116
SERVICING
AUXILIARY LIMIT
CONTROL
AUXILIARY LIMIT
CONTROL
BLOWER HOUSING
Auxiliary Limit Control Location
(80 % and 90% Two-Stage Furnaces)
The
80% two-stage
furnaces auxiliary limit control (auto-
matic reset) is connected in series with the gas valve and
the integrated ignition control. If its temperature should be
exceeded, it will open, interrupting the voltage to the gas
valve causing it to close.
If the auxiliary limit control opens, the air circulation blower
will run continuously and the diagnostic light will flash (1)
one time.
The auxiliary limit control is designed to prevent furnace op-
eration in case of main blower failure in horizontal or
counterflow installations. It may also open if the power sup-
ply 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.
The
90% two-stage
and single-stage counterflow
furnaces
auxiliary limit control (manual reset) is connected in series
with the primary limit control wiring to the integrated ignition
control. If its temperature should be exceeded, it will open,
interrupting the voltage to the gas valve causing it to close.
If this limit control opens, the air circulation blower will run
continuously and the diagnostic light will flash (4) four times.
These symptoms are identical to a trip of the primary limit
control.
The auxiliary limit control is designed to prevent furnace op-
eration in case of main blower failure in horizontal or
counterflow installations. It may also open if the power sup-
ply 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
HIGH
VOLTAGE
D
ISCONNECT
ALL
POWER BEFORE SERVICING OR
INSTALLING THIS UNIT.
M
ULTIPLE POWER SOURCES MAY
BE PRESENT.
F
AILURE TO DO SO MAY CAUSE PROPERTY
DAMAGE, PERSONAL INJURY OR DEATH.
1. Remove blower compartment door to gain access to
the auxiliary limit control which is located on the front of
the blower housing on all 80% single-stage furnaces
and on the blower deck of the 90% single-stage upflow
furnaces. The two-stage 80% and 90% furnaces and
90% single-stage counterflow furnaces have (2) auxil-
iary limits, one located on each side of the blower hous-
ing.
2. Remove the wires from the auxiliary limit control termi-
nals.
3. Using an ohmmeter, test for continuity across the two
terminals (refer to the following auxiliary limit control fig-
ure). No reading indicates the control is open. Push red
manual reset button (only on 90% furnaces), test again
- if still open, replace the control.
VOLT / OHM
METER
RED
RESET
BUTTON
COLOR
IDENTIFYING
TAB
Testing Auxiliary Limit Control
(80% & 90% Furnaces)
WARNING
T
O AVOID POSSIBLE FIRE, ONLY RESET THE AUXILIARY LIMIT CONTROL
ONCE.
I
F IT SHOULD OPEN A SECOND TIME, A QUALIFIED SERVICER
MUST DETERMINE WHY THE AUXILIARY LIMIT OPENED BEFORE
RESETTING AGAIN.
NOTE:
If it becomes necessary to slide the blower assem-
bly out of any 90% single-stage upflow furnace, the auxiliary
limit control must be removed from the blower deck before
the blower assembly can be removed. After the blower as-
sembly is reinstalled, the auxiliary limit must be reinstalled.