Lennox F8AUH series Service Manual page 25

PROBLEM 3: UNIT FAILS TO FIRE IN THE HEATING MODE, COMBUSTION AIR BLOWER DOES
Condition
3.3
− Unit operates with a cooling and con-
tinuous fan demand.
− Combustion air blower will not start
with a Heating demand.
− Diagnostic lights flash the pressure
switch failure code 2.5 minutes after switch failure code 2 5 minutes after
heating demand.
LED#1−Off,
LED#2−Slow Flash
PROBLEM 4: UNIT FAILS TO FIRE IN THE HEATING MODE, COMBUSTION AIR BLOWER
Condition
4.1
− Unit operates with a cooling and
continuous fan demand.
− Combustion air blower energizes
with a heating demand with a heating demand.
− Diagnostic lights flash the pressure
switch failure code 2.5 minutes after
heating demand.
LED#1−Off
LED#2−Slow Flash LED#2 Slow Flash
NOT ENERGIZE (CONT.).
Possible Cause
3.3.1
Miswiring of furnace or improper con-
nections to combustion air blower.
3.3.2
Combustion air blower failure.
ENERGIZES, IGNITOR IS NOT ENERGIZED.
Possible Cause
4.1.1
Pressure switch does not close due
to incorrect routing of the pressure
switch tubing.
4.1.2
Pressure switch does not close due
to obstructions in the pressure tub-
ing.
4.1.3
Pressure switch tubing damaged
4.1.4
Pressure switch does not close due
to a low differential pressure across
the pressure switch.
4.1.5
Wrong pressure switch installed in
the unit, or pressure switch is out of
calibration.
4.1.6
Miswiring of furnace or improper con-
nections at pressure switch.
4.1.7
Pressure switch failure.
Page 25
Corrective Action/Comments
ACTION 1 − Check for correct wiring and loose
connections. Correct wiring and/or replace any
loose connections.
ACTION 1 − If there is 120V to combustion air
blower and it does not operate, replace combus-
tion air blower.
Corrective Action/Comments
ACTION 1 − Check that the pressure switch tub-
ing is correctly routed. Correctly route pressure
switch tubing.
ACTION 1 − Remove any obstructions from the
pressure tubing and/or taps.
ACTION 1 − Check pressure switch tubing for
leaks. Replace any broken tubing.
ACTION 1 − Check the differential pressure
across the pressure switch. This pressure
should exceed the set point listed on the
switch.
ACTION 2 − Check for restricted inlet and ex-
haust vent. Remove all blockage.
ACTION 3 − Check for proper vent sizing and
run length. See installation instructions.
ACTION 4 − Check voltage to combustion air
blower.
ACTION 1 − Check that the proper pressure
switch is installed in the unit. Replace pressure
switch if necessary.
ACTION 1 − Check for correct wiring and loose
connections. Correct wiring and/or replace any
loose connections.
ACTION 1 − If all the above modes of failure have
been checked, the pressure switch may have
failed. Replace pressure switch and determine if
unit will operate.