7
U.S.A. home manufacturers shall comply with all of the
following conditions to have acceptable return air systems
for closet installed forced air heating appliances:
• Thereturnairopeningintotheclosetshallnotbeless
thanspeciedintheappliance’slisting.
• The cross-sectional area of the return duct system
leading into the closet, when located in the oor or
ceiling shall not be less than 235 in
2
(or 390 in
2
for 5
ton ready M1/M5 Furnaces).
• Meansshallbeprovidedthatpreventinadvertentclosure
ofatobjectsplacedoverthereturnairopeninglocated
intheoorofthecloset(versustheverticalfrontorside
wall).
• Thetotalfreeareaofopeningsintheoororceiling
registers serving the return air duct system must be at
least 235 in
2
. At least one register should be located
where it is not likely to be covered by carpeting, boxes
and other objects.
• Materialslocatedinthereturnductsystemmusthavea
amespreadclassicationof200orless.Thisincludes
a closet door if the furnace is in a closet.
• Noncombustiblepanshaving1”upturnedangesare
locatedbeneathopeningsinaoorductsystem.
• Wiringmaterialslocatedinthereturnductsystemshall
conform to Articles 300-22 of the National Electrical
Code (ANSI C1/NFPA-70).
• Gaspipingisnotruninorthroughthereturnductsystem.
water column.
• Testthenegativepressureintheclosetwith 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.
• Airconditioningsystemsmayrequiremoreductregister
andopenlouverareatoobtainnecessaryairow.Use
NORDYNE’s certiduct program to determine proper
duct sizes for air conditioning.
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.
Location, size, and number of registers should be selected
onthebasisofbestairdistributionandoorplanofthe
home.
Three typical
distribution systems are shown in Figure 5
Figure 5
Figure 5.
A Single trunk duct
B
Dual trunk duct
w/crossover connector
C
Transition duct
w/branches
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