Lennox LGH094U Unit Information page 35

4-Low Ambient Switches S11, S84
The low ambient switch is an auto‐reset SPST N.O. pres­
sure switch which allows mechanical cooling operation
at low outdoor temperatures. On standard and high effi­
ciency units, the switches are located in each liquid line
prior to the indoor coil section. On ultra high efficiency
units, S11 (only) is located on the common liquid line pri­
or to the indoor coil section.
On standard and high efficiency units, S11 and S84 are
wired to the A55 Unit Controller which cycles outdoor fans
via K10 (outdoor fan 1) and K68 (outdoor fan 2). On ultra
high efficiency units, S11 is wired to the A55 Unit Controller
which cycles outdoor fan 1 (outdoor fans 2 and 3 are de-en­
ergized during low ambient operation).
When liquid pressure rises to 450 ± 10 psig (3102 ± 69
kPa), the switch closes and the condenser fan is ener­
gized. When discharge pressure in one refrigerant circuit
drops to 240 ± 10 psig (1655 ± 69 kPa), the switch opens
and the condenser fan in that refrigerant circuit is de-ener­
gized. This intermittent fan operation results in higher evap­
orating temperature allowing the system to operate without
icing the evaporator coil and losing capacity.
5-Service Valve (optional)
Non-reheat, fin/tube coil condenser units may be
equipped with service valves located in the discharge
and liquid lines. The service valves are manually operat­
ed valves used for service operation.
6-Filter Drier
LGH units have a filter drier located in the liquid line of each
refrigerant circuit. See figure 12, 13, or 14. The drier re­
moves contaminants and moisture from the system.
7-Low Pressure Switches S87, S88
The low pressure switch is an auto‐reset SPST N.O. switch
(held N.C. by refrigerant pressure) which opens on a pres­
sure drop. All units are equipped with this switch. The
switch is located in the compressor suction line.
On standard and high efficiency units, S87 (compressor
one) and S88 (compressor two) are wired to A55 Unit Con­
troller. On ultra high efficiency units, S87 (only) is located on
the common suction line and is wired to A55 Unit Controller.
A55 governs the low pressure switches by shunting the
switches during start up until pressure is stabilized. After
the shunt period, the control has a three‐strike counter, dur­
ing first thermostat demand, before the compressor is
locked out. The control is reset by breaking and remaking
the thermostat demand or manually resetting the control.
When suction pressure drops to 40 + 5 psig (276 ± 34
kPa), (indicating low pressure), the switch opens and the
compressor(s) is(are) de-energized. The switch auto­
matically resets when pressure in the suction line rises to
90 + 5 psig (620 ± 34 kPa) due to many causes such as
refrigerant being added.
8-Condenser Fans B4, B5, B21
See SPECIFICATIONS tables at the front of this manual for
specifications of condenser fans used in all units. All con­
denser fans have single-phase motors. The fan assembly
may be removed for servicing and cleaning.
Ultra High Efficiency Units Only
Ultra high efficiency units are equipped with electronically
commutated condenser fan motors (ECM). The ECM mo­
tors are wired directly to 230VAC power but do not operate
until a pulse width modulated (PWM) control signal is sent
from the A55 Unit Controller. The PWM signal determines
the condenser fan speed. All three fans will operate in low
speed with a Y1 demand; all three fans will operate in high
speed with a Y2 demand.
Both low and high voltage plugs are located at the top of the
blower compartment in the indoor section of the unit. Con­
denser fan motors B4, B5 and B21 high voltage plugs are
J86, J87 and J88 respectively. Low voltage plugs are J336,
J337 and J338 respectively. Refer to wiring markings to
identify plugs. Use figure 15 to identify fan motors.
If an ECM fan is not operating:
1- Check to make sure high voltage is present before
checking low voltage.
2- Read the voltage at the appropriate high voltage fan
motor plug (J86, J87, or J88) using the VAC meter set­
ting.
3- If high voltage is present, check the low voltage plug
(J336, J337, or J338) for a signal from the Unit Con­
troller. Use either the duty cycle (%) or a VDC meter
setting.
Note - The VDC reading may fluctuate and is normal for a
PWM signal.
ULTRA FAN MOTOR IDENTIFICATION
Compressors
1
2
Page 35
Condenser Fans
2
B5
1
B4
3
B21
FIGURE 15