Combustion Air Supply - Carrier AERO 39MN Installation, Start-Up And Service Instructions Manual

a39-4404
High Limit Switch — The furnace module is equipped with a
fixed-temperature high limit switch mounted on the vestibule
panel. This switch shuts off gas to the heater through the ignition
control module in the event of reduced circulating airflow over
the heat exchanger. Reduced airflow can be caused by motor
failure of the circulating air blower, dirty or blocked filters, or re-
striction of the air inlet or outlet to the unit. The high limit switch
will automatically reset when the temperature drops to 30° F
(16.7° C) below the set point. Determine the cause of the re-
duced airflow and correct.
Induced Draft Air Pressure Switch — An air pressure switch is
provided as part of the control system to verify combustion air-
flow through the induced-draft fan. The pressure switch moni-
tors the difference in pressure between the induced-draft fan and
the atmosphere. If sufficient negative pressure is not present, the
switch opens, shutting off gas supply though the ignition control
module. The air pressure switches have fixed settings and are
not adjustable.
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.
Fig. 103 — Gas-Fired Furnace Components (Rack System)
UNIT LOCATION AND CLEARANCES — For both out-
door and indoor installations, observe these requirements:
• Be sure unit is located with respect to building construction
and other equipment to provide ready access and clearance to
access panels or doors that must be opened to permit adjust-
ment and servicing of the heating section.
• The heating section needs an ample supply of air for proper
and safe combustion of the fuel gas. Do not block or obstruct
air openings to the area where the heating unit is installed.
Combustion air openings in the cabinet should be sized to
provide 1 sq in. of free area per 4000 Btuh of input.
• Do not install unit where it may exposed to potentially explo-
sive or flammable vapors.
• Do not locate unit in areas where corrosive vapors (such as
chlorinated, halogenated, or acidic vapors) are present in the
atmosphere or can be mixed with combustion air entering
heater.
Outdoor Installations — Observe the following additional re-
quirements for outdoor installations:
• Provide at least 6 feet clearance to side of the unit, where the
combustion air inlet or vent (flue) gas discharge is located,
from walls, parapets or adjacent buildings or equipment.
• Unit must be installed with combustion air openings located
at least one (1) ft above the average snow depth for the loca-
tion.
• Do not locate unit near building ventilators or exhausts, or
areas where corrosive chemical vapors can be drawn into

combustion air supply.

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