SYSTEM OPERATION
22
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Integrated control module performs safety circuit
checks.
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Induced draft blower is energized on high speed for a
15-second prepurge.
•
Induced draft blower steps to low speed following
prepurge. Low stage pressure switch contacts are
closed.
•
Igniter warm up begins upon Inducer draft blower step
to low speed and presence of closed low stage pressure
switch contacts.
•
Gas valve opens at end of igniter warm up period,
delivering gas to burners and establishing flame.
•
Integrated control module monitors flame presence.
Gas valve will remain open only if flame is detected.
•
Based on the furnace internal control algorithms the gas
valve and induced draft blower may continue operating
on low stage or both will change to high stage. After
a power cycle, the first heat call will result in high
stage operation.
•
Circulator blower is energized on heat speed following
the selected blower on delay and will begin to ramp
up. Electronic air cleaner terminal is energized with
circulator blower.
•
Furnace is now operating on the specified stage
determined by the internal control algorithm.
•
Furnace runs, integrated control module monitors
safety circuits continuously.
•
If the internal algorithm changes the call from low
heat to high heat, the integrated control module will
immediately switch the induced draft blower, gas valve,
and circulator blower to their high stage settings.
•
If the internal algorithm changes the call from high
heat to low heat, the control will immediately switch the
induced draft blower and gas valve to their low stage
settings. The circulator blower will remain on high
heating speed for thirty (30) seconds before switching
to the low heat circulating speed.
•
W thermostat contacts open, completing the call for
heat.
•
Gas valve closes, extinguishing flame.
•
Induced draft blower is de-energized following a 15
second post purge.
•
Circulator blower continues running for the selected
heat off delay period. The speed run during this period
depends on the last heat call provided by the thermostat.
If the last call for heat was a call for low heat, the air
circulator motor will run on low heat speed for the
duration of the heat off delay period. If the last call for
heat was a call for high heat, the air circulating motor will
run on the high heating speed for thirty (30) seconds and
then switch to the low heating speed for the remainder
of the heat off delay period.
•
Circulator blower and electronic air cleaner terminal
are de-energized.
•
Circulator blower shuts off after the heat off delay period
expires.
•
Furnace awaits next call from thermostat.
Operational Checks
The burner flames should be inspected with the burner
compartment door installed. Flames should be stable,
quiet, soft, and blue (dust may cause orange tips but
they must not be yellow). Flames should extend directly
outward from the burners without curling, floating, or lifting
off. Flames must not impinge on the sides of the heat
exchanger firing tubes.
Safety Circuit Description
A number of safety circuits are employed to ensure safe
and proper furnace operation. These circuits serve to
control any potential safety hazards and serve as inputs
in the monitoring and diagnosis of abnormal function.
These circuits are continuously monitored during furnace
operation by the integrated control module.
Integrated Control Module
The integrated control module is an electronic device
which, if a potential safety concern is detected, will take the
necessary precautions and provide diagnostic information
through an LED.
Primary Limit
The primary limit control is located on the partition panel
and monitors heat exchanger compartment temperatures.
It is a normally-closed (electrically), automatic reset,
temperature-activated sensor. The limit guards against
overheating as a result of insufficient conditioned air
passing over the heat exchanger.
Auxiliary Limit
The auxiliary limit controls are located on or near the
circulator blower and monitors blower compartment
temperatures. They are a normally-closed (electrically),
automatic-reset sensors. These limits guard against
overheating as a result of insufficient conditioned air
passing over the heat exchanger.
Rollout Limit
The rollout limit controls are mounted on the burner/
manifold assembly and monitor the burner flame. They
are normally-closed (electrically), manual-reset sensors.
These limits guard against burner flames not being properly
drawn into the heat exchanger.
Pressure Switches
The pressure switches are normally-open (closed during
operation) negative air pressure-activated switches. They
monitor the airflow (combustion air and flue products)
through the heat exchanger via pressure taps located on
