48
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. Al-
low adequate time for thermometer readings to stabi-
lize.
4. Adjust temperature rise by adjusting the circulator blower
speed. Increase blower speed to reduce temperature
rise. Decrease blower speed to increase temperature
rise. Refer to
Circulator Blower Speed
section in the
Product Design section of this manual for speed chang-
ing details. Temperature rise is related to the BTUH out-
put of the furnace and the amount of air (CFM) circu-
lated over the heat exchanger. Measure motor current
draw to determine that the motor is not overloaded dur-
ing adjustments.
S-300 CHECKING PRIMARY LIMIT CONTROL
Primary limit controls are nonadjustable, automatic reset,
bi-metal type limit control. Refer to the following drawing
for the location of the primary limit.
*
*
*
*
*
*
*
2
PRIMARY LIMIT
CONTROL
Primary Limit Control Location
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
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 tem-
perature is above limit control setting. If not as above,
replace the control.
VOLT / OHM
METER
Testing Primary Limit Control
