Carrier Aero 39MN Series Installation, Start-Up And Service Instructions Manual page 171

Rollout Switch (Manual Reset) — 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 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 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
panel. This switch shuts off gas to the heater through the igni-
tion 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 fil-
ters, or restriction 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 reduced airflow and correct.
Induced Draft Air Pressure Switch — An air pressure switch
is provided as part of the control system to verify combustion
airflow 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
Fig. 146 — Gas-Fired Rack Furnace Components
171
ignition control module. The air pressure switches have fixed
settings and are not adjustable.
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
available to the heater, poor combustion and inefficient opera-
tion will result.
For outdoor installations, the heating unit cabinet combus-
tion 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.
UNIT LOCATION AND CLEARANCES — For both out-
door and indoor installations, observe these requirements:
• Be sure unit is located with respect to building construc-
tion 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.
gas-fired furnaces