61
SERVICING
SUPPLY
AIR
RETURN
AIR
Checking Temperature Rise
3. Subtract the return air temperature from the supply air
temperature to determine the air temperature rise. Allow
adequate time for thermometer readings to stabilize.
S-300 CHECKING PRIMARY LIMIT CONTROL
All primary limit controls are nonadjustable, automatic re-
set, bi-metal type limit control. Refer to the following illus-
tration for the location of the primary limit.
PRIMARY LIMIT
CONTROL
Primary Limit Control Location
(90% Upflow Furnace Shown)
The following illustration indicates the style of limit switches
used on the 90% furnaces.
ENCLOSED DISK
FRONT VIEW
SIDE VIEW
Primary Limit Control Style
(90% Furnaces)
WARNING
HIGH
VOLTAGE
D
ISCONNECT
ALL
POWER BEFORE SERVICING OR
CHANGING ANY ELECTRICAL WIRING.
M
ULTIPLE POWER
SOURCES MAY BE PRESENT.
F
AILURE TO DO SO MAY CAUSE
PROPERTY DAMAGE, PERSONAL INJURY OR DEATH.
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 temperature is
above limit control setting. If not as above, replace the
control.
LIMIT FAULT CODES
If the control detects the high limit circuit open, the RED
LED will flash FOUR times and energize the inducer and
indoor blower.
If the limit circuit opens five times within a single call for
heat, the furnace will go into lockout for one hour. The RED
LED will display a FOUR flash code during this time. The
control board can be reset by cycling 115 volt power to the
furnace.
If the limit circuit opens and does not close within five min-
utes, the control assumes the blower has failed the RED
LED will flash ELEVEN times.
If the limit circuit opens and does not close within fifteen
minutes, the control assumes the manual rollout or fuse
has opened and the RED LED will flash FIVE times. The
control will enter a one hour lockout and the inducer will run
continuously.
In an open limit condition the red LED will flash as described
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 resets in less than 5 minutes a four flash code will be
stored. If the limit resets in more than 5 but less than 15
minutes, an eleven flash code will be stored. If the limit is
still open after 15 minutes, a five flash code will be stored.
