28
P
RIMARY
L
IMIT
The primary limit control guards against overheating resulting from
insufficient conditioned air passing over the heat exchanger. If the
primary limit control does not function during this test, the cause
must be determined and corrected. Function of this control should
be verified by gradually blocking the furnace return air after the
furnace has been operating (burners firing) for approximately ten
minutes. Check the control as follows:
1. Allow the furnace to operate with burners firing continuously
for approximately ten minutes.
2. Gradually block the return air to furnace. Remove airflow
blockage when limit control is activated and turns off burners.
Airflow blockage causes unit overheating and will produce
the following reactions:
• The gas valve to close and extinguish flame,
• The induced draft blower to be de-energized after a fifteen
second postpurge, and
• The circulator blower to remain energized continuously until
limit control resets.
3. Remove the return air blockage to clear overheating
condition. After an acceptable temperature is reached
during the cool down period, the limit control will reset and
allow the furnace to resume normal operation.
WARNING
T
O PREVENT PREMATURE FAILURE OF HEAT EXCHANGER, PROPERTY DAMAGE,
PERSONAL INJURY OR DEATH, DO NOT ADJUST THE LIMIT CONTROL (FACTORY
SET).
XIV. SAFETY CIRCUIT DESCRIPTION
IMPORTANT NOTE:
This unit must not be used as a construction
heater during the finishing phases of construction of a new structure.
This type of use may result in premature failure due to extremely
low return air temperatures and exposure to corrosive or very dirty
atmospheres.
These checks establish that the primary limit control is functioning
and will respond to a restriction in the return air, or a circulator
blower failure. If the primary limit control does not function during
this test, the cause must be determined and corrected.
G
ENERAL
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.
I
NTEGRATED
C
ONTROL
M
ODULE
The integrated control module is an electronic device which controls
all furnace operations. Responding to the thermostat, the module
initiates and controls normal furnace operation, and monitors and
addresses all safety circuits. If a potential safety concern is
detected, the module will take the necessary precautions and
provide diagnostic information through an LED.
P
RIMARY
L
IMIT
The primary limit control is located on the partition panel and
monitors heat exchanger compartment temperatures. It is an
automatic reset, temperature sensor. The limit guards against the
overheating as a resulting of insufficient air passing over the heat
exchanger.
A
UXILIARY
L
IMIT
The auxiliary limit control is located either on or near the circulator
blower and monitors heat exchanger compartment temperatures.
The control is a temperature sensor. It guards against overheating
resulting from insufficient air passing over the heat exchanger.
R
OLLOUT
L
IMITS
The rollout limit controls are mounted on the burner/manifold
assembly and monitor the burner flame. They are manual-reset,
temperature sensors. This limit guards against burner flames not
being properly drawn into the heat exchanger.
P
RESSURE
S
WITCHES
The pressure switches are normally-open, 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. These switches guard against
insufficient airflow (combustion air and flue products) through the
heat exchanger.
F
LAME
S
ENSOR
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.
XV. TROUBLESHOOTING
E
LECTROSTATIC
D
ISCHARGE
(ESD) P
RECAUTIONS
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 procedure
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.
D
IAGNOSTIC
C
HART
WARNING
T
O AVOID ELECTRICAL SHOCK, INJURY OR DEATH, DISCONNECT ELECTRICAL
POWER BEFORE PERFORMING ANY SERVICE OR MAINTENANCE.
