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Marvair GAA A/C w/Gas Heat I&O Manual
06/2023 Rev.5
1.5 Sequence of Operation - Heating Mode
1. On a call for heat, two stage thermostat contact( s ) close, providing 24 VAC to “4” terminal on
the ignition control.
2. Draft inducer is energized at high speed ( B-W leads ) at line voltage.
3. Air Pressure Switch ( APS ) closes, initiating 30 second prepurge.
4. At end of 30 second prepurge period, Spark & Gas Valve are energized for up to 5 second
ignition trial. Gas valve will open in either low or high fire position, depending upon the heat
requirements and contact closure in the thermostat.
5. Burners ignite and carryover.
6. Flame is detected by flame sensor and control operates in steady state heating position.
7. The two stage thermostat powers the low fire contact continuously on a call for heat and will
close the high fire contact if necessary to meet heat requirements. Unit continues in operation
in either low or high fire until the thermostat set point is reached.
8. Low fire contact in thermostat opens, interrupting power to “4” terminal on control and closing
the gas valve.
9. The Draft inducer continues to run for a 60 second post purge period.
10. The control system is capable of three retrials if the above sequence is interrupted at any point.
If the above sequence does not occur then:
11. If ignition is not achieved with in 5 seconds, the gas valve is shut off, the inducer keeps running for
an inter purge period of 60 seconds and additional trials follow the specified sequence. If all 3 trials
for ignition have occurred without proper ignition and flame detection, the control is locked out.
12. Control may be brought out of lock out by a. turning the thermostat to OFF for a minimum of
5 seconds and then ON, or b. turning the disconnect to OFF for a minimum of 5 seconds and
then ON. Control will automatically also attempt another ignition sequence one hour after a
lockout occurrence.
13. If flame is lost once it has been established, the control will shut off the gas supply within .8
seconds and enter the interpurge period. Control will initiate up to 3 additional trials per normal
sequence of operation. ( To restart, refer to item 12 ).
14. If flame sensor indicates presence of a flame during the purge period when no flame is present,
the inducer will remain energized, but the gas valve will not be energized until the cause of the
false flame is removed.
15. If the air pressure switch ( APS ) is closed when the inducer is energized or does not close after
the inducer is energized, the control will wait one minute for the air switch to open or close and
then lock out. ( To restart, refer to item 12 ).
16. If the control detects power to the gas valve when the it should be off or no power when it
should be on, the control will go into lockout with all outputs off. ( To restart, refer to item 12 ).
