Carrier AERO 39MN Installation, Start-Up And Service Instructions Manual page 113

WARNING
Gas-fired furnaces are not designed for use in hazardous
atmospheres containing flammable vapors or combustible
dust, in atmospheres containing chlorinated or halogenated
hydrocarbons, or in applications with airborne substances
containing silicone.
CONDENSATE DRAINS — In furnace applications operating
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 operation
of the air conditioning, resulting in condensation from warm,
moist air in the heat exchanger tubes and flue collector. This con-
densate is not harmful to the heat exchanger provided it is
drained continuously. For these applications a ¼ NPT connec-
tion is provided for attachment of condensate drain line to re-
move condensate from heat exchanger.
A P-trap is recommended as the system operates under a neg-
ative pressure. The use of a "Tee" fitting is recommended to al-
low for priming and cleaning the trap. Use a plug in the cleanout
opening. See Fig. 95.
CAUTION
Do not use copper tubing for condensate drain lines. Flue
gas condensate is slightly acidic.
4.00"
Fig. 95 — Condensate Drain
FURNACE COMPONENT IDENTIFICATION — See Fig.
96 for single-unit gas furnace components.
OPERATING AND SAFETY CONTROLS — The following
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 air-
flow through induced-draft fan (ID fan) by monitoring the differ-
ence in pressure between the ID fan and the atmosphere. If suffi-
cient 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 pressure switch is
used, monitoring high and low speed pressures. The air pressure
switches have fixed settings and are not adjustable.
Rollout Switch (Manual Reset) —
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.
COLLECTOR BOX
PLUG
TEE FITTING
2.00"
a39-4394
The duct furnace
Flame rollout can be caused by insufficient airflow for the burner
firing rate (high gas pressure), blockage of the vent system 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 button 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 vestibule
panel. This switch will shut off gas to the heater through the ig-
nition control module before the air temperature reaches 250 F
(121.1 C). Reduced airflow may be caused by restrictions up-
stream 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 automatically reset when the tem-
perature drops to 30° F (16.7° C) below the set point, initiating a
furnace ignition. The furnace will continue to cycle on limit until
the cause of the reduced airflow is corrected.
Ignition Control Module — Monitors furnace function and pro-
vides safety interlocks. Ignition control modules are available
having a number of different operating functions. Refer to Se-
quence of Operation and Control Diagnostic data sheets provid-
ed 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 information,
to contact your Carrier service representative.
COMBUSTION AIR SUPPLY — All gas-fired furnaces need
an ample supply of air for proper and safe combustion of the fuel
gas. If sufficient quantities of combustion air are not available to
the heater, poor combustion and inefficient operation will result.
For outdoor installations, the heating unit cabinet combustion
air openings should be sized to provide 1 sq in. of free area per
4000 Btuh of input. Combustion air inlet and flue gas outlet must
be located in the same pressure zone to minimize effects of wind
on burner and heater performance.
For indoor installations, locate heating unit to ensure an ade-
quate supply of fresh air to replace air used in the combustion
and ventilation process. Install air openings that provide a total
free area in accordance with the National Fuel Gas Code (ANSI
Z223.1 or NFPA 54) in the United States or CAN/CGA B-149
Installation Code in Canada.
Do not install unit in a confined space (a space whose volume
is less than 50 cubic feet per 1000 Btuh of furnace input rating)
without providing wall openings leading to and from the space.
Provide openings near floor and ceiling to provide air for com-
bustion and ventilation. Size openings for total Btuh of all appli-
ances in confined space and provide openings as follows:
• Air from inside building — openings 1 sq in. free area per
1000 Btuh, but never less than 100 sq in.
• Air from outside through a duct — openings 1 sq in. free area
per 2000 Btuh
• Air direct from outside — openings 1 sq in. free area per
4000 Btuh
VENTING — Safe operation of indirect-fired gas furnaces
requires a properly operating venting system which vents all the
products of combustion (flue gases) to the outside atmosphere.
Failure to provide proper venting affects furnace perfor-
mance and may result in a health hazard which could cause
serious personal injury or death.
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WARNING