S8XB-SVX001-1A-EN
75
Sequence of Operation S8B1/S8X1/S8X2
N
No
otte
e:: The seven-segment LED readout is based on
thermostat input. For the S8X2, during
simultaneous call for W1 and W2, the seven-
segment LED will read “
Xt2
”, although the IFC
will process the call for 1
st
stage heat first.
E
EA
AC
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nd
d H
HU
UM
M T
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•
EAC relay closes approximately 2 seconds after the
blower starts.
•
EAC relay opens when the blower motor stops.
•
HUM relay closes on any heating call (HP/Gas)
approximately 1 second after the blower motor
starts.
•
HUM relay opens when any heating call (HP/Gas) is
removed.
1
1
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1. R – W1 contacts close on the thermostat sending
24VAC to the W1 low voltage terminal of the IFC.
Technician should read 24VAC from W1 to B/C. The
seven segment LED will read: .
•
Xt
= S8B1/S8X1
•
Xt1
= S8X2
2. The IFC performs a self-check routine and then
confirms:
a. Flame roll-out switches (FRS) 1 & 2, main
thermal limit (TCO), and any reverse air flow
(RAF) switches are closed by sending 24VAC out
the HLO terminal and monitoring the HLI input.
b. Pressure switch 1 (PS1) and pressure switch 2
(PS2–S8X2 only) are opened by sending 24VAC
out the HLO terminal, through the limit
switches, and monitoring the PS1 (3) and PS2
(7) inputs.
N
No
otte
e:: If a thermal limit is open, 24VAC will not
be present at either pressure switch.
3. After steps a & b are confirmed, the inducer relay is
closed, energizing the inducer motor.
4. As the inducer ramps up, PS1 will close.
N
No
otte
e:: S8X2 units will start the inducer motor on high
speed for approximately 6 seconds, then switch
to low speed. If PS1 does not close within 60
seconds, the control will report a
e3
.
1
error and
increase the inducer to high speed in an attempt
to close PS1.
5. When PS1 closes, the igniter relay on the IFC will
close. The igniter is energized. The igniter warm up
is approximately 17 seconds.
6. After the igniter warm up, the 1
st
stage gas valve
relay is closed, which energizes the 1
st
stage gas
valve solenoid to allow ignition.
7. The first burner will ignite, and flame will crossover
to the remaining burners establishing current to the
flame sensor. Flame sensing must be established
within 4 seconds.
N
No
otte
e:: There are two flame sense pads located on the
IFC marked as “FP”. To measure the flame
current, use a VOM set to DC volts. 1VDC = 1
micro-amp. Flame current will vary depending
on the type of meter used. Typical flame current
ranges from 0.75 – 3.0 micro-amps (0.75 – 3 VDC).
8. Once the flame sense has been established, a timer
on the IFC starts and the indoor blower will energize
at 1
st
stage speed after the blower “Heat On Delay”
has completed. The seven-segment LED for
example will alternately read:
•
Xt
= Gas heating = S8B1/S8X1
•
Xt1
= Gas heating, stage 1 = S8X2
•
tP3
= Tap 3
9. When the temperature raises enough to satisfy the
thermostat setting, contacts R-W1 will open.
10. The gas valve relay will open, closing the gas valve.
The inducer will continue to run for approximately 5
seconds to remove any combustion byproducts
from inside the furnace.
11. The indoor blower continues to run to remove heat
from the heat exchangers. The “blower off” time is
field adjustable through the IFC menu setup option.
The seven-segment LED will read “
IDL
” = Idle, no
thermostat demand.
2
2
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S8
8X
X2
2 O
On
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1. See sequence of operation for 1
st
stage gas heating
operation above (steps 1–7)
N
No
otte
e:: 2
nd
stage heating cannot operate without 1
st
stage operation.
2. R-W2 contacts close on the thermostat sending
24VAC from to the W2 low voltage terminal of the
IFC. Technician should read 24VAC from W2 to B/C.
The seven-segment LED will read
Ht2
.
3. The IFC checks to ensure that PS2 is open, and then
energizes the 2
nd
stage inducer relay. The inducer
is energized on high speed, and the second stage
gas valve relay on the IFC closes, energizing second
stage gas valve. The indoor blower motor will ramp
up to the 2
nd
stage gas heating speed. The seven-
segment LED for example will alternately read:
•
Xt2
= Gas heating = stage 2
•
tP5
= Tap 5
4. The IFC monitors PS2 for closure and if PS2 does
not close within 45 seconds, a PS2 open error will
be declared and the furnace will operate in 1
st
stage. If PS2 closes, 2
nd
stage gas heating will
continue until the thermostat R-W2 contacts open.
N
No
otte
e:: If PS2 does not close within the 45 seconds, the
IFC will wait 10 minutes and repeat steps 3 & 4. If
on the third attempt during the same heating call,
PS2 does not close within the 45 second proving
time, the unit will run in 1
st
stage until the
thermostat R-W2 contacts open.
