49
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
Do not bypass safety devices.
WARNING
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,
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),
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
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
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.
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 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.
Checking Flame Signal
Flame current can be measured in two ways:
1. Putting a D.C. microamp meter in series with the
flame rod
2. Putting a D.C. volt meter across the two solder pads
in the flame current section of the control board.
3. 1 D.C. volt = 1 microamp. See Figure 46.
