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:
Un-insulated 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 un-insulated, 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.
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