I Unit Components - Lennox 80MGF Series Manual

I−UNIT COMPONENTS (Figures 1, 2, 3)
80MGF unit components are shown in figure 1. The blower
controls, gas valve and burners can be accessed by re-
moving the front access panel. A separate blower access
door is located behind the front access panel. Electrical
control components are mounted to the blower access
door.
80MGF units are factory equipped with bottom return air pan-
els. The panels are designed to be field removed as required
for bottom air return. Indentations on side of units, show
where side air return opening should be cut during installation.
A−Blower Door Components (Figure 4)
Electrical burner control and blower control components
are located on the outside surface of the blower access
door. Jackplugs allow the blower door to be easily re-
moved for blower service.
Located on the blower door are the unit transformer (T1),
the furnace control (A3) and door interlock switch (S51).
Furnace control (A3) combines the function of a burner
ignition control and blower control.
1− Control Transformer (T1)
A transformer located on the blower door provides power
to the low voltage section of the unit. Transformers on all
models are rated 30VA with a 120V primary and a 24V sec-
ondary.
2−Door Interlock Switch (S51)
A door interlock switch rated 16A at 125VAC is located on
the blower access door. The switch is wired in series with
line voltage. When the blower door is removed the unit will
shut down.
3− Ram Control (A3)
The furnace control combines burner ignition functions
with blower control functions.
Some early model 80MGF units utilize a furnace control
manufactured by RAM Electronics Corporation. The RAM"
board is a printed circuit board which controls the blower, gas
valve, combustion air blower and ignition spark. It also moni-
tors the flame, limit and gas valve operation. The control has
a non−adjustable, factory preset on" fan timing (45 sec-
onds). Fan off" timings are adjustable. The board utilizes
both 120 and 24VAC. See figure 5. The board is also
equipped with a diagnostic LED for use when troubleshoot-
ing the unit.
When the furnace is idle (blower off and no heating or cool-
ing demand), the diagnostic LED flashes at a slow steady
rate. On a call for heat, the diagnostic LED begins flashing
at a fast rate and the combustion air blower is energized.
The LED flashes different codes to indicate problem con-
ditions. The diagnostic LED lights red (not flashing) to indi-
cate control board failure. Table 3 shows how to interpret the
other LED modes.
Pre-Purge
On a call for heat, the combustion air blower begins oper-
ating. If the combustion air prove switch closes, the com-
bustion air blower continues to operate for 45 seconds
(pre-purge) before allowing ignition. Pre-purge allows the
heat exchanger to be cleared of combustion products and
to introduce fresh air for combustion. If the combustion air
prove switch does not close, the combustion air blower
continues to run indefinitely (until the prove switch closes).
Post-Purge
After a demand, the combustion air blower continues to
operate for 5 seconds (post-purge) before stopping. Post-
purge allows the heat exchanger to be cleared of combus-
tion products.
Ignition Control
The ignition control is a direct spark ignition control module
integral to the furnace control. See figure 5. When there is a
call for heat, the control delays ignition until combustion air
blower operation has been proved and pre-purge period
has elapsed. It then opens the gas valve and generates a
spark to ignite the burners. Trial for ignition lasts for 7 sec-
onds. At the same time, the control begins monitoring the
flame sensor. If the flame current is too weak (less than 1
microamp) or if the burners do not ignite (within the 7 sec-
ond ignition trial), the control will shut off spark and the com-
bustion air blower and de-energize the gas valve. Flame
current should be between 1 and 5 microamps to keep the
gas valve open. See figure 23.
The control will attempt to ignite the burners up to two more
times. Each time the control restarts the ignition sequence,
it begins with a 45 second pre-purge. If flame is not sensed
after the third trial, the control locks out. Lockout means that
the control shuts off the gas valve, spark and combustion air
blower for 60 minutes. At the end of 60 minutes the control
completely resets and will attempt ignition up to three times.
The control can be manually reset before the end of 60 min-
utes by momentarily turning off power to the unit.
Shock hazard.
Spark related components contain high voltage.
Disconnect power before servicing. Control is
not field repairable. If control is inoperable, sim-
ply replace entire control.
Can cause injury or death. Unsafe operation will
result if repair is attempted.
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