System Operation; Defrost System - Lennox Merit 13HPX Series Installation Instructions Manual

System Operation

UNIT COMPONENTS
Some scroll compressor have internal vacuum protector
that will unload scrolls when suction pressure goes
below 20 psig. A hissing sound will be heard when the
compressor is running unloaded. Protector will reset
when low pressure in system is raised above 40 psig. DO
NOT REPLACE COMPRESSOR.
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.
Bi−Flow Liquid line Filter Drier
The unit is equipped with a large−capacity bi−flow 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 (S87)
The 13HPX is equipped with an 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 40 + 5 psig (276
+ 34 kPa) and opens at 25 + 5 psig (172 + 34 kPa). It is not
adjustable.
Low Pressure Switch Bypass (S41) (Optional)
For use in applications where the heat pump is operated in
outdoor ambient temperatures below 15°F.
S
Prevents nuisance trips form the low pressure switch
S
Wired in parallel with the low pressure switch
High Pressure Switch (S4)
The 13HPX 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 + 15 psig (4068 + 103 kPa).
NOTE − A Schrader core is under the pressure switches.
Defrost Thermostat (S6)
The defrost thermostat is located on the liquid line between
the check/expansion valve and the distributor. When
defrost thermostat senses 42°F (5.5°C) or cooler, the
thermostat contacts close and send a signal to the defrost
control to start the defrost timing. It also terminates defrost
when the liquid line warms up to 70°F (21°C).
Crankcase Heater (HR1) and Thermostat Switch
All models sizes are equipped with a belly band type
crankcase heater. HR1 prevents liquid from accumulating
in the compressor. The HR1 is controlled by a single pole,
single throw thermostat switch (S40) located on the liquid
line (see figure 1 for location). On all other units, the heater
is on when there is no compressor operation.
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IMPORTANT
(S40) (−60 model only)
search engine
Thermal Protection Switch (S173)
Some XC13 units are equipped with a compressor
mounted normally closed temperature switch that
prevents compressor damage due to overheating caused
by internal friction. The switch is located on top of the
compressor casing (see figure 1). This switch senses the
compressor casing temperature and opens at 239−257°F
(115°C−125°C) to shut off compressor operation. The
auto−reset switch closes when the compressor casing
temperature falls to 151−187°F (66°C−86°C), and the
compressor is re−energized. This single−pole, single−throw
(SPST) bi−metallic switch is wired in series with the 24V Y
input signal to control compressor operation.

Defrost System

The defrost system includes a defrost thermostat (S6) and
a defrost control (CMC1).
DEFROST CONTROL (CMC1)
This defrost control includes the combined functions of a
time/temperature defrost control, defrost relay, time delay,
diagnostic LEDs, and a terminal strip for field wiring
connections.
DEFROST TIMING
PINS (P1)
TEST
PINS
COMPRESSOR
DELAY PINS
REVERSING
VALVE
S87
LOW PRESSURE
SWITCH
DEFROST
THERMOSTAT (S6)
S4
HIGH PRESSURE
SWITCH
Figure 24. Defrost Control (CMC1)
The defrost control provides automatic switching from
normal heating operation to defrost mode and back. When
the defrost thermostat is closed, the control accumulates
compressor run time at 30, 60 or 90 minute field adjustable
intervals. When the selected compressor run time interval
is reached, the defrost relay is energized and defrost
begins.
Defrost Control Timing Pins (P1)
Each timing pin selection provides a different
accumulated compressor run time period for one defrost
cycle. This time period must occur before a defrost cycle
is initiated. The defrost interval can be adjusted to 30
(T1), 60 (T2), or 90 (T3) minutes (see figure 24). The
maximum defrost period is 14 minutes and cannot be
adjusted.
Page 29
Compressor
Mounted
DIAGNOSTIC
LEDS
24V TERMINAL
STRIP
CONNECTIONS
SERVICE LIGHT
CONNECTIONS
13HPX SERIES