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BURNER BOX
This furnace must use indoor air for combustion. It is not a
direct vent furnace, and it cannot be installed as a direct
vent furnace. The burner box (on some models) is present
only to reduce the burner sound transmission.
AUXILIARY LIMIT CONTROL (FIGURE 35)
A manual reset limit is located on the blower side of the
blower deck, near the center. To access this auxiliary limit,
disconnect the electrical power and remove the blower
door. If the limit control opens, the air circulation blower will
run continuously. The diagnostic light will flash four times.
These symptoms are identical to a trip of the primary limit
control. See Section X,
Abnormal Operation - Integrated
Ignition Control
for diagnosis.
The auxiliary limit control is designed to prevent furnace
operation in case of main blower failure on horizontal and
counterflow installations. It may also open if the power
supply is interrupted while the furnace is firing.
The auxiliary limit control is suitable for both horizontal right
and horizontal left installations. Regardless of airflow direc-
tion, it does not need to be relocated.
WARNING
To prevent death, personal injury or property
damage due to fire, follow these directions
for the auxiliary limit control. If the auxiliary
limit control opens, it may be reset one time
only.
(SERVICER’S NOTE: If it becomes necessary to slide the
blower assembly out of the furnace, the auxiliary limit
control should be removed first. After the blower assembly
is reinstalled, the auxiliary limit must be reinstalled.)
Figure 35
Auxiliary Limit Control
ABNORMAL OPERATION - INTEGRATED IGNITION
CONTROL
The following presents the probable causes of question-
able furnace operation and how to fix them. Look through
the observation window in the blower access door and
make a note of the number of flashes in sequence between
pauses. Next, refer to the
Diagnostic Signal Chart
for an
interpretation of the signals and to this section for a descrip-
tion.
1. Internal Control Failure with Integrated Ignition
Control
An internal control failure occurs when the integrated
ignition control senses an internal problem and stops
the unit.
To solve this problem, replace the ignition control.
2. System Lockout
A system lockout occurs when the integrated ignition
control determines that a measurable combustion can-
not be established or maintained after three consecu-
tive tries (four, if flame is established then lost) to turn
on the furnace.
If a flame is not sensed during the first six seconds after
a gas valve has been energized, the ignition control will
internally turn off the gas. After 60 seconds, during
which time the induced draft blower purges the heat
exchanger, the ignitor will re-energize and preheat for
27 seconds. The gas valve is then re-energized. If a
flame is not sensed again in six seconds, the gas valve
will de-energize and another purge is performed. The
ignition control will cycle the gas valve
three times
before it determines it cannot establish measurable
combustion and enter a lockout state. If a flame is
sensed but lost after 10 seconds, the control will cycle
four more times
before locking out. A lockout stops
ignition attempts and causes the air blower to run
continuously. The diagnostic light will indicate either
condition with one short flash followed by a longer off.
The control can be reset and brought out of lockout
mode by turning the thermostat off and then back on. It
can also reset by turning off the electrical disconnect
switch to the furnace for 30 seconds. The control will
reset after one hour.
IMPORTANT: If you have to frequently reset your furnace,
it means that a problem exists that should be corrected.
Contact a qualified servicer for further information.
3. Pressure Switch Stuck Closed
A sticking pressure switch can be caused by either a
faulty pressure switch, faulty wiring, a disconnected
hose, or a restricted intake or flue piping. In the case
of a pressure switch sticking closed, the probable
cause is a faulty pressure switch or wiring.
If the ignition control senses that the pressure switch
is closed and the induced draft blower is off, it will shut
down the unit until the fault is corrected. The light code
for this problem is two short flashes followed by a
longer pause.
4. Pressure Switch Stuck Open
A sticking open pressure switch can be caused by a
faulty pressure switch, a disconnected hose to the
pressure switch, a restricted air intake or flue piping, or
a faulty wiring.
If the ignition control senses that the induced draft
blower is energized but the pressure switch is not
closed, the control will keep the blower on and wait for
the switch to close. The diagnostic light code for this
problem is three short flashes followed by a pause.