51
F
URNACE
C
ONTROL
B
OARD
The furnace control board is an electronic device which, if a
potential safety concern is detected, will take the necessary
precautions and provide diagnostic information through an
LED display.
P
RIMARY
L
IMIT
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.
A
UXILIARY
L
IMIT
The auxiliary limit controls are located on or near the circula-
tor blower and monitors blower compartment temperatures.
They are a normally-closed (electrically), auto-reset sensors.
These limits guard against overheating as a result of insuffi-
cient conditioned air passing over the heat exchanger.
R
OLLOUT
L
IMIT
The rollout limit controls are mounted on the burner/mani-
fold assembly and monitor the burner flame. They are nor-
mally-closed (electrically), manual-reset sensors. These lim-
its guard against burner flames not being properly drawn into
the heat exchanger.
•
Furnace circulator blower and outdoor cooling unit run
their appropriate speeds, integrated control module
monitors safety circuits continuously.
•
R and Y1/G or Y2/G thermostat contacts open,
completing the call for cool.
•
Outdoor fan and compressor are de-energized.
•
Circulator blower continues running during a cool off
delay period. The OFF delay time and airflow level are
determined by the selected ramping profile.
• Electronic air cleaner terminal and circulator blower
are de-energized.
•
Furnace awaits next call from thermostat.
F
AN
O
NLY
M
ODE
The normal operational sequence in fan only mode is as fol-
lows:
•
R and G thermostat contacts close, initiating a call for
fan.
•
Integrated control module performs safety circuit
checks.
•
Circulator blower is energized on continuous fan speed
(25%, 50%, 75% or 100% DIP switch selectable when
using a conventional thermostat). Electronic air
cleaner terminal is energized.
•
The furnace control board HUM dry contacts close.
•
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. Electronic air cleaner
terminal is de-energized.
•
Furnace awaits next call from thermostat.
O
PERATIONAL
C
HECKS
The burner flames should be inspected with the burner com-
partment 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 burn-
ers without curling, floating, or lifting off. Flames must not
impinge on the sides of the heat exchanger firing tubes.
Check the
Burner Flames for:
1. Stable, soft and blue.
2. Not curling, floating
or lifting off.
Burner Flame
Figure 54
S
AFETY
C
IRCUIT
D
ESCRIPTION
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 con-
tinuously monitored during furnace operation by the integrated
control module.
F
URNACE
C
ONTROL
B
OARD
The furnace control board is an electronic device which, if a
potential safety concern is detected, will take the necessary
precautions and provide diagnostic information through an
LED display.
P
RIMARY
L
IMIT
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.
A
UXILIARY
L
IMIT
The auxiliary limit controls are located on or near the circula-
tor blower and monitors blower compartment temperatures.
They are a normally-closed (electrically), auto-reset sensors.
These limits guard against overheating as a result of insuffi-
cient conditioned air passing over the heat exchanger.
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