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 low-heat speed. Electronic
air cleaner terminals are energized.
A number of circuits are employed to ensure proper furnace operation. These circuits serve to control any potential 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 concern is detected, will take the necessary precautions
and provide diagnostic information through an LED.
Primary Limit Switch
The primary limit switch 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 switch guards against overheating as
a result of insufficient conditioned air passing over the heat
Auxiliary Limit Switches
The auxiliary limit switches are located on or near the circulator
blower and monitors blower compartment temperatures. They
are normally-closed (electrically), manual-reset sensors. These
limit switches guard against overheating as a result of insufficient
conditioned air passing over the heat exchanger.
Rollout Limit Switches
The rollout limit switches are mounted on the burner/manifold
assembly and monitor the burner flame. They are normally-
closed (electrically), manual-reset sensors. These limit switches
guard against burner flames not being properly drawn into the
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
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.
Circulator blower runs; integrated control module monitors
safety circuits continuously.
R and G thermostat contacts open, completing the call for
Circulator blower is de-energized. Electronic air cleaner
terminals are de-energized.
Furnace awaits the next call from thermostat.
Circulator Blower Speeds
This furnace is equipped with a multispeed circulator blower. This
blower provides ease in adjusting blower speeds. The
Specification Sheet applicable to your model provides an airflow
table, showing the relationship between airflow (CFM) and
external static pressure (E.S.P.), for the proper selection of
heating and cooling speeds.
The cooling blower speed is shipped set on HIGH, and the
heating blower speed is set as indicated in the Specification
Sheet applicable to your model. These blower speeds should be
adjusted by the installer to match the installation requirements so
as to provide the correct heating temperature rise and correct
To adjust the circulator blower speed, proceed as follows:
1. Turn off power to the furnace.
2. Select the heating and cooling blower speeds that match the
installation requirements from the airflow table in the
3. Relocate the desired motor leads to the circulator blower heat
and cool speed terminals on the integrated control module.
(Terminals are identified as LO HEAT-H, HI HEAT-H and
COOL-H [hot]). If a heating speed and the cooling blower
speed are the same, a jumper wire must be used between the
heat and cool terminals.
4. Connect all unused blower motor leads to the "PARK"
terminals on the integrated control module. Any leads not
connected to the "PARK" terminals must be taped.
5. Turn on power to furnace.
6. Verify the proper temperature rise as outlined in "Measure
Temperature Rise" in "Complete Installation."
Circulator Blower Speeds