Lennox F8AUH series Service Manual page 27

PROBLEM 6: BURNERS LIGHT WITH HEATING DEMAND BUT UNIT SHUTS DOWN
Condition
6.3
− Combustion air blower energizes
with a heating demand.
− Burners light.
− Roll−out switch trips during the
heating demand.
− Diagnostic lights flash roll−out fail-
ure. ure.
LED#1−On
LED#2−Slow Flash
6.4
− Combustion air blower energizes
with a heating demand.
− Burners light roughly and the unit
fails to stay lit.
− Diagnostic lights flash watchguard
flame failure. flame failure.
LED#1−Alternating Slow Flash
LED#2−Alternating Slow Flash
PREMATURELY (CONT.)
Possible Cause
6.3.1
Unit is firing above 100% of the
nameplate input.
6.3.2
Gas orifices leak at the manifold con-
nection.
6.3.3
Insufficient flow through the heat ex-
changer caused by a sooted or re-
stricted heat exchanger.
6.3.4
Burners are not properly located in
the burner box.
6.4.1
Recirculation of flue gases. This
condition causes rough ignitions and
operation. Problem is characterized
by nuisance flame failures.
6.4.2
Improper burner cross−overs
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Corrective Action/Comments
ACTION 1 − Check that the manifold pressure
matches value listed on nameplate. See installa-
tion instructions for proper procedure.
ACTION 2 − Verify that the installed orifice size
match the size listed on the nameplate or instal-
lation instructions.
ACTION 3 − Check gas valve sensing hose to in-
sure no leaks are present.
ACTION 4 − Check the input rate to verify rate
matches value listed on nameplate.
ACTION 1 − Tighten orifice until leak is sealed.
NOTE: Be careful not to strip orifice threads.
ACTION 2 − Check for gas leakage at the
threaded orifice connection. Use approved
method for leak detection (see unit instructions).
ACTION 1 − Check for sooting deposits or other
restrictions in the heat exchanger assembly.
Clean assembly as outlined in instruction manu-
al.
ACTION 2 − Check for proper combustion and
flow. CO2 should measure between 6.0% and
8.0% for natural and 6.5% and 8.5% for L.P. CO
should measure below .04% (400PPM) in an air−
free sample of flue gases.
ACTION 1 − Check that the burners are firing into
the center of the heat exchanger openings. Cor-
rect the location of the burners if necessary.
ACTION 1 − Check for proper flow of exhaust
gases away from intake vent. Remove any ob-
stacles in front of the intake and exhaust vent
which would cause recirculation.
ACTION 2 − Check for correct intake and ex-
haust vent installation. See instructions
ACTION 1 − Remove burner and inspect the
cross−overs for burrs, or any restriction or if
crossover is warped. Remove restriction or re-
place burners.