Suburban SF-20F Service Manual

31
Start
Thermostat Calls
for Heat
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
then resume normal operation.
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
Cycle
The module board will then verify that the sail switch circuit is closed and motor is up
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
Cycle and Flame
Sense
The module board will energize the gas valve and enable the high voltage spark output
to the electrode for 7 seconds of ignition time.
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.
2
nd
and 3
rd
Ignition
Cycles if Required
3 Try Ignition Board
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
Down
When the thermostat has reached its set point and the demand for heat ends, the gas
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 24 VAC Fan Control Module Board
Part Number 520947
Time Line
Description - Sequence of Events
Start
Thermostat Calls
for Heat
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 must have a minimum 18 volts for normal operation.
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.

Suburban SF-20F Specifications

General

Brand	Suburban
Model	SF-20F
Category	Furnace
Language	English