MGD Gas Furnace
Service Manual
If the high temperature limit or auxiliary limit opens while the furnace is in heating mode and the
thermostat is calling for heat (W), and the switch remains open for a continuous duration of 5
minutes the control enters a fan failure lockout mode. While in this mode the control does not
allow the next ignition cycle to start until 20 minute4s after the limit recluses. Once the ignition
cycle is started the control only allows the furnace to burn for a 30 second duration, at which point
the main valve is shut off and the control starts another 20 minute ignition lockout and 30 second
burn cycle. This pattern repeats indefinitely or until the thermostat call for heat is removed, or
power to the control is removed. During this mode all other unsafe conditions (such as flame
sense, main valve energized, limit open) are still monitored and are not superseded except 5 limit
cycles does not result in a 1-hour lockout. However, any other failures do restart the lockout
interval.
The proper polarity of the AC line is tested upon power up. If the AC “hot” line is connected to L2
then a “rapid flash code” will be indicated on the LED. Normal operation will still occur; however, it
is highly improbable that flame sense will be detected. This condition will persist until power is
cycled to the board.
RECYCLES
A recycle occurs when ignition has occurred and flame failure, flame circuit failure or a main valve
failure during a call for heat is detected. If a recycle occurs before the heat fan delay ON has
completed, the delay ON is bypassed and the heat fan comes ON immediately and performs the
selected delay OFF. Up to five recycles may occur in a given operation. Following the fifth recycle
the furnace enters a one-hour lockout.
SOFTWARE TIMING TABLE
Pre-purge 30
sec.
Inter-purge 30
sec.
Igniter Warm-up Timer (1st. Pass)
30 sec.
Igniter Warm-up Timer (Retry)
30 sec.
Ignition Activation Period
5 sec.
Flame Establishing Period
2 sec.
Flame Stabilization Period
10 sec.
Flame Failure Timer (During Flame Stab.)
2 sec.
Flame Failure Response Time
0.75 sec.
Heat Fan Delay ON
selectable 60, 75, 90 sec.
Heat Fan Delay OFF
selectable 120, 150, 180 sec.
Cool Fan Delay OFF
18 sec.
Post-Purge 15
sec.
S/W Watchdog Timer
1.5 sec.
Ignition Lockout Timer
1 hr.
NPC Failure Timer 1 (Stuck Closed)
30 sec.
NPC Failure Timer 2 (Stuck Open)
30 sec.
NOTE: All timings given are nominal values.
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