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SEQuEnCE OF OpEraTiOn
FOr FurnaCES EQuippED WiTh TiME DElaY
TiME linE
DESCripTiOn
Start Time
Thermostat Calls for Heat
The wall thermostat controls the operation of the furnace by reacting to room temperature upon
a call from the thermostat, this allows current to flow through the ON/OFF switch then to the
relay.
The relay receives the power and allows power to pass through to the switch within the relay.
This is done by a heater coil within the relay which actuates a bi-metal disc closing the relay
circuit.
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 Second Purge Cycle
The limit switch protects the furnace from over heating conditions. The contacts in the limit
switch open at a given temperature setting, shutting off power to the ignition system which
controls the gas valve.
As the room air wheel comes up to speed, air flow closes the sail switch completing the
circuit. The sail switch is placed into the system as a safety to prove there is adequate air for
combustion.
7 Seconds
Ignition Cycle and
Flame Sense
The next operation is controlled by the Direct Spark ignition (DSi) system as power is applied
to the DSi Board. The system will do the following:
1. The board has a timing circuit which allows the blower to purge the chamber of any products
of combustion or gas.
2. The board will then apply power to the gas valve. At the same time it produces a high
voltage power supply to the electrode producing spark at the burner.
3. The board will also confirm the presence of a flame. if the flame is not sensed after seven
(7) seconds, the module will try two (2) more times and then go into lock out mode. The
flame is sensed through the spark at the burner.
2 & 3 Ignition Cycles
If Needed
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 motor will continue to operate.
Heating Cycle
if during the heating cycle, the limit switch circuit opens and remains open, the gas valve will
close and the blower motor will continue to run.
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.
NOTE: On some models, Sail Switch is before limit switch.
