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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 contin-
uously 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 circula-
tor blower and monitors blower compartment temperatures.
They are a normally-closed (electrically), manual-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 normal
-
ly-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 oper-
ation) negative air pressure-activated switches. They monitor
the airflow (combustion air and flue products) through the
heat exchanger via pressure taps located on the induced draft
blower and the coil front cover. These switches guard against
insufficient airflow (combustion air and flue products) through
the heat exchanger and/or blocked condensate drain conditions.
Flame Sensor
The flame sensor is a probe mounted to the burner/manifold
assembly which uses the principle of flame rectification to
determine the presence or absence of flame.
Cooling Mode
Low stage cooling Mode sequence:
• On a call for lo cool, the YLO and G thermostat
contacts close signaling the control module.
• The LED will display a “GREEN”1 flash
• The compressor and condenser fan are energized.
The compressor is connected to the YLO terminal
of the control module.
•
The circulator fan is energized at lo cool speed after
a cool on delay. The electronic air cleaner will also
be energized.
• After the thermostat is satisfied, the compressor is
de-energized and the 45 second fan off delay period
begins.
• Following the 45 second cool mode fan off delay
period, the cool circulator and air cleaner relay are
de-energized.
Single cooling stage thermostat or 2
nd
stage cooling
Mode sequence:
•
On a call for cool, the Y and G thermostat contacts
close signaling the control module.
• The LED will display a “GREEN”2 flash
• The compressor and condenser fan are energized.
The circulator fan is energized at cool speed after
a cool on delay. The electronic air cleaner will also
be energized.
• After the thermostat is satisfied, the compressor is
de-energized and the fan off delay period begins.
• Following the 45 second fan off delay period,
the cool circulator and air cleaner relay are de-
energized.
Note:
If a call for both YLO and Y are present, Y will have
priority.
Fan Only Mode
MANUAL FAN ON:
•
Call for continuous fan (24 volts on the furnace “G”
terminal.)
• The LED will display rapid “GREEN”.
•
The circulator blower and the electronic air cleaner
terminal are energized.
Operational Checks
The burner flames should be inspected with the burner compart
-
ment 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.
