Suburban DD-17DSI Service Manual page 26

Sequence of Operations for SHD-2542
1st Stage Call for Heat
The wall therm ostat controls the operation of the dual stage furnace by reacting to room tem perature. This allows current to
flow through the On/Off switch to the Blower Speed Control Board then to the Module Board.
The m odule board constantly checks for a m inim um 9.5 volts. If there is not 9.5 volts, the m odule board will go into a standby
m ode until adequate power is supplied. It will then resum e norm al operation.
Upon a call from the therm ostat the first stage will be activated. The m odule board therm ostat circuit will go active and the sail
switch is verified as being open. The blower output is energized. The blower m otor starts.
15 Second Purge Cycle
The m odule board will then verify that the sail switch circuit is closed and m otor is up to speed. If this circuit rem ains open for
30 seconds after the blower m otor starts, the m odule board will go into lockout and shut down the blower m otor.
The m odule board checks that the gas valve relay contacts (located on the m odule board) are open before the igniting
sequence starts.
The board has a pre-purge tim ing circuit of approxim ately 15 seconds. This allows the com bustion cham ber to purge.
7 Second Ignition Cycle and Flam e Sense
The m odule board will energize the gas valve and enable the high voltage spark output to the electrode for a seven second
ignition period.
The m odule board will then check for flam e sense to verify successful lighting of the m ain burner flam e. Sparking of the igniter
is then term inated. The gas valve and blower outputs will rem ain energized.
If ignition is successful the m odule board will m onitor the flam e sense, sail switch, and the lim it switch circuits. The flam e is
sensed through the spark wire and electrode therefore, it is essential that the electrode is properly positioned in the burner
flam e.
3 try Ignition Board
If the flam e is NOT sensed after seven (7) seconds, or if the unit fails to ignite, a second fifteen (15) purge cycle will begin
followed by second trial for ignition sequence. After three (3) trial for ignition attem pts with no ignition of the m ain burner, the
m odule board will de-energize the gas valve im m ediately and the blower will run for three (3) m inutes and then go into lockout
m ode.
nd
2 Stage Heating
If during the therm ostat cycle the dem and for heat drops below the second stage therm ostat setting ( 3 to 5 degrees variance
depending on therm ostat m anufacturer), the blower speed control board will be energized. The blower speed control board
will energize the additional gas valve solenoid and increase m otor speed to the high position. Furnace will rem ain in the second
stage operation until therm ostat is satisfied.
90 Second Shut Down
W hen the therm ostat has reached its set point and the dem and for heat ends, the gas valve will be de-energized and the flam e
will go out. The post com bustion purge period of 90 seconds begins. W hen it tim es out, the blower m otor output is rem oved,
and the blower stops.
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