Daikin Skyline OAH003GDAC Installation And Maintenance Manual page 49

5:1 Modulating Furnace
Sequence
When system is powered, 24 VAC will be applied to the ignition
control (IC) across terminals 24VAC/GND, to the SC40 (Blue
LED lit) and to the EXA Valve. The ignition control will reset,
perform a self-check routine, initiate full time flame sensing,
flash the diagnostic LED for up to four seconds and enter the
thermostat scan standby state.
Call for Heat
1. Thermostat (or heat enable) closes (T1/T2) on call for
heat.
2. 24 VAC is supplied to IC terminal T'STAT, provided the
high limit switch is in closed position.
3. The control will check that pressure switch contacts are
open (IC terminal APS (PSW) is not powered).
4. The Induced Draft Fan (ID Fan) is then energized at high
speed. SC40 Green (AFS) LED is lit.
5. Air Switch (APS-1 Low) closes initiating a 15 second pre-
purge period.
6. At end of pre-purge period, the spark commences and
the combination gas valve is energized for a trial for
ignition period.
7. The SC40 will output 5 VDC to the EXA modulating
control valve regardless of the analog input signal to
SC40 terminals T10 & T11.
8. Burners ignite and cross light, operating at the adjusted
mid-fire input rate
9. When flame is detected by flame sensor, the spark is
shut-off immediately and gas valve and ID Fan remain
energized.
10. SC40 is powered (terminal T4) simultaneously (Yellow
(SU) LED lit) and begins timing a 30 second warm-up
period while maintaining the ID Fan at high speed (Green
AFS LED remains lit).
11. When the initial timer in SC40 times out, control will
respond to analog input at terminals T10 and T11. Green
(Mod) LED is lit.
12. If the controller is providing an analog signal between 0.5
and 5.0 VDC to the SC40 control, relay R1 is energized.
RED (R1) LED is lit.
13. The ID Fan will run at low speed and the EXA modulating
valve will be powered proportional to the analog input
voltage signal and will open or close modulating the gas
input between low and mid-fire.
14. If the signal is 5.1 VDC or higher, the SC40 relay R1 is
de-energized and ID Fan goes to high speed.
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15. Provided the High Air Switch (APS-2) contacts are
closed, with analog inputs between 5.1 and 10.0 VDC
the EXA Valve will modulate the gas input between mid-
fire and high fire.
16. During heating operation, the thermostat, pressure
switch and main burner flame are constantly monitored
by the IC to assure proper system operation.
17. Operation continues in the High fire mode until the
controller input signal to the SC40 control drops to 4.8
VDC. At this point the SC40 R1 relay circuit is energized
and switches the ID Fan to low speed operation. Low-fire
modulation will continue for analog input voltages from
4.7 VDC.
18. When the thermostat (temperature controller) is satisfied
and the demand for heat ends, the heat enable contact
opens, power is removed from the T'stat terminal on
IC and the combination gas valve is de-energized
immediately and a 30 second post-purge occurs (at high
speed) before de-energizing the ID Fan.
Ignition and Operational failures during a call for heat result in
"lockout" of the ignition control
1. If flame is lost during an operational cycle, the control will
respond within 0.8 seconds. The spark will be energized
for a trial for ignition period to attempt to relight burners
and prove flame sensor. If flame is re-established,
normal operation resumes
2. If the burners fail to light or carryover during a trial for
ignition, the control will attempt two additional ignition
trials. If no flame is present at the flame sensor within 10
seconds, the spark and gas valve will be de-energized. A
15 second inter-purge period begins and the combustion
blower continues to run. After the inter-purge period
another ignition trial will take place.
3. If burner fails to light or prove the flame sensor following
the two additional trials the control will go into lockout.
The valve relay in the IC will be de-energized shutting
of the gas valve immediately and the induced draft fan
following a 30 second post-purge.
49 IM 777-18 • SKYLINE OUTDOOR AIR HANDLER
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