SERVICING
48
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2
PRIMARY LIMIT
CONTROL
Primary Limit Control Location
(90% Upflow Furnace Shown, Counterflow Similar)
The following drawing illustrates the style of limit switches
used on the 90% furnaces.
ENCLOSED DISK
FRONT VIEW
SIDE VIEW
Primary Limit Control Style
(90% Furnaces)
WARNING
HIGH
VOLTAG
E
D
IS
C
ONN
EC
T
ALL
POW
ER
B
E
FO
RE
S
ER
VI
C
ING
O
R
C
HANGING
ANY
E
L
EC
T
R
I
C
AL
WI
R
ING
.
M
ULTIPL
E
POW
ER
SOU
RCE
S
MAY
B
E
P
RE
S
E
NT
.
F
AILU
RE
TO
DO
SO
MAY
C
AUS
E
P
R
OP
ER
TY
DAMAG
E
,
P
ER
SONAL
IN
J
U
R
Y
O
R
D
E
ATH
.
1. Remove burner compartment door to gain access to
the primary limit.
2. Remove low voltage wires at limit control terminals.
3. With an ohmmeter, test between these two terminals
as shown in the following drawing. The ohmmeter
should read continuous unless heat exchanger tem-
perature is above limit control setting. If not as above,
replace the control.
COLOR
IDENTIFYING
TAB
Testing Primary Limit Control
(90% Furnaces)
4. After completing check and/or replacement of primary
limit control, reinstall burner compartment door.
5. Turn on electrical power and verify proper unit operation.
To aid in identifying these controls, refer to the
Primary
Limit Charts
in furnace Technical Manual for part number,
temperature setting and color(s) code.
CHECKING AUXILIARY LIMIT CONTROL
The auxiliary limit control is designed to prevent furnace
operation in case of main blower failure in horizontal or
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
direction, it does not need to be relocated. The (2) two auxiliary
limits are located on the blower housing (one on each side),
as shown in the following illustration.
LIMIT
AUXILIARY LIMIT
CONTROL
AUXILIARY LIMIT
CONTROL
BLOWER HOUSING
Auxiliary Limit Control Location
(Select 90% / 95% Furnaces)
