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OPERATION SEQUENCE
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Line voltage is sent to the transformer
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Terminal branches off line voltage to ignition control terminal L1
Transformer reduces line voltage to 24 VAC
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24 VAC is sent to thermostat
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When the thermostat calls for heat, Thermostat closes and returns 24V to terminal W1 on ignition control
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Red light on ignition control is illuminated
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Air proving circuit is tested
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Ignition control module begins ignition trial sequence
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Ignition control sends 120V from terminal IND to motor.
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Motor starts
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Air proving switch closes and 24V are returned to terminal TH of ignition control
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The ignition module, after a pre-purge period of approximately 30 seconds, energizes the igniter.
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Additionally, the gas valve is energized for this ignition trial period of 15 seconds.
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If a flame is detected, the gas valve remains open, and sparking stops.
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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
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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,
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the burner and fan shut off with no post purge.
Series AA250
Page 23
LT252 Oct 2019
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