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A number of safety circuits are employed to ensure safe and proper furnace operation. These circuits serve to control
any potential safety hazards and, as inputs in the monitoring and diagnosis of abnormal function. These circuits are
continuously monitored by the integrated control module.
Integrated Control Module
The integrated control module is an electronic device
which regulates all furnace operations. Responding to
the thermostat, the module initiates and controls normal
furnace operation, and monitors and addresses all safety
circuits. If a potential safety concern is detected, the
module will take the necessary precautions and provide
diagnostic information through an LED located on the
module. See troubleshooting chart on page 14.
Primary Limit
The primary limit control is located on the partition panel
and monitors heat exchanger compartment tempera-
tures. It is a normally closed (electrically), automatic re-
set, temperature activated sensor. This limit guards
against overheating as a result of insufficient conditioned
air passing over the heat exchanger.
Auxiliary Limit
The auxiliary limit control is located on the blower deck
and monitors heat exchanger compartment tempera-
tures. It is a normally closed (electrically), manual reset,
temperature activated sensor. This limit guards against
overheating as a result of insufficient conditioned air
passing over the heat exchanger.
Rollout Limit(s)
The rollout limit is a normally-closed (electrically),
manual-reset, temperature-activated sensor. It is
mounted on the burner/manifold assembly and monitors
the burner flame. If there is an improper draw of burner
flames into the heat exchanger, the rollout limit will detect
it and shutdown gas flowing to the burners.
Pressure Switches
The pressure switches are normally-open (closed during
operation), single-pole single-throw, negative air pressure
activated switches. They monitor the airflow (combustion
air and flue products) through the heat exchanger via
pressure taps located on the induced draft blower and the
coil front cover. These switches guard against insufficient
airflow (combustion air and flue products) through the
heat exchanger and blocked condensate drain conditions.
Flame Sensor
The flame sensor is a probe mounted to the burner/mani-
fold assembly which uses the principle of flame rectifica-
tion to determine the presence or absence of flame.
Safety Circuits
ROLLOUT
LIMIT(S)
PRESSURE
SWITCH
CIRCULATOR
BLOWER
Resetting From Lockout
Furnace lockout is characterized by a non-functioning
furnace (circulator blower may be running continuously)
providing a one flash diagnostic LED code. Lockout
results when a furnace is unable to achieve ignition after
three attempts, or when it has lost flame five times
during a single call for heat. If the furnace is in "lockout",
it may be reset by any of the following methods:
1. One-hour automatic reset. Control will automatically
reset itself and attempt to resume normal operations
following a one hour lockout period.
2. Power interruption. Interrupt 115 volt power to the
furnace for between 0 and 20 seconds.
3. Thermostat cycle. Interrupt thermostat signal to the
furnace for between 0 and 20 seconds.
If the condition which originally caused the lockout still
exists, the control will return to lockout. Refer to the
"Troubleshooting" section for aid in determining the
cause. If your furnace frequently locks out, a problem
exists which must be corrected. Contact a qualified
servicer.
11
PRIMARY LIMIT
FLAME SENSOR
AUXILIARY LIMIT
(UNDERSIDE OF BLOWER DECK)
Safety Circuits
Upflow Shown
PRESSURE
SWITCH
BLOWER
DECK
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