System Operation; Defrost System - Lennox Merit Series 14HPX Installation Instructions Manual

System Operation

The outdoor unit and indoor blower cycle on demand from
the room thermostat. If the thermostat blower switch is in
the ON position, the indoor blower operates continuously.
FILTER DRIER
The unit is equipped with a large−capacity biflow filter drier
which keeps the system clean and dry. If replacement is
necessary, order another of the same design and capacity.
The replacement filter drier must be suitable for use with
HFC−410A refrigerant.
LOW PRESSURE SWITCH (OPTIONAL)
The 14HPX may be equipped with an optional auto−reset
low pressure switch which is located on the vapor line. The
switch shuts off the compressor when the vapor pressure
falls below the factory setting. This switch, which is ignored
during defrost operation, closes at pressures at or above
55 psig and opens at 25 psig. It is not adjustable.
HIGH PRESSURE SWITCH
The 14HPX is equipped with an auto-reset high pressure
switch (single−pole, single−throw) which is located on the
liquid line. The switch shuts off the compressor when
discharge pressure rises above the factory setting. The
switch is normally closed and is permanently adjusted to
trip (open) at 590 + 10 psig (4412 + 69 kPa).
NOTE − A Schrader core is under the pressure switches.

Defrost System

DEFROST SYSTEM DESCRIPTION
The demand defrost controller measures differential
temperatures to detect when the system is performing
poorly because of ice build−up on the outdoor coil. The
controller self−calibrates" when the defrost system starts
and after each system defrost cycle. The defrost control
board components are shown in figure 12.
The control monitors ambient temperature, outdoor coil
temperature, and total run time to determine when a
defrost cycle is required. The coil temperature probe is
designed with a spring clip to allow mounting to the outside
coil tubing. The location of the coil sensor is important for
proper defrost operation.
NOTE − The demand defrost board accurately measures
the performance of the system as frost accumulates on the
outdoor coil. This typically will translate into longer running
time between defrost cycles as more frost accumulates on
the outdoor coil before the board initiates defrost cycles.
DIAGNOSTIC LEDS
The state (Off, On, Flashing) of two LEDs on the defrost
board (DS1 [Red] and DS2 [Green]) indicate diagnostics
conditions that are described in table 13.
506377−01 11/09
TEST PINS
DEFROST
TERMINATION
PIN SETTINGS
SENSOR
PLUG IN
(COIL & AM-
BIENT
SENSORS)
DELAY
PINS
REVERSING
VALVE
PRESSURE
SWITCH CIR-
CUIT CONNEC-
TIONS
Figure 12
DEFROST BOARD PRESSURE SWITCH
CONNECTIONS
The unit's automatic reset pressure switches (LO PS − S87
and HI PS − S4) are factory−wired into the defrost board on
the LO−PS and HI−PS terminals, respectively.
(OPTIONAL) Low Pressure Switch (LO−PS) When the
low pressure switch trips, the defrost board will cycle off the
compressor, and the strike counter in the board will count
one strike. The low pressure switch is ignored under the
following conditions:
S
during the defrost cycle and 90 seconds after the
termination of defrost
S
when the average ambient sensor temperature is
below 15° F (−9°C)
S
for 90 seconds following the start up of the compressor
S
during "test" mode
High Pressure Switch (HI−PS) When the high pressure
switch trips, the defrost board will cycle off the compressor,
and the strike counter in the board will count one strike.
DEFROST BOARD PRESSURE SWITCH SETTINGS
High Pressure (auto reset) − trip at 590 psig; reset at 418.
Low Pressure (auto reset) − trip at 25 psig; reset at 40.
5−STRIKE LOCKOUT FEATURE
The internal control logic of the board counts the pressure
switch trips only while the Y1 (Input) line is active. If a
pressure switch opens and closes four times during a Y1
(Input), the control logic will reset the pressure switch trip
counter to zero at the end of the Y1 (Input). If the pressure
switch opens for a fifth time during the current Y1 (Input),
the control will enter a lockout condition.
The 5−strike pressure switch lockout condition can be reset
by cycling OFF the 24−volt power to the control board or by
shorting the TEST pins between 1 and 2 seconds. All timer
functions (run times) will also be reset.
If a pressure switch opens while the Y1 Out line is
engaged, a 5−minute short cycle will occur after the switch
closes.
Page 22
LOW
AMBIENT
THERMOSTAT
PINS
DIAGNOSTIC
LEDS
24V TERMINAL
STRIP
CONNECTIONS
Note − Compo-
nent Locations
Vary by Board
Manufacturer.
.
Defrost Control Board