Carrier Aero 39MN Series Installation, Start-Up And Service Instructions Manual page 163

• Furnace is installed in orientation marked on vestibule.
Orientation is specific to airflow direction through the
heating section of the unit.
• There is an adequate supply of fresh air for the combus-
tion and ventilation process. Combustion air openings in
the cabinet should be sized to provide 1 sq in. of free area
per 4000 Btuh of input.
CAUTION
The presence of chlorine vapors in the combustion air
supplied to gas-fired heaters presents a substantial cor-
rosion hazard.
• A properly designed vent system is connected to the
heating unit to convey the products of combustion (flue
gases) outside the building. For outdoor applications be
sure the flue gases are directed away from any combus-
tion air inlets.
• Duct furnace assembly is installed in a non-combustible
duct or cabinet on the positive pressure side of the circu-
lating air blower.
• An air flow proving switch is installed and wired to
prove operation of the system circulating air blower.
• An auxiliary manual reset limit switch is installed to shut
off furnace in the event of low airflow conditions due to
filter blockage, coil blockage and or damper failure.
• A drain tube is installed for disposal of condensate, if the
heating unit is equipped with modulating controls or is
located downstream of cooling system.
• Equipment access panels and doors are sized and located
to provide easy access for servicing, adjustment and
maintenance of the heating units installed.
WARNING
Gas-fired furnaces are not designed for use in hazardous
atmospheres containing flammable vapors or combusti-
ble dust, in atmospheres containing chlorinated or halo-
genated hydrocarbons, or in applications with airborne
substances containing silicone. Use in these environ-
ments may result in damage to equipment and personal
injury.
CONDENSATE DRAINS — In furnace applications operat-
ing with modulating controls, temperature rises below 40°F
(4.4°C), or with outside make-up air, some condensation may
occur in the heating cycle. In these applications connection of a
condensate drain line is required to avoid condensate build-up
and possible heat exchanger damage.
If heating section is located downstream of a refrigeration
system or cooling coil, condensation can occur during opera-
tion of the air conditioning, resulting in condensation from
warm, moist air in the heat exchanger tubes and flue collector.
This condensate is not harmful to the heat exchanger provided
it is drained continuously. For these applications a ¼-in. NPT
connection is provided for attachment of condensate drain line
to remove condensate from heat exchanger.
A P-trap is recommended as the system operates under a
negative pressure. The use of a "Tee" fitting is recommended
to allow for priming and cleaning the trap. Use a plug in the
cleanout opening. See Fig. 135.
CAUTION
Do not use copper tubing for condensate drain lines.
Flue gas condensate is slightly acidic and may corrode
lines.
4.00"
Fig. 135 — Condensate Drain
FURNACE COMPONENT IDENTIFICATION — See
Fig. 136 for single-unit gas furnace components.
OPERATING AND SAFETY CONTROLS — The follow-
ing controls are provided for the duct furnace:
Combustion Air Pressure Switch — An air pressure switch
is provided as part of the control system to verify combustion
airflow through induced-draft fan (ID fan) by monitoring the
difference in pressure between the ID fan and the atmosphere.
If sufficient negative pressure is not present, indicating lack of
proper air movement through heat exchanger, the switch opens
shutting off gas supply though the ignition control module. On
units with two-speed draft inducer operation, a dual air pres-
sure switch is used, monitoring high and low speed pressures.
The air pressure switches have fixed settings and are not ad-
justable.
Rollout Switch (Manual Reset) — The duct furnace is
equipped with manual reset rollout switch(es) in the event of
burner flame rollout. The switch will open on temperature rise
and shut off gas supply through the ignition control module.
Flame rollout can be caused by insufficient airflow for the
burner firing rate (high gas pressure), blockage of the vent sys-
tem or in the heat exchanger. The duct furnace should not be
placed back in operation until the cause of rollout condition is
identified. The rollout switch can be reset by pressing the but-
ton on the top of the switch.
Primary High Limit Switch — To prevent operation of the
duct furnace under low airflow conditions, the unit is equipped
with a fixed temperature high limit switch mounted on the ves-
tibule panel. This switch will shut off gas to the heater through
the ignition control module before the air temperature reaches
250°F (121.1°C). Reduced airflow may be caused by restric-
tions upstream or downstream of the circulating air blower,
such as dirty or blocked filters or restriction of the air inlet or
outlet to the unit. The high limit switch will shut off the gas
when the temperature reaches its set point and then automati-
cally reset when the temperature drops to 30°F (16.7°C) below
the set point, initiating a furnace ignition. The furnace will con-
tinue to cycle on limit until the cause of the reduced airflow is
corrected.
Ignition Control Module — Monitors furnace function and
provides safety interlocks. Ignition control modules are avail-
able having a number of different operating functions. Refer to
Sequence of Operation and Control Diagnostic data sheets pro-
vided in the instruction package for a detailed description of the
control features, operation and troubleshooting for the model
control installed. Check the furnace section for this informa-
tion, or contact your Carrier service representative.
COMBUSTION AIR SUPPLY — All
need an ample supply of air for proper and safe combustion of
the fuel gas. If sufficient quantities of combustion air are not
163
COLLECTOR BOX
PLUG
TEE FITTING
2.00"
a39-4394
gas-fired furnaces