21
Verifying and Adjusting Temperature Rise
Confi rm the temperature rise through the furnace is
within the limits specifi ed on the furnace rating plate. Any
temperature rise outside the specifi ed limits could result
in premature failure of the heat exchanger.
1.
Place thermometers in the return and supply air stream
as close to the furnace as possible. The thermometer
on the supply air side must be shielded from direct
radiation from the heat exchanger to avoid false
readings.
2. Adjust all registers and duct dampers to the desired
position and run the furnace for 10 to 15 minutes before
taking any temperature readings. The temperature
rise is the difference between the supply and return
air temperatures.
For typical duct systems, the temperature rise will fall
within the limits specifi ed on the rating plate with the blower
speed at the factory recommended setting. If the measured
temperature rise is outside the specifi ed limits, it may be
necessary to change the speed of the blower.
The *XC furnace is equipped with a multi-speed motor.
Heating, cooling, and fan speed selection is made by
moving the switch located on the motor control board
(Figure 24).
NOTE:
Lowering the blower speed will
increase the temperature rise and a higher blower speed
will decrease the temperature rise.
Verifying Burner Operation
CAUTION:
The door over the burners may only be open
for inspection purposes only. The door must be
installed during unattended operation.
1. Remove the burner compartment door.
2. Set the thermostat above room temperature and
observe the ignition sequence.
The burner fl ame
should carry over immediately between all burners
without lifting off, curling, or fl oating. The fl ames
should be blue, without yellow tips.
3. After validating the fl ame, change thermostat setting
to below room temperature.
4. Verify the burner fl ame is completely extinguished.
5. Replace the burner compartment door.
Verifying Operation of the Supply Air Limit Switch
A properly functioning limit switch should turn off the
gas valve when the return is blocked (time depends on
how well the return air is blocked). The circulating air and
combustion blowers should continue to run when the limit
switch opens.
1. Check the blower door and verify that it is securely
mounted in place and that there is power to the
furnace.
2. Block the return airfl ow to the furnace by installing
a close-off plate in place of or upstream of the
fi lter(s).
3. Set the thermostat above room temperature and
observe the Operating Sequence (page22).
4. Remove the close-off immediately after the limit
switch opens. If the furnace continues to operate
with no return air, set the thermostat below room
temperature, shut off the power to the furnace, and
replace the limit switch.
GREEN
RED
STATUS
NO
T USED
NOT USED
CONNECTION TO
FURNACE CONTROL
BOARD
MOTOR WIRE
HARNESS
R
C
Y1
TWIN
DHUM
W1
THERMOST
A
T
EXPANSION
PORT
COOL
HEAT
FAN SPEED
1 2 3 4 5 6 7 8
OFF ON
C
ONNECTION
TERMINAL T
O
T
W
IN FURNA
CES
Figure 24. Motor Control Board