36
Step 19 — Install Supply-Air Thermistors (Modu-
lating Gas Units Only)
Supply-air thermistors are a field-installed, factory-provided com-
ponent. Three supply-air thermistors are shipped with modulating
gas units inside the unit control box. Thermistor wires must be
connected to the SGC (staged gas control) in the unit control box.
See Table 9. The supply-air thermistors should be located in the
supply duct with the following criteria:
• downstream of the heat exchanger cells
• equally spaced as far as possible from heat exchanger cells
• a duct location where none of the supply air thermistors
are within sight of the heat exchanger cells
• a duct location with good mixed supply air
Step 20 — Install Unit Accessories
For applications requiring accessories, the following packages are
available:
All units:
• space temperature sensor
• CO
2
sensor
• space temperature sensor with CO
2
sensor
• airflow switch
• filter switch
• smoke detector
Refer to the individual accessory installation instructions in each
accessory package for information on installing accessories.
Two-Piece Unit Fit Up
1. Locate and remove split parts package from return section of
unit. Open package and remove split unit parts. See
Tables 10 and 11 for parts list.
2. Rig main unit section and return section separately.
3. Do not assemble main unit section and return section on
ground and attempt to lift. Unit pieces must be assembled on
roof curb.
4. Insert base rail guides into the base rails of the main unit sec-
tion. Do not attach with screws yet. See Fig. 33 and 34.
5. Insert top rail guides into top rails of the main unit section.
Do not attach with screws yet. See Fig. 34 and 35.
6. Rig and set the main unit section on the unit roof curb. Align
the unit bulkhead with the supply opening in the roof curb.
7. Remove the roof cap, top cover and L brackets from the fil-
ter end of the return section. Save these parts for later instal-
lation. See Fig. 36 and 37.
8. Rig and set return section onto the roof curb using rail guides
as an alignment aid. The return section must be within 2 in.
of the main unit section. Slide base rail and top rail guides
from main unit section rails partially into return section rails.
9. Install tar tape on edge of main unit base rails to seal base rail
splice. See Fig. 38.
10. Using ¾-in. x 30-in. threaded rod, nuts, washers and backing
plates provided, pull and secure the base rails together. See
Fig. 39.
11. Attach (4) brackets to top rails. Using
3
/
8
-in. x 6-in. threaded
rods, nuts, washers and backing plates provided, pull the top
rails together and secure with top rail guide from Step 5. See
Fig. 40. Remove threaded rods and brackets after securing
top rail guide.
12. Place seal strip along both sides of basepan joint. Attach
basepan covers to basepan joint. See Fig. 41.
13. Install L brackets, top cover and top cap joining main unit
section and return section top cover assemblies. See Fig. 42
and 43.
14. Connect control wiring plugs located at unit split. All same
color plugs should be connected together.
On units equipped with return air fan or power exhaust fan
option, locate black, yellow, and blue power wires located at
the split in main section. Run the wires through the grommet
below the return air filters and through the grommet at the
economizer damper. See Fig. 44. Run wires through wire
trays, separate from control wires, to the return fan/power
exhaust fan VFD on back wall of return section. Connect
wires to VFD power terminal connections: Black to UL1,
Yellow to V1, and Blue to W1. On units equipped with VFD
bypass option connect the wires to the power distribution in
the VFD bypass box: Black to L1, Yellow to L2, and Blue to
L3. See Fig. 45.
Connect air pressure tubes located at split. All same num-
bered tubes should be connected together. See Table 12 for
tubes associated with unit options.
15. Install side post on far side of unit. See Fig. 46.
16. Install side panels at main unit section/return section split.
Install panel side brackets, 2 on each panel. See Fig. 46.
Table 9 — SGC Thermistor Designations
THERMISTOR PIN CONNECTION POINT
FUNCTION AND LOCATION
PART NO.
Thermistors
SAT1 J8 – 1,2 (SGC)
Supply-Air Thermistor (SAT) — Inserted into supply section underneath
the gas heat section (factory-provided, field-installed)
HH79NZ034
SAT2 J8 – 3,4 (SGC)
Supply-Air Thermistor (SAT) — Inserted into supply section underneath
the gas heat section (factory-provided, field-installed)
SAT3 J8 – 5,6 (SGC)
Supply-Air Thermistor (SAT) — Inserted into supply section underneath
the gas heat section (factory-provided, field-installed)
LIMTTEMP J8 – 15,16 (SGC)
Limit switch Thermistor (LIMTTEMP) — Inserted next to the lower limit switch
(factory-installed)
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