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I−UNIT COMPONENTS
G71MPP unit components are shown in figure 1. The gas
valve, combustion air inducer and burners can be ac-
cessed by removing the burner access panel. Electrical
components are in the control box (figure 2) found in the
blower compartment.
G71MPP units are factory equipped with a bottom return air
panel in place. The panel is designed to be field removed as
required for bottom air return. Markings are provided for side
return air and may be cut out in the field.
CAUTION
Electrostatic discharge can affect electronic com-
ponents. Take precautions during furnace instal-
lation and service to protect the furnace’s elec-
tronic controls. Precautions will help to avoid con-
trol exposure to electrostatic discharge by putting
the furnace, the control and the technician at the
same electrostatic potential. Neutralize electro-
static charge by touching hand and all tools on an
unpainted unit surface, such as the gas valve or
blower deck, before performing any service pro-
cedure.
ELECTROSTATIC DISCHARGE (ESD)
Precautions and Procedures
A−Control Box
1. Control Transformer (T1)
A transformer located in the control box provides power to
the low voltage section of the unit. Transformers on all
models are rated 40VA with a 120V primary and a 24V sec-
ondary.
2. Door Interlock Switch (S51)
A door interlock switch rated 14A at 125VAC is wired in se-
ries with line voltage. When the blower door is removed the
unit will shut down.
FIGURE 2
CONTROL BOX G71MPP
SURELIGHT®
INTEGRATED
CONTROL
TRANSFORMER
CIRCUIT
BREAKER
DOOR INTERLOCK
SWITCH
3. Circuit Breaker (CB8)
A 24V circuit breaker is also located in the control box.
The switch provides overcurrent protection to the trans-
former (T1). The breaker is rated 3A at 32V. If the current
exceeds this limit the breaker will trip and all unit opera-
tion will shutdown. The breaker can be manually reset
by pressing the button on the face. See figure 3.
FIGURE 3
CIRCUIT BREAKER CB8
PRESS TO RESET
WARNING
Shock hazard.
Disconnect power before servicing. Integrated
Control Board is not field repairable. If control is in-
operable, simply replace entire control.
Can cause injury or death. Unsafe operation will re-
sult if repair is attempted.
4. Integrated Control Board(A92)
G71MPP units are equipped with the Lennox SureLight
®
variable capacity integrated control. The system consists
of an ignition / blower control (figures 4 and 5) with control
pin designations in tables 12 and 13 and ignitor (figure 12).
The control provides gas ignition, safety checks and in-
door blower control with variable capacity rate gas heat-
ing. The furnace combustion air inducer, gas valve and in-
door blower are controlled in response to various system
inputs such as thermostat signal, prove and limit switch
signal and flame signal. The control operates with a con-
ventional single or two−stage thermostat. The board fea-
tures a seven segment LED display, indicating furnace
status (including indoor blower) and error codes. The LED
flashes in single digits. For example using table 25 under
LIMIT CODE, a 2" followed by 5" followed by 0", the limit
switch circuit is open. The board also has two 120 volt ac-
cessory terminals (used for a humidifier and electronic air
cleaner) rated at (1) one amp each.
