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LAARS Heating Systems
it is flashing three times, for ignition control lockout.
If it is, then that is a combustion lockout.
a.
Poor Combustion: Poor combustion should be
suspected if there is a strong flue gas odor. The
odor may result from either an improper gas/air
ration (high or low CO
2
) or contamination of the
combustion air supply. If an improper gas/air
ratio is suspected refer to “gas valve calibration
(offset adjustment)” in Section 10. If
contamination of the air source is suspected,
check the vent system for leaks back into the
combustion air.
b.
Poor flame sensor signal: The boiler control
board will lockout if the flame sensor is
grounded or if the flame signal it receives is less
than 1
P
A dc. To check for a grounded sensor,
remove the flame sensor wire (yellow) from the
boiler control board and check for continuity
between the yellow wire terminal and the
combustion chamber bottom pan. If there is
continuity, the flame sensor is grounded. An
alternate way to check for grounding of the
flame sensor is to check the ohms resistance
between the yellow wire terminal and the
combustion chamber bottom pan, using a meter
set for at least 100,000 ohms. The resistance
must be more than 250,000 ohms (some
technicians have been confused by meters which
report infinite resistance with what appears to
them as double zeros; an infinite resistance
reading means there is no grounding of the flame
sensor.) If there is continuity, or the ohms
resistance is less than 250,000 ohms, debris or
moisture in the combustion chamber may be the
cause. To correct, carefully remove the ignitor
and flame sensor assembly. Look for debris on
the assembly where the flame rod exits the
ceramic tube. If debris exists, remove it. If this
area is clean, insert a pencil or other round object
into the flame sensor hole in the bottom
combustion chamber pan and roll it around to
clear any debris that may exist inside the
combustion chamber. Replace the ignitor/flame
sensor assembly, reconnect the yellow wire and
start the boiler. If lockout persists, check the
flame signal. The minimum flame current
necessary to keep the system from lockout is 1
microamp (
P
A).
There is no need to disconnect
anything to measure the flame current signal
.
To measure the flame signal, connect an analog
DC microammeter to the FC terminals on the
ignition control, per Figure 37. The meter should
read 1
P
A or higher. (If the meter reads below
“0” on the scale, the meter leads are reversed.
Disconnect and reconnect the meter for correct
polarity.) If the meter reading is less than 1
P
A,
see the “poor combustion” section. (Check for air
leaks at inlet to blower.)
c.
Inadequate gas supply: Before proceeding,
ensure that the gas supply has not been shutoff or
the LP tank (LP boilers) is not empty. Then, reset
the boiler and observe the operational cycle.
Approximately 20 seconds after the start of the
cycle, the GV LED on the boiler control should
light. If it does not, reset the boiler and check for
24 VAC at the boiler control. If the voltage is
less than 20 VAC, and transformer voltage is 24
VAC replace the boiler control. If the voltage is
correct, connect a pressure gauge to the offset
pressure port on the gas valve. Reset the boiler
and observe the offset pressure at startup and
after 20 seconds. The pressure will be ~ -2 in.
w.c. at startup (during prepurge) and should
become less negative (-0.1 in. w.c.) when the gas
valve opens and gas flow starts. If the pressure
remains at ~ -2 in. w.c. throughout the cycle
replace the gas valve.
d.
Ignitor failure: If the boiler goes through a
normal start cycle but combustion does not
occur, ignitor failure should be suspected.
Check the ignitor by unplugging the ignitor plug
and measuring the ignitor resistance. It should be
50-100 (
:
) ohms. If the resistance is not 50-100
ohms (
:
), replace the ignitor. If the resistance is
correct, reset the boiler and check for 120 VAC
at the ignitor plug during the start cycle. If there
is no voltage, replace the boiler control or faulty
ignitor wires.
e.
Blocked or contaminated air source: Before
proceeding, ensure that the vent terminal (Direct
Vent installations) or the air inlet (Category IV
installations) is not obstructed. In general, only
direct vent installations have the potential for
cross contamination (flue products entering the
combustion air stream) of the combustion air
source. This may be caused by leaky joints in the
3" flue pipe located inside the 5" combustion air
pipe. If this is suspected, remove the front panel
from the appliance, reset the boiler and allow it
Figure 37. Flame Sensor Current Check.
