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The furnace module 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 igni
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tion control module. 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 furnace module
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.
High Limit Switch — The furnace module is equipped with a
fixed-temperature high limit switch mounted on the vestibule pan
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el. This switch shuts off gas to the heater through the ignition con
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trol 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 restriction of
the air inlet or outlet to the unit. The high limit switch will auto
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matically reset when the temperature drops to 30°F (16.7°C) be
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low the set point. Determine the cause of the reduced airflow and
correct.
Induced Draft Air Pressure Switch — An air pressure switch is
provided as part of the control system to verify combustion air
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flow through the induced-draft fan. The pressure switch monitors
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 combus
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tion air are not available to the heater, poor combustion and ineffi
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cient 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 lo
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cated in the same pressure zone to minimize effects of wind on
burner and heater performance.
UNIT LOCATION AND CLEARANCES
For both outdoor and indoor installations, observe these require
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ments:
• 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
adjustment 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
explosive 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.
Fig. 154 —
Gas-Fired Rack Furnace Components