Lennox G27M Series - Page 27

Page 27
PROBLEM 6: BURNERS LIGHT WITH HEATING DEMAND BUT UNIT SHUTS DOWN
PREMATURELY (CONT.)
Condition Possible Cause Corrective Action/Comments
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
failure.
6.3.1
Unit is firing above 100% of the
nameplate input.
ACTION 1 – Check that the manifold pressure
matches value listed on nameplate. See
installation instructions for proper procedure.
ACTION 2 – Verify that the installed orifice size
match the size listed on the nameplate or
installation instructions.
ACTION 3 – Check gas valve sensing hose to
insure no leaks are present.
ACTION 4 – Check the input rate to verify rate
matches value listed on nameplate.
LED#1–On
LED#2–Slow Flash
6.3.2
Gas orifices leak at the manifold
connection.
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 instruc-
tions).
6.3.3
Air leakage at the connections be-
tween the primary heat exchanger,
secondary heat exchanger, and
combustion air blower.
ACTION 1 – Check for air leakage at all joints
in the heat exchanger assembly. Condition
will cause high CO2 with high CO.
ACTION 2 – Seal leakage if possible, replace
heat exchanger if necessary, tag and return
heat exchanger to proper Lennox personnel.
6.3.4
Insufficient flow through the heat
exchanger caused by a sooted or
restricted heat exchanger.
ACTION 1 – Check for sooting deposits or oth-
er restrictions in the heat exchanger assem-
bly. Clean assembly as outlined in instruction
manual.
ACTION 2 – For G27M gas furnace (NG only)
combustion should measure between 7.1%
and 8.3% (high fire). CO should measure be-
low .040% (400PPM).
6.3.5
Burners are not properly located in
the burner box.
ACTION 1 – Check that the burners are firing
into the center of the heat exchanger open-
ings. Correct the location of the burners if nec-
essary.
6.4
– Combustion air blower energizes
with a heating demand.
– Burners light roughly and the
unit fails to stay lit.
– Diagnostic lights flash watch-
guard flame failure.
6.4.1
Recirculation of flue gases. This
condition causes rough ignitions
and operation. Problem is charac-
terized by nuisance flame failures.
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
g
LED#1–Alternating Slow Flash
LED#2–Alternating Slow Flash
6.4.2
Improper burner cross–overs
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.