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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)

Summary of Contents for PT9 Series

Page 1: ...IST OF SECTIONS SAFETY 1 DUCTWORK 3 FILTERS 6 GAS PIPING 6 ELECTRICAL POWER 8 COMBUSTION AIR AND VENT SYSTEM 9 CONDENSATE PIPING 15 SAFETY CONTROLS 16 START UP AND ADJUSTMENTS 16 WIRING DIAGRAM 23 LIS...

Page 2: ...ational vehicles Failure to carefully read and follow all instructions in this manual can result in furnace malfunction death personal injury and or property damage Furnaces for installation on combu...

Page 3: ...required for passage to the front when servicing or for inspection or replacement of flue vent connections In all cases accessibility clearances shall take precedence over clear ances for combustible...

Page 4: ...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 instal...

Page 5: ...eal the furnace to plenum connections to prevent air leakage Refer to Figure 1 for unit and plenum dimensions Attic installations must meet all minimum clearances to combustibles and have floor suppor...

Page 6: ...nd in the codes The furnace rating plate and the instructions in this section specify the type of gas approved for this furnace only use those approved gases The instal lation of a drip leg and ground...

Page 7: ...nal pressure Never apply a pipe wrench to the body of the gas valve when installing piping A wrench must be placed on the octag onal hub located on the gas inlet side of the valve Placing a wrench to...

Page 8: ...onnect the green furnace lead equipment ground to the power supply ground An alternate wiring method is to use a field provided 2 5 08 cm x 4 10 2 cm box and cover on the outside of the furnace Route...

Page 9: ...mplete duct system and its own wall thermostat SECTION VI COMBUSTION AIR AND VENT SYSTEM COMBUSTION AIR AND VENT SAFETY This Category IV dual certified direct vent furnace is designed for resi dential...

Page 10: ...els column then look to the right in those rows to the five elbows column It shows N A in the 1 1 2 3 8 cm row so 1 1 2 5 1 cm pipe cannot be used the 2 5 1 cm pipe row shows 40 ft 12 2 m so up to 40...

Page 11: ...0 kW 36 inches 91 cm for models 100 000 BTUH 30 kW Two pipe direct vent applications 9 inches 23 cm for models 50 000 BTUH 15 kW 12 inches 30 cm for models 50 000 BTUH 15 kW Single pipe applications 4...

Page 12: ...stion air systems must be installed so the vent and the combustion air pipes terminate in the same atmospheric zone Refer to Figures 13 or 14 VENTING MULTIPLE UNITS Multiple units can be installed in...

Page 13: ...ical vent Care should be taken to locate side vented systems where trees or shrubs will not block or restrict supply air from entering the terminal Also the terminal assembly should be located as far...

Page 14: ...ht FIGURE 19 Outside and Ambient Combustion Air GABLE VENT GAS VENT SOFFIT VENT VENTILATED ATTIC TOP ABOVE INSULATION OPTIONAL INLET a OUTLET AIR a VENTILATED CRAWL SPACE GAS WATER HEATER VENTILATED A...

Page 15: ...Air The ventilated attic space or a crawl space from which the combustion air is taken must comply with the requirements specified in AIR SOURCE FROM OUTDOORS in this instruction or in Section 5 3 Air...

Page 16: ...CONTROLS These controls are mounted on the burner box assembly If the temper ature in the burner box exceeds its set point the ignition control and the gas valve are de energized The operation of thi...

Page 17: ...ll start and come up to speed Shortly after inducer start up the hot surface igniter will glow for about 17 seconds b After this warm up the ignition module will energize open the main gas valve c Aft...

Page 18: ...3600 then divided by the time it took to measure 0 10 m3 of gas from the gas meter For Propane LP Gas multiply the Heat content of the gas MJ m3 or Default 93 15 times 0 10 m3 of gas measured at the...

Page 19: ...shown in Figure 23 The procedure below is used to adjust either the high fire manifold pressure or the low fire manifold pressure IMPORTANT The cap for the pressure regulator must be removed entirely...

Page 20: ...The fan off timing may be adjusted by positioning the jumper on two of the four pins as shown in Figure 25 The furnace control board also allows user selection of the blower speed that is energized d...

Page 21: ...1225 35 1175 33 1105 31 1045 30 990 28 905 25 890 25 790 22 100 1600 C 80 1600 C HIGH 1960 56 1955 55 1925 55 1890 54 1830 52 1765 50 1695 48 1615 46 1600 45 1485 42 M HI 1565 44 1560 44 1560 44 1575...

Page 22: ...otal system static is 0 58 w c To determine the system airflow complete the follow ing steps Obtain the airflow values at 0 50 w c 125 Pa 0 60 w c 150 Pa ESP Airflow 0 50 2285 CFM 64 70 m3 min Airflow...

Page 23: ...106633 UIM A 0205 Unitary Products Group 23 SECTION X WIRING DIAGRAM FIGURE 26 Wiring Diagram...

Page 24: ...change without notice Printed in U S A 106633 UIM A 0205 Copyright by York International Corp 2005 All rights reserved Supersedes 035 20417 001 Rev C 0904 Unitary 5005 Norman Product York OK Group Dr...

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