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S9240F2051 MODULATING INTEGRATED FURNACE CONTROL
Light Off
Refer to Fig. 9 on page 18 (Fig. 12 on page 21 for single stage
thermostat), for the operating sequence during light off. Light
off occurs in the following manner for each call for heat and
any other situation requiring light off.
The following describes the operation of the IFC during light
off:
— The IFC conducts a safe start check that consists of
an internal hardware/software self-check and
verification there is no main burner flame present.
— The IFC verifies the limit and rollout switch circuits are
closed.
— The IFC verifies the low and high pressure switch
circuits are open.
— The IFC energizes the inducer and adjusts to the
ignition firing rate speed.
— When the low pressure switch closes, Prepurge
begins.
— After the completion of Prepurge, the HSI energizes
for the Igniter Warm-up Period.
— When the Igniter Warm-up Period expires, the gas
valve energizes, the Inducer remains at the ignition
firing rate, gas flow begins, and the Trial for Ignition
(TFI) begins.
• During TFI, the HSI remains energized.
• After TFI, the HSI de-energizes, the gas valve
remains energized, and the Inducer remains at the
ignition firing rate for the flame recognition period.
— The heat fan on delay is started upon entering Trial for
Ignition.
— When flame is sensed during Trial for Ignition, the IFC
de-energizes the HSI.
— When the ignition stabilization delay expires, the IFC
moves to the ignition firing rate.
— After successful lightoff, the IFC moves to the target
firing rate (see below) based on the current thermostat
inputs and the selected operating sequence.
Firing Rate
There are three target firing rates:
1. Low Fire Rate, which is about 40% of appliance full rate.
2. Mid Fire Rate, which is about 70% of appliance full rate.
3. High Fire Rate, which is about 100% of appliance full
fire rate.
Normal Heat Mode - Conventional 2-
Stage Thermostat (standard 2-stage
operation selected)
Refer to figures 9-14 beginning on page 18 for the operating
sequence diagrams.
Call for Stage 1 Heat
A call for 1st stage heat is signaled by the thermostat closing
the contacts between R and W1.
Upon a call for 1st stage heat:
— After a successful Light Off and expiration of the
ignition stabilization period, the Inducer adjusts to the
mid firing rate speed.
69-2807—01
— After expiration of the heat fan on delay, the IFC
adjusts the circulator to the appropriate speed (CFM).
— If a discharge air sensor is present, the circulator
adjusts to maintain the target discharge air
temperature.
— The IFC remains in this state until the thermostat
generates a 2nd stage call for heat or the 1st stage
call for heat is removed.
Call for Stage 2 Heat
A call for 2nd stage heat is signaled by the thermostat
simultaneously closing the contacts R, W1, and W2.
On the first call for 2nd stage heat, the request is ignored until
the 2nd stage recognition delay expires.
When a 2nd stage recognition delay expires (and on
subsequent calls for 2nd stage heat in the same heating
cycle), the following occurs:
— The Inducer adjusts to the high firing rate speed.
— The IFC adjusts the circulator speed to the appropriate
speed (CFM).
— If a discharge air sensor is present, the circulator
adjusts to maintain the target discharge air
temperature.
— The IFC remains in this state until the 2nd stage call
for heat or the 1st stage call for heat is removed.
2nd Stage Heat Satisfied, Call for 1st Stage
Heat Remains
When the 2nd stage heat request is removed and there is still
a demand for heat on the 1st stage, the following occurs:
— The Inducer adjusts to the mid firing rate speed.
— The IFC adjusts the circulator speed to the appropriate
speed (CFM).
— If a discharge air sensor is present, the circulator
adjusts to maintain the target discharge air
temperature.
— The IFC remains in this state until the thermostat
generates a 2nd stage call for heat or the 1st stage
call for heat is removed.
Call for 1st Stage Heat Satisfied
When the 1st stage is satisfied:
— The gas valve de-energizes.
— When the loss of flame is detected, the Postpurge and
circulator fan off delay timings begin.
— When the Postpurge time completes, the inducer
de-energizes.
— When the circulator fan off delay expires, the circulator
de-energizes.
1st and 2nd Stages Satisfied
Simultaneously
When both 1st and 2nd stages are satisfied simultaneously:
— The gas valve de-energizes.
— When the loss of flame is detected, the Postpurge and
circulator fan off delay timings begin.
— When the Postpurge time completes, the inducer
de-energizes.
— When the circulator fan off delay expires, the circulator
de-energizes.
12
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