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Fan Only Mode
The normal operational sequence in fan only mode is as follows:
•
R and G thermostat contacts close, initiating a call
for fan.
•
Integrated control module performs safety circuit
checks.
•
Circulator blower is energized on heat speed.
•
EAC terminal is energized.
•
Circulator blower runs, integrated control module
monitors safety circuits continuously.
•
R and G thermostat contacts open, completing the
call for fan.
•
Circulator blower is de-energized.
•
EAC terminal is de-energized.
•
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
T
he primary limit control is located on the partition panel and
monitors heat exchanger compartment temperatures. It is
a normally-closed (electrically), automatic reset, tempera
-
ture-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), manual-reset sensors. These
limits guard against overheating as a result of insufficient condi
-
tioned air passing over the heat exchanger.
Burner Temperature Switch
The burner temperature switch is mounted on the burner as
-
sembly to monitor the burner box temperature. It is normally
closed (electrically), auto-reset sensor. This switch guards
against the 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 insuffi
-
cient 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 assembly
which uses the principle of flame rectification to determine the
presence or absence of flame.
Pressure Sensor
Pressure sensor is mounted on the induced draft blower. Its
function is to regulate the induced draft blower’s speed in
order to maintain proper air-fuel ratio for clean and reliable
combustion.
Troubleshooting
Electrostatic Discharge (ESD) Precautions
NOTE:
Discharge body’s static electricity before touching
unit. An electrostatic discharge can adversely affect electrical
components.
Use the following precautions during furnace installation and
servicing to protect the integrated control module from damage.
By putting the furnace, the control, and the person at the same
electrostatic potential, these steps will help avoid exposing the
integrated control module to electrostatic discharge. This proce
-
dure is applicable to both installed and uninstalled (ungrounded)
furnaces.
1. Disconnect all power to the furnace. Do not touch the
integrated control module or any wire connected to the
control prior to discharging your body’s electrostatic
charge to ground.
2. Firmly touch a clean, unpainted, metal surface of the
furnace away from the control. Any tools held in a
person’s hand during grounding will be discharged.
3. Service integrated control module or connecting wiring
following the discharge process in step 2. Use caution
not to recharge your body with static electricity; (i.e., do
not move or shuffle your feet, do not touch ungrounded
objects, etc.). If you come in contact with an ungrounded
object, repeat step 2 before touching control or wires.
4. Discharge your body to ground before removing a new
control from its container. Follow steps 1 through 3 if
installing the control on a furnace. Return any old or
new controls to their containers before touching any
ungrounded object.
