12. HVAC Ductwork
Problems
The owner converted the original duct work system to
make it run over the attic to distribute air. It’s nothing
wrong to run air duct over attic, but difficult to do it
right. When air ducts penetrate interior (warm - room
temperature) and attic (cold - exterior temperature),
condensation is very easy cause problem in ductwork
and ceiling penetrations, especially in winter
cold climate days. To eliminate the condensation
problem, it has to rely on the insulation around the air
duct (air tight seal to ductwork and the vertical
penetration between interior and attic). However, an
air tight seal is very difficult to achieve or overlooked
during construction.
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57. HVAC Maintenance Issues -
1
MAINTENANCE DIFFICULTIES: -
1
In many attics the air handler
system is installed in the corner,
sometimes behind the supply and
return ductwork. The only way to get
to the filter for regular cleaning or
replacement (monthly) is to climb
over the ductwork, sometimes
damaging the ducts.
58. HVAC Maintenance Issues -
2
Most homeowners don’t even go into the
attic to change the filter, even if the air
handler is easy to access. Because of this
hard to access filter, most filters are not
changed on a regular basis, causing the
filter and cooling coil to become dirty and
clogged, reducing the efficiency and air
circulation of the already inefficient
system. The house will have increased
operational costs and the furthest rooms
from the air handler will not be cooled or
heated efficiently or adequately.
59. HVAC Maintenance Issues -
3
SOLUTIONS - 1
1.Keep the air handler unit and ductwork
inside the thermal envelope.
2.Installing the air handler in a first or second
floor closet with proper insulation and
access for service.
3.Install a proper ductwork distribution system.
4.Install the full supply ductwork system below
the upper floor ceiling, boxed into a soffit
that would run down the center of the
hallway of the house.
60. HVAC Maintenance Issues -
4
SOLUTIONS - 2
1. All ductwork joints should be sealed.
2. All attic ductwork should be sealed and
double insulated to reduce heat and
cooling losses into the attic.
3. An air handler that must be installed in the
attic should be installed inside an attic
room that has insulated stud walls and
ceiling, as well as sheathing on the exterior
of the room.
4. Also, install an exterior grade door for
access and to maintain a proper thermal
envelope
61. HVAC Attic Ductwork - 1
Ductwork in the attic runs from
two sources:
1. From the furnace installed
anywhere in the house or in the
attic.
2. From the air handler installed
in the house or in the attic.
62. HVAC Attic Ductwork - 2
There are several reasons for
running ductwork in the attic:
1. House is very big and spacious
and there not enough walls for all
heat runs and cold air returns.
2. It is an existing house.
3. High level of incompetence.
63. HVAC Attic Ductwork - 3
Here is how one mechanical inspector
described this kind of problematic
installation:
“Ductwork in an attic is normally the
largest energy problem in the home or
building. The reasons why this practice
should be avoided are comfort complaints,
heating & cooling losses in the attic
ductwork and air handler, higher energy
bills, maintenance difficulties and system
failures.”
64. HVAC Attic Ductwork - 4
SYSTEM DESIGN – 1 – Winter
Climate
In a winter climate, average winter
temperature is approx. 30 degrees and the
average summer temperature is
approximately 90 degrees. Average
temperature in a properly ventilated attic
should be within 20 degrees of the outside
temperature. (50 in the winter – 110 in the
summer)
65. HVAC Attic Ductwork - 5
SYSTEM DESIGN – 2 – Winter
Climate
The average temperature of the air in the
supply ducts in the winter is 120 degrees
and 60 degrees in the summer. This is a
temperature differential between the attic
air and the air inside the ductwork of 70
degrees in the winter and 50 degrees in
the summer.
66. HVAC Attic Ductwork - 6
To have a correct thermal envelope
would be to have the highest point of
the house be the highest R value. Attic
insulation of 10+ inches (R 30), would
be the best. Also all penetrations into
the attic should be sealed and all
joints in the ductwork and attic air
handler should be sealed. However,
most attics are not built with any
regard to proper sealing and
ventilation or heat and cooling losses
67. HVAC Attic Ductwork - 7
Most HVAC contractors install cooling
systems in the attic based on the
theory that cold air falls down.
However, the real reason most of them
install them in the attic, is that it is a
lot easier and less expensive to install
the system. There are a number of
problems with this type of installation.
68. HVAC Attic Ductwork - 8
1. The attic air handler and ductwork system
is normally insulated to R-4 and is installed
above the attic insulation causing minimal
resistance to heat and cooling losses
through the ductwork and air handler.
2. The size and cost of the system will have
to be increased due to the cooling and
heating losses through the thinner attic
ductwork insulation. A larger system will be
required to compensate for this
inefficiency.
3. The systems will have to run longer to
make up for the losses in the attic
ductwork.
69. HVAC Attic Ductwork - 9
1. There is also inefficiency, due to the fact
that the cooling system is generating
cooling in the hottest part of the house and
the heating system is generating heat in
the coldest part of the house in the winter.
2. On a typical attic installation the
temperature differential from the attic air
handler unit to the supply register on the
other side of the house can be as much as
8 degrees. This amount of cooling loss
cannot be made up with just over sizing the
system.
70. HVAC Attic Ductwork - 10
1. Additionally, most attic access
doors/hatches are not insulated
which will allow more winter heat to
escape up into the attic.
2. The attic heat in the summer time will
also migrate down into the house
through the ducts and attic access,
making the system have to run
longer to cool the warmer air.
71. 1. This design is very wasteful in the winter.
The house heat will rise up by stack effect
into the supply and return ducts in the
winter, making the 10 inches of attic
insulation mostly ineffective. This air will
flow into attic ducts and air handler to be
lost through the thinner insulation and
leakage points in the ductwork.
2. The heating system will have to recycle on
and off more often to make up for this
stack loss.
HVAC Attic Ductwork - 11
72. HVAC Attic Ductwork - 12
1. Delivery duct leakage into attics increases
the cost of operation because the system
has to run longer to make up for the
leakage lost to the attic.
2. Return duct leaks allow frigid winter air to
be introduced into the system, increasing
the heating load.
3. Return duct leakage in the summer pulls
very hot humid air into the system,
increasing cooling and dehumidification
loads.
