MG3P Installation Manual

13
Plenums & Air Ducts
• Plenums and air ducts must be installed in accordance with the
Standard for the Installation of Air Conditioning and Ventilating
Systems (NFPA No. 90A) or the Standard for the Installation
of Warm Air Heating and Air Conditioning Systems (NFPA
No. 90B).
• Table 9 (page 37), Table 10 (page 38), Table 11 (page
39), Table 12 (page 40), & Table 13 (page 41) contain
the maximum airflow and temperature rise data. If the maximum
airflow is 1,600 CFM or more, it is recommended that two
openings be used for return air on upflow furnaces. Downflow
furnaces can only use one return opening.
• It is recommended that the outlet duct contain a removable
access panel. The opening should be accessible when the
furnace is installed in service and shall be of a size that
smoke or refelcted light may be observed inside the casing
to indicate the presence of leaks in the heat exchanger. The
cover for the opening shall be attached in such a manner as
to prevent leaks.
• If outside air is used as return air to the furnace for ventilation
or to improve indoor air quality, the system must be designed
so that the return air is not less than 60° F (15° C) during
operation. If a combination of indoor and outdoor air is used,
the ducts and damper system must be designed so that the
return air supply to the furnace is equal to the return air supply
under normal, indoor return air applications.
• When a cooling system is installed which uses the furnace
blower to provide airflow over the indoor coil, the coil must
be installed downstream (on the outlet side) of the furnace
or in parallel with the furnace.
• If a cooling system is installed in parallel with the furnace, a
damper must be installed to prevent chilled air from entering
the furnace and condensing on the heat exchanger. If a
manually operated damper is installed, it must be designed
so that the furnace will not operate when the damper is in
the cooling position or when in heating position, the cooling
system is inoperable.
• It is good practice to seal all connections and joints with
industrial grade sealing tape or liquid sealant. Requirements
for sealing ductwork vary from region to region. Consult with
local codes for requirements specific to your area.
Return Air Connections
In applications where the supply ducts carry heated air to areas
outside the space where the furnace is installed, the return
air must be delivered to the furnace by duct(s) secured to the
furnace casing, running full size and without interruption. Do
not use the back of the furnace for return air.
• Upflow furnaces draw the return air from the base of the
furnace. A stand or return air duct must be supplied to the
furnace to provide the required return air.
• Downflow models draw the return air from the top of the furnace.
The minimum required clearance to the top of the furnace is
detailed on the furnace rating plate. Additional clearance may
be required depending upon filter accessibility.
For each U.S.A. application, the home manufacturer shall comply
with all of the following conditions to have acceptable return
air systems for closet installed forced air heating appliances:
• Regardless of the location, the return air opening into the
closet shall not be less than specified in the appliance’s listing.
• Means shall be provided to prevent inadvertent closure by a
flat object placed over the return air opening when it is located
in the floor of the closet (versus the vertical front or side wall).
• The cross-sectional area of the return duct system leading
into the closet shall not be less than 390 in
2
.
• The total free area of openings in the floor or ceiling registers
serving the return air duct system must be at least 352 in
2
.
At least one register should be located where it is not likely
to be covered by carpeting, boxes and other objects.
• Materials located in the return duct system must have a flame
spread classification of 200 or less. This includes a closet
door if the furnace is in a closet.
• Noncombustible pans having 1" upturned flanges are located
beneath openings in a floor duct system.
• Wiring materials located in the return duct system shall conform
to Articles 300-22 of the National Electrical Code (ANSI C1/
NFPA-70).
• Gas piping is not run in or through the return duct system.
• Test the negative pressure in the closet with the air-circulating
fan operating at high speed and the closet closed. The negative
pressure is to be no more negative than minus 0.05 inch water
column.
• Air conditioning systems may require more duct, register and
open louver area to obtain necessary airflow. Use Nortek’s
certiduct program to determine proper duct size for A/C.
• For floor return systems, the manufactured home manufacturer
shall affix a prominent marking on or near the appliance where
it can be easily read when the closet door is open. The marking
shall read:
CAUTION:
HAZARD OF ASPHYXIATION: Do not cover or
restrict return air opening.
Supply Air Connections
For proper air distribution, the supply duct system must be
designed so that the static pressure measured external to the
furnace does not exceed the listed static pressure rating shown
on the furnace rating plate. The supply air must be delivered to
the heated space by duct(s) secured to the furnace casing,
running full size and without interruption.
Three typical distribution systems are shown in Figure 11. The
location, size, and number of registers should be selected on
the basis of best air distribution and floor plan of the home.
Acoustical Treatments
Damping ducts, flexible vibration isolators, or pleated media-
style filters on the return air inlet of the furnace may be used to
reduce the transmission of equipment noise eminating from the
furnace. These treatments can produce a quieter installation,
particularly in the heated space. However, they can increase
the pressure drop in the duct system. Care must be taken to
maintain the proper maximum pressure rise across the furnace,
temperature rise and flow rate. This may mean increasing the
duct size and/or reducing the blower speed. These treatments
must be constructed and installed in accordance with NFPA and
SMACNA construction standards. Consult with local codes for
special requirements. For best sound performance, install all
the needed gaskets and grommets around penetrations into the
furnace, such as for electrical wiring.
A Single trunk duct
B
Dual trunk duct
w/crossover connector
C
Transition duct
w/branches
Figure 11. Typical Supply Duct System