SERVICING
34
SUPPLY
AIR
RETURN
AIR
Temperature Rise Measurement
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.
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.
ENCLOSED DISK
FRONT VIEW
SIDE VIEW
Primary Limit Control Style
WARNING
High Voltage!
Disconnect all power before servicing
or installing this unit. Multiple power
sources may be present. Failure 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.
COLOR
IDENTIFYING
TAB
Testing Primary Limit Control
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.
