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Gas Furnace Identification
The gas heat section(s) for 39M air-handling units offer two styles
of gas furnace: the gas-fired duct furnace and the gas-fired rack
system. Air-handling units with the duct heater can have 1 or 2 gas
sections with a single duct furnace in each section. Air-handling
units with the rack system can have 1 or 2 gas sections with 1 to 5
racks in each section (up to 10 racks total). Duct furnaces are
Carrier Gas Heat Codes 11-35. Rack furnaces are Carrier Gas
Heat Codes 36-62, and 70. Refer to Fig. 10.
To determine the type of gas heating system in the air-handling
unit, open the burner side door to view the burner assembly. Gas-
fired duct furnaces have the burners arranged vertically. See
Fig. 140. Rack system furnaces have the burners arranged hori-
zontally. See Fig. 141.
BURNERS
Fig. 140 — Gas-Fired Duct Furnace Burner Assembly
Fig. 141 — Gas-Fired Rack System Burner Assembly
Follow the appropriate installation instructions below for the style
of gas furnace included in the air-handling unit.
a39-4406
a39-
4405
BURNERS
Gas-Fired Duct Furnaces
IMPORTANT: This furnace is not listed or suitable for
separated combustion air supply.
IMPORTANT: This furnace is not listed or suitable for
drying or process applications. Use in such applications
voids any warranty and manufacturer disclaims any
responsibility for the duct furnace and/or application.
INSTALLATION REQUIREMENTS
Gas-fired duct furnaces are factory installed. If circumstances re-
quire field installation of a gas furnace, it can only be installed in a
factory-supplied gas heater section. All unit installations must be
in accordance with the National Fuel Gas Code ANSI Z223.1
(NFPA 54) in the United States and Can/CGA-B149 Installation
Code in Canada, and all other applicable local codes and ordinanc-
es. These requirements include but are not limited to:
• Combustion air supply to the heating equipment
• Venting of the products of combustion (flue gases)
• Gas supply, piping and connections
• Unit location and clearances
All electrical equipment must be grounded and wired in accor-
dance with the National Electric Code (ANSI/NFPA 70) in the
United States, and the Canadian Electric Code (CSA C22.1), in
Canada.
Control Wiring (Duct System Furnace)
Multiple-heater systems are controlled by a Vernier stage control-
ler. This control provides modulation of the lead gas furnace and
staging control of additional furnaces in the assembly. The stage
controller requires a Heat Enable contact closure and a 0 to 10
VDC analog input to operate the system. Figure 142 shows the
typical terminal connections for the Heat Enable and the Analog
Input signal. Also, refer to the wiring diagrams provided for elec-
trical connections included in the furnace section. A Control In-
struction Sheet with the Sequence of Operation for the system un-
der the supervision of the stage controller is also available in the
information packet. See Fig. 142 for a typical wiring diagram.
HEAT ENABLE
T2
T1
Fig. 142 — Gas-Fired Duct System Furnace Typical
Wiring Diagram
VERIFICATION
Verify the following before placing the equipment into service:
• Electrical supply matches the heater voltage marked on the
duct furnace Rating Plate.
187
ANALOG INPUT
(0-10 vdc)
–
+
GROUND
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