Sequence Of Operation (Fan Control Module Board (520820) Equipped Furnaces) - Suburban DD-17DSI Service Manual

Time Line
Start The wall thermostat controls the operation of the furnace by reacting to room
temperature, this allows current to flow through the On/Off switch to the module board.
The module board constantly checks for a minimum 9.5 volts. If there is not 9.5 volts,
the module board will go into a stand by mode until adequate power is supplied. It will
Thermostat Calls
then resume normal operation.
for Heat
Upon a call from the thermostat, the module board thermostat circuit will go active. The
sail switch circuit is verified as being open. The blower output is energized. Blower
motor starts.
15 Seconds Purge The module board will then verify that the sail switch circuit is closed and motor is up
Cycle to speed. If this circuit remains open for 30 seconds after the blower motor starts, the
module board will go into lock out and shut down the blower motor.
The module board checks that the gas valve relay contacts (which are located on the
module board) are open before the ignition sequence starts.
The board has a pre-purge timing circuit of (approximately 15 seconds). This allows the
chamber to purge.
7 Seconds Ignition The module board will energize the gas valve and enable the high voltage spark output
Cycle and Flame to the electrode for 7 seconds of ignition time.
Sense
The module board will then check for flame sense to verify successful lighting of the
main burner flame. Sparking will then be terminated and the gas valve and blower
outputs will remain energized.
If ignition is successful the module board will monitor the flame sense, sail switch and
limit switch circuits, and the thermostat inputs during the heating period.
The flame is sensed through the spark wire and electrode. Therefore, it is essential that
the electrode is properly positioned in the burner flame.
nd rd
2 and 3 Ignition 3 Try Ignition Board
Cycles if Required
If the flame is not sensed after seven (7) seconds, a second 15 second purge cycle will
begin followed by a second Trial-For-Ignition sequence. After three (3) Trial-For-Ignition
attempts with no ignition of the main burner, the module board will de-energize the gas
valve immediately and blower will run for 3 minutes and then shutdown in lockout.
Heating Cycle If during the heating cycle, the limit switch circuit opens and remains open for 5
minutes, the module board will go into lock out and shut down the blower motor. If this
occurs, the thermostat will need to be reset for the furnace to operate.
90 Second Shut
When the thermostat has reached its set point and the demand for heat ends, the gas
Down
valve will be de-energized and the flame will go out. The post purge period of 90
seconds begins. When it times out, the blower motor output is removed, and the blower
stops.
Sequence of Operation for Fan Control Module Board
Part Number 520820
Description - Sequence of Events
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