape with acrylic adhesive performed well in test conducted by Lawrence Berkeley
National Labortory (LBNL). Brands include Fasson 0800, Fasson 0810, Ideal Seal 587A/B, Shurtape
AF100, Tyco Polyken 337, and Venture 1581.
Foil-backed butyl tape is expensive but performs well, according to LBNL testing. A well-known brand
is Hardcast Foil-Grip 1402-181BFX.
Duct tightness specifications. Specifications for a forced-air system should include a duct
tightness specification. The specification usually requires Duct Blaster testing after the duct seams
have been sealed with mastic.
One typical specification reads, "Ductwork should be sealed tightly with mastic and pass a level of
duct tightness of no more than 5% of the floor area @ 25 Pa. For example, a 1,000-square-foot
house can have no more than 50 cfm of leakage."
Another specification reads, "Duct leakage shall not exceed 5% [or 7%, or 10%] of high-speed fan
ABOUT DUCT TESTING
The only way to verify that a duct system is tight is to test the duct system with a duct blower. The
2009 International Residential Code requires all residential duct systems except those located
completely within a home's thermal envelope to be tested for tightness.
For more information about measuring duct leakage, see “Duct Leakage Testing.”
ABOUT BALANCED SYSTEMS
Provide a pathway for replacement air
Many forced-air distribution systems create unwanted pressure imbalances
within the building envelope — higher air pressure in some parts of the house,
low air pressure in others. This problem can be minimized by providing
adequate return ducts to move air back to the air handler or furnace. (For more
information, see "Return-Air Problems.")
There are two general types of return systems: a central return, which serves a
limited number of rooms, or individual room returns, which are designed to
return air from each conditioned room.
Individual air returns are usually quieter and do a better job of minimizing
pressure imbalances. But they are also more expensive to install, and they may require a larger
blower motor in the air handler to overcome the increased friction of air moving through a larger
number of ducts.
Central air returns can be effective, as long as provisions have been made for a free flow of air
from each conditioned room to the central return. Leaving a healthy gap beneath interior doors isn’t
enough to accomplish this, so transfer grilles or jumper ducts are required.
For a forced-air system to work efficiently, return ducts must be able to move the same volume of air
as the supply ducts. The pressure differential between the common central space and any
conditioned room with a closed door should be no more than 2.5 Pascals. When a return register
can't be provided in every conditioned room, jumper ducts or transfer grilles are good alternatives.
Jumper ducts (also called crossover ducts) connect a bedroom grille with a grille in a common area.
As long as there are no doors between the common area and the centrally located return air grille,
the jumper duct helps equalize the pressure between the bedroom and the common area. Jumper
ducts range in size from 6 inches to 10 inches in diameter.
Transfer grilles generally connect two back-to-back wall-mounted grilles in adjacent rooms. One
side of the transfer grille is installed in a bedroom wall, for example, with the other side in a hallway.
Transfer grilles do the same thing as jumper ducts, but without the ductwork. They can incorporate
baffles to muffle sound and provide privacy. Site-built transfer grilles, however, are often undersized.
A good rule of thumb is to make the grille so it has between 70 and 100 square inches of free area
for every 100 cubic feet per minute of supply air that's ducted to the room.
Tamarack Technologies (West Wareham, MA) sells a well designed transfer grille called the Zenon.
Avoid halfway measures. The strategy of undercutting a bedroom door is inadequate, since a gap
of 1 inch between the finish floor and a 30-inch-wide door can handle only 47 cfm of return air at a
maximum pressure differential of 2.5 Pascals — adequate for only a tiny room measuring about 75
Designers should also be cautious about using a stud bay as a duct connecting a high grille in a
bedroom wall with a low grille in an adjacent hallway, since a 3 1/2 inch by 15 inch stud cavity can
handle only 52 cfm of return air.
Unbalanced HVAC systems make a house uncomfortable. Rooms with undersized returns
become pressurized, forcing air into wall and ceiling cavities. During the winter, warm interior air can
carry moisture into the walls where it condenses on cooler surfaces. This may lead to the growth of
mold. A room with a large return-air grille but an inadequate supply-air register can become
depressurized, drawing outside air into building cavities.
These problems can be avoided when the volume of air supplied by the furnace is balanced by an
equal volume of air being drawn into the return ductwork.
Sizing jumper ducts:
HVAC System Pressure Relief by Paul h. Raymer and Neil Moyer.
Duct sealing retrofit:
Carrier Aeroseal, P.O. Box 4802, Syracuse, NY 13221
Ceiling design elements:
Rooms without Walls by Sarah Susanka
Best Practices Guide for Residential HVAC Retrofits
Duct Leakage Testing
Duct Tape and Mastic
Related Topics in the Green Building Encyclopedia
Green Basics Index
Main topic areas:
Design, Planning, and Building Science
Building Lot & Siting
Envelope and Structure
Green Remodeling Projects
Christopher Clapp / Fine Homebuilding 107
Krysta Doerfler / Fine Homebuilding
Fine Homebuilding 120
TAGS: FORCED AIR, (NOT-IN-USE, FORMALLY FURNITURE)