Installation manual

66840/035-20003-001 Rev. B (1205)
Unitary Products Group 5
Example: The furnace input is 80,000 BTUH, 1,200 CFM. The recom-
mended duct area is 280 sq.in, there are two 8 x 14 rectangular ducts
attached to the plenum and there are two 7 inch round ducts attached to
the furnace.
1. Take 8 x 14, which equals 112 sq.in. X 2, which equals 224 square
inch then go to round duct size located in Table 2.
2. The square inch area for 7 inch round pipe is 38.4, multiply by 2 for
two round ducts which equals 76.8 square inch.
3. Then take the 224 square inch from the rectangular duct and add it
to the 76.8 sq.in. of round duct. The total square inch of duct
attached to the furnace plenum is 300.8 square inch. This exceeds
the recommended 280 square inch of duct.
In this example, the duct system attached to the plenum has a sufficient
area so that the furnace operates at the specified external static pres-
sure and within the air temperature rise specified on the nameplate.
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.
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.
I
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.
TABLE 2:
Minimum Duct Sizing For Proper Airflow
Input Airflow
Return
1
1. Maximum return air velocity in rigid duct @ 700 feet per minute (19.82 m
3
/
minute).
Rectangular
2
2. Example return main trunk duct minimum dimensions.
Round
2
Supply
3
3. Maximum supply air velocity in rigid duct @ 900 feet per minute (25.49 m
3
/
minute).
BTU/H
(kW)
CFM
(m³)
In²
(cm²)
in. x in.
(cm x cm)
in.
(cm) dia.
In²
(cm²)
75000
(21.98)
1,200
(33.98)
280
(711)
14 x 20
(35.6 x 50.8)
18
(45.7)
216
(549)
75000
(21.98)
1,600
(45.31)
360
(914)
18 x 20
(45.7 x 50.8))
22
(55.8)
280
(711)
100000
(29.31)
1,600
(45.31)
360
(914)
18 x 20
(45.7 x 50.8)
22
(55.8)
280
(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.
TABLE 3: External Static Pressure Range
Input Output
Nominal
Air Flow
Ext. Static Pressure
Minimum Maximum
MBH kW MBH kW CFM cmm In.W.C kPa In.W.C kPa
75 22.0 60 17.6 1200 34.0 0.12 0.0299 0.50 0.1245
75 22.0 60 17.6 1600 45.3 0.12 0.0299 0.50 0.1245
100 29.3 80- 23.4 1600 45.3 0.15 0.0374 0.50 0.1245
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.