Lennox LGA Series Unit Information page 15

11-Power Exhaust Relay K65 (PEF units)
Power exhaust relay K65 is a DPDT relay with a 24VAC
coil. K65 is used in all LGA/LCA units equipped with the
optional power exhaust fans. K65 is energized by the
economizer control panel (A56), after the economizer
dampers reach 50% open (adjustable in ECTO). When
K65 closes, the exhaust fan B10 is energized.
12-Blower Motor Overload Relay S42 (units
with high efficiency motors)
The blower motor overload relay is used in all L series units
equipped with high efficiency motors. The relay (S42) is con-
nected in line with the blower motor to monitor the current flow
to the motor. When the relay senses an overload condition, a
set of normally closed contacts open to de-energize pin #9 in
plug 110 of the A55 main control module. A55 de-energizes all
outputs. The overload relay has an adjustable setting and is
set per the nameplate current of the motor. All units are
equipped with a relay manufactured by Siemens which is de-
tailed in figure 5.
13-Start Capacitor C7 (single-phase)
Single-phase units use a start capacitor (C7) wired in
parallel with the compressor side of the dual capacitor.
The start capacitor is located to the side of the control-
panel. C7 is engaged during compressor start-up and is
switched off by the potential relay as the compressor
nears full speed. Capacitor ratings may be different for
each motor, but the rating and repair part number will be
printed on the side of the capacitor.
14-Potential Relay K31 (single-phase)
Single-phase units use a potential relay which controls the op-
eration of the starting circuit. The potential relay is located to
the side of the unit control panel. The relay is normally closed
when the compressor (contactor K1) is de-energized. Capaci-
tor (C7) is connected to a set of N.C. K31 contacts and is used
to assist the compressor in starting. When K1 energizes, the
compressor immediately begins start-up. K31 remains de-en-
ergized during compressor start-up and the start capacitor
(C7) remains in the circuit. As the compressor gains speed,
K31 is energized by electromotive forces generated by the
compressor. When K31 energizes, its contacts open to take
the start capacitor out of the circuit.
ELECTROSTATIC DISCHARGE (ESD)
Precautions and Procedures
Electrostatic discharge can affect electronic
components. Take precautions during furnace
installation and service to protect the furnace's
electronic controls. Precautions will help to avoid
control exposure to electrostatic discharge by
putting the furnace, the control and the techni-
cian at the same electrostatic potential. Neutral-
ize electrostatic charge by touching hand and all
tools on an unpainted unit surface, such as the
gas valve or blower deck, before performing any
service procedure.
INTEGRATED MODULAR CONTROL BOARDS
The Integrated Modular Control (IMC) is a series of control
boards which integrates most control functions required for the
LGA/LCA units. The control boards are located in the upper
left hand corner of the control box. The control includes
complete unit diagnostics with permanent code storage,
field programmable control parameters and control op-
tions, on-site testing, and serial communications. Two
different printed circuit boards (see figure 6) make-up the
modular configurations for the LGA/LCA units. See table
2 for a list of control panels used for each unit. For further
information refer to Integrated Modular Control Guide sent with
each unit.
UNIT UNIT
LGA
LCA
15-Main Control Module A55 (all units)
The main control module A55 is the heart of the system. It
controls one compressor, one single or two stage gas
valve, one bank of electric heat, one outdoor fan, and one
blower. A55 includes the thermostat inputs, serial commu-
nications ports, diagnostic code display, control pushbutton,
system configuration dip switches, and four expansion ports. A
diagnostic code list is located on the back side of the access
panel.
16-Economizer Control Module A56
(Economizer only)
The economizer control module A56 controls the econo-
mizer. A56 has four different cooling modes, sensible
temperature, outdoor enthalpy, differential enthalpy, and
global control.
Page 15
CAUTION
TABLE 2
CONTROL PANELS
A55
X
X
A56
OPT
OPT