SERVICING
50
The following drawing illustrates the style of limit switches
used on the 90% furnaces.
ENCLOSED DISK
FRONT VIEW SIDE VIEW
G
TG
T
R BR RG R
GG TR RG
T R
R B RT
R T
RRT G R JR R T
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.
If the control detects the high limit circuit open, the RED LED
blower.
the furnace will go into lockout for one hour. The RED LED
board can be reset by cycling 115 volt power to the furnace.
the control assumes the blower has failed the RED LED will
minutes, the control assumes the manual rollout or fuse
control will enter a one hour lockout and the inducer will run
continuously.
above but the error code will not be stored in memory until
the limit resets or until 15 minutes has elapsed. So if the limit
If the limit resets in more than 5 but less than 15 minutes, an
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.
The auxiliary limit control is designed to prevent furnace
operation in case of main blower failure in horizontal
installations. It may also open if the power supply is interrupted
The auxiliary limit control is suitable for both horizontal
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
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