SERVICING
33
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 tem-
perature 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 relat-
ed to the BTUH output of the furnace and the amount
of air (CFM) circulated over the heat exchanger. Mea
-
sure motor current draw to determine that the motor is
not overloaded during adjustments.
CHECKING PRIMARY LIMIT CONTROL
All use a nonadjustable, automatic reset, bi-metal type limit
control. Refer to the following drawing for the location of the
primary limit.
Location of Primary Limit
Primary Limit Control Location
(80% Upflow Furnace Shown, Counterflow Similar)
Style 1 drawing illustrates the Primary Limit used on the
80% furnaces.
Style 1
WARNING
HIGH VOLTAGE
Disconnect ALL power before servicing or
changing any electrical wiring. Multiple pow
-
er 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 tem
-
perature is above limit control setting. If not as above,
replace the control.
COLOR
IDENTIFYING
TAB
Testing Primary Limit Control
WARNING
Do not bypass any safety limit connection.
CHECKING AUXILIARY LIMIT CONTROL
BLOWER HOUSING
AUXILIARY LIMIT CONTROL
Auxiliary Limit Control Location
