Series AA250
Page 23
LT252 Oct 2019
OPERATION SEQUENCE
•
Line voltage is sent to the transformer
•
Terminal branches off line voltage to ignition control terminal L1
•
Transformer reduces line voltage to 24 VAC
•
24 VAC is sent to thermostat
•
When the thermostat calls for heat, Thermostat closes and returns 24V to terminal W1 on ignition control
•
Red light on ignition control is illuminated
•
Air proving circuit is tested
•
Ignition control module begins ignition trial sequence
•
Ignition control sends 120V from terminal IND to motor.
•
Motor starts
•
Air proving switch closes and 24V are returned to terminal TH of ignition control
•
The ignition module, after a pre-purge period of approximately 30 seconds, energizes the igniter.
Additionally, the gas valve is energized for this ignition trial period of 15 seconds.
o
If a flame is detected, the gas valve remains open, and sparking stops.
o
If no flame is detected, the gas valve closes and a 30 sec inter-purge period begins. After the inter-
purge, the module repeats the trial for ignition period. If flame is still not established, a third and
final inter-purge followed by a final ignition trial cycle begins. After three trials, the module will
lockout for a period of approximately 1 hour or until reset.
(Reset is accomplished by removing
power from the module for at least 5 seconds.)
After this 1-hour period, the module re-attempts the
full ignition sequence.
•
When using a 24V thermostat and the heat requirement has been met and the thermostat opens, the
burner shuts off but the fan continues to run for approximately 30 seconds. This is referred to as a post
purge. This allows the products of combustion to be removed from the heat and increase heat chamber life.
•
When using a line voltage thermostat and the heat requirement has been met and the thermostat opens,
the burner and fan shut off with no post purge.
