60
SERVICING
1. Operate furnace with burners firing for approximately ten
minutes. Check BTU input to furnace - do not exceed
input rating stamped on rating plate. Ensure all registers
are open and all duct dampers are in their final (fully or
partially open) position.
2. Place thermometers in the return and supply ducts as
close to the furnace as possible. Thermometers must not
be influenced by radiant heat by being able to “see” the
heat exchanger.
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.
*
*
*
*
*
*
*
2
PRIMARY LIMIT
CONTROL
Primary Limit Control Location
(90% Upflow Furnace Shown, Counterflow Similar)
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.
