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Introduction
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 section is meant to help you understand the
problems that can affect the duct system and how you can:
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Save money
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Improve comfort
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Protect health
Components of
the Duct System
A duct system is a
branching network of round or rectangular tubes —generally constructed
of sheet metal, fiberglass board, or a flexible plastic-and- 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.)
Energy Losses and Costs
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.
Air Leakage
Another way that ducts lose energy is
through air leak-age. 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.
Leaky Supply Ducts
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
leak-ing 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.
Leaky Return Ducts
Suppose the supply ducts are tight but
the returns leak . 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
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 duct-work typically
have an area that is 10 to 20 percent of the leakage area of the
house. Over the course of a heat-ing season, the energy losses from
ducts when the fan is off can be nearly as great as when the fan is
on!
INSPECTION OF
THE DUCT SYSTEM
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.
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. 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 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.
Now that you know
where 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 crawl-space 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.
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.
Get A
Professional
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. 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. 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?
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.
For more information Please
Contact our engineers, call at 88 00 44
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