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98618/035-20009-001
Rev. B (1205)
Consideration
should be given to the heating capacity required and also
to the air quantity (CFM) required. These factors can be determined
by
calculating the heat loss and heat gain of the home or structure. If these
calculations
are not performed and the furnace is over-sized,
the follow-
ing may result:
1.
Short cycling of the furnace.
2.
Wide temperature fluctuations
from the thermostat setting.
3.
Reduced overall operating efficiency of the furnace.
The supply and return duct system
must be of adequate
size and
designed such that the furnace will operate within the designed air tem-
perature rise range and not exceed the maximum designed static pres-
sure. These values are listed in the table below.
TABLE 2: Minimum Duct Sizing For Proper Airflow
Input
Airflow
Return 1
Rectangular 2
Round 2
Supply 3
BTU/H
CFM
In2
in. x in.
in.
In 2
(kW)
(m 3)
(cm 2)
(cm x cm)
(cm) dia.
(cm 2)
75000
1,600
360
18 x 20
22
280
(21.98)
(45.31)
(914)
(45.7 x 50.8))
(55.8)
(711)
NOTE: This chart does not replace proper duct sizing calculations
or take into
account static pressure drop for run length and fittings. Watch out for the temper-
ature rise and static pressures.
1. Maximum return air velocity in rigid duct @ 700 feet per minute (19.82 m3 /
minute).
2. Example return main trunk duct minimum dimensions.
3. Maximum supply air velocity in rigid duct @ 900 feet per minute (25.49 m 3 /
minute).
TABLE 3: External Static Pressure Range
Input
MBH
kW
75
22.0
Output
MBH
kW
60
17.6
Nominal
Air Flow
CFM cmm
1600 45.3
Ext. Static Pressure
Minimum
Maximum
In.W.C
kPa
In.W.C
kPa
0.12
0.0299
0.50
0.1245
TABLES 2 and 3 are to be used as a guide only to help the installer
determine if the duct sizes are large enough to obtain the proper air flow
(CFM) through the furnace. TABLES 2 and 3 ARE NOT to be used to
design ductwork for the building where the furnace
is being installed.
There are several variables associated
with proper duct sizing that are
not included in the tables. To properly design the ductwork for the build-
ing, Refer
to the ASHRAE
Fundamentals
Handbook,
Chapter
on
"DUCT DESIGN" or a company that specializes
in Residential and Mod-
ular Home duct designs.
IMPORTANT:
The minimum
plenum height is 12" (30.5). The furnace
will not operate properly on a shorter plenum height. The minimum rec-
ommended rectangular
duct height is 4 inches (10 cm) attached to the
plenum.
IMPORTANT:
The air temperature
rise should be taken only after the
furnace has been operating for at least 15 minutes. Temperatures
and
external static pressures should be taken 6" (15 cm) past the first bend
from the furnace in the supply duct and the return duct. If an external fil-
ter box or an electronic air cleaner is installed, take the return air read-
ings before the filter box or air cleaner.
kWARNING
The supply air temperature
MUST NEVER exceed the Maximum
Supply
Air Temperature,
specified on the nameplate.
Operating the furnace above the maximum
supply air temperature
will cause the heat exchanger to overheat, causing premature heat
exchanger
failure. Improper
duct sizing, dirty air filters, incorrect
manifold pressure, incorrect gas orifice and/or a faulty limit switch
can cause the furnace to operate above the maximum
supply air
temperature.
Refer to sections II, III and IX for additional informa-
tion on correcting the problem.
If a matching cooling coil is used, it may be place directly on the furnace
outlet and sealed to prevent leakage.
Follow the coil instructions
for
installing the supply plenum. On all installations without a coil, a remov-
able access panel is recommended
in the outlet duct such that smoke
or reflected light would be observable
inside the casing to indicate the
presence of leaks in the heat exchanger.
This access cover shall be
attached in such a manner as to prevent leaks.
20
"19-1/2
14
I
21
FIGURE 1: Dimensions
TABLE 4: Cabinet and Duct Dimensions
BTUH (kW)
Input/Output
75/60 (22.0/17.6) 1
BTUH (kW)
Input/Output
75/60 (22.0/17.6) 1
1. 4-position models ma
Cabinet
"
" =
"tm31mm/
Size
A(in.)
A(cm)
B(in.)
CFM
1600 (45.31)
C
21
53.3
20
Cabinet
"
"
"
"(m3/mm)
Size
F(in.)
F(cm)
G(in.)
CFM
1600 (45.31)
C
18 1/4
46.35
18 3/4
Cabinet Dimension
B(cm)
C(in.)
C(cm)
D(in.)
D(cm)
E(in.)
E(cm)
50.8
20 3/8
51.8
20.0
50.8
19 1/2
49.5
KVent
KVent
G(cm)
H(in.)
H(cm)
J(in.)
J(cm)
(in.)
(cm)
47.62
18 5/8
47.3
19.0
48.2
42
10.162
be factory configured
as upflow (MU) or downfiow (MD).
Unitary Products Group
5
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