106633-UIM-A-0205
4
Unitary Products Group
DUCTWORK INSTALLATION AND SUPPLY PLENUM
CONNECTION
A proper Heat Loss/Gain Calculation should be done on all
installations for proper application of equipment. From this
the ductwork sizing can be calculated, ACCA Manual J and
D and industry standards are helpful.
Attach the supply plenum to the furnace or coil outlet duct
connection flanges. This is typically through the use of S
cleat material when a metal plenum is used. The use of an
approved flexible duct connector is recommended on all installations to
prevent noise transmission. All connections should be sealed to prevent
air leakage. Sheet metal should be crosshatched to eliminate any pop-
ping when the indoor fan is energized.
When replacing an existing furnace, if the existing supply plenum is not
the same size as the new furnace then the existing plenum must be
removed and a new plenum installed that is of the proper size for the
new furnace. The minimum plenum height is 12” (30 cm). If the plenum
is shorter than 12” (30 cm) the turbulent air flow may cause the limit
controls not to operate as designed if at all.
The duct system is a very important part of the installation.
If the duct
system is improperly sized the furnace will not operate properly.
The ducts attached to the furnace plenum should be of sufficient size so
that the furnace operates at the specified external static pressure and
within the air temperature rise specified on the nameplate.
Table 2 is a guide for determining whether the rectangular duct system
that the furnace is being connected to be of sufficient size for proper fur-
nace operation.
Use the Example below to help you in calculating the duct area to deter-
mine whether the ducts have sufficient area so that the furnace oper-
ates at the specified external static pressure and within the air
temperature rise specified on the nameplate.
The following are general duct sizing guidelines that may not serve to
requirements of every application.
Example: The furnace input is 80,000 BTUH, 1,200 CFM blower
requirement. The recommended duct area is 216 sq.in, there are two 8
x 12 rectangular ducts attached to the plenum and there are two 7 inch
round ducts attached to the furnace.
1.
Take 8 x 12, which equals 96 square inch x 2 = 192 square inches
then go to round duct size located in Table 3.
2.
The square inch area for 7 inch round ducts, 38.4 square inch x 2
= 76.8 square inches,
3.
Then take the 192 square inch from the rectangular duct and add it
to the 76.8 square inch of round duct. The total square inch of duct
attached to the furnace supply plenum is 268.8 square inches.
This exceeds the recommended 216 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.
Providing the return duct is properly sized as well.
1.
The Air Temperature Rise is determined by subtracting the Return
Air Temperature Reading from the Supply Air Temperature Read-
ing.
2.
The External Static Pressure is determined by adding the Supply
Duct Static Pressure reading to the Return Duct Static Pressure
reading and adding the pressure drop across any applied coil.
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 cm). The furnace
will not operate properly on a shorter plenum height. The minimum rec-
ommended rectangular duct height is 4” (10 cm) attached to the ple-
num.
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.
The duct system must be properly sized to obtain the correct airflow
for the furnace size that is being installed.
Refer to Table 6 and the furnace rating plate for the correct rise
range and static pressures
If the ducts are undersized, the result will be high duct static pres-
sures and/or high temperature rises which can result in a heat
exchanger OVERHEATING CONDITION. This condition can result
in premature heat exchanger failure, which can result in personal
injury, property damage, or death.
TABLE 2:
Minimum Duct Sizing For Proper Airflow
Input
Airflow
Return
1
Rectangular
2
Round
2
Supply
3
Rectangular
2
Round
2
BTU/H (kW)
CFM (m³)
In² (cm²)
in. x in. (cm x cm)
in. (cm) dia.
In² (cm²)
in. x in. (cm x cm)
in. (cm) dia.
40,000(17.6)
1,200 (34.0)
280 (711)
14 x 20 (35.6 x 50.8)
18 (45.7)
216 (549)
12 x 18 (30.5 x 45.7)
16 (40.6)
60,000 (17.6)
1,200 (34.0)
280 (711)
14 x 20 (35.6 x 50.8)
18 (45.7)
216 (549)
12 x 18 (30.5 x 45.7)
16 (40.6)
80,000 (23.4)
1,200 (34.0)
280 (711)
14 x 20 (35.6 x 50.8)
18 (45.7)
216 (549)
12 x 18 (30.5 x 45.7)
16 (40.6)
80,000 (23.4)
1,600 (45.3)
360 (914)
18 x 20 (45.7 x 50.8)
22 (55.8)
280 (711)
14 x 20 (35.6 x 50.8)
18 (45.7)
100,000 (29.3)
1,600 (45.3)
360 (914)
18 x 20 (45.7 x 50.8)
22 (55.8)
280 (711)
14 x 20 (35.6 x 50.8)
18 (45.7)
100,000 (29.3)
2,000 (56.6)
440 (1,118)
20 x 22 (50.8 x 55.8)
24 (60.9)
390 (991)
16 x 22 (40.6 x 55.8)
22 (55.8)
120,000 (35.2)
2,000 (56.6)
440 (1,118)
20 x 22 (50.8 x 55.8)
24 (60.9)
390 (991)
16 x 22 (40.6 x 55.8)
22 (55.8)
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 temperature rise and static
pressures.
1. Maximum return air velocity in rigid duct @ 700 feet per minute (19.82 m
3
/ 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:
Round Duct Size
Round Duct Size
Calculated Area For Each Round Duct Size
inches (cm)
Sq.in (cm
2
)
5 (13)
19.6 (126)
6 (15)
28.2 (182)
7 (18)
38.4 (248)
8 (20)
50.2 (324)
9 (23)
63.6 (410)
10 (25)
78.5 (506)
11 (28)
95 (613)
12 (30)
113.1 (730)
13 (33)
132.7 (856)
14 (36)
153.9 (993)
