SERVICING
38
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.
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 changing details.
Temperature rise is related to the BTUH output of the
furnace and the amount of air (CFM) circulated over the
heat exchanger. Measure motor current draw to determine
that the motor is not overloaded during adjustments.
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
The following drawing illustrates the style of limit switches
used on the 90% furnaces.
ENCLOSED DISK
FRONT VIEW SIDE VIEW
WARNING
IG
V
L
AGE
D
ISCNNEC
ALL
PWER ERE SERVICING R
CANGING AN ELECRICAL WIRING
M
LIPLE PWER
SRCES MA E PRESEN
AILRE D S MA CASE
PRPER DAMAGE PERSNAL INJR R DEA
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.
RED
RESET
BUTTON
COLOR
IDENTIFYING
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.
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