System Operation; Defrost System - Lennox Merit Series13HPD Units Installation Instructions Manual

System Operation

The outdoor unit and indoor blower cycle on demand from
the room thermostat. When the thermostat blower switch
is in the ON position, the indoor blower operates
continuously.
LIQUID LINE BI−FLOW FILTER DRIER
The pre−installed liquid line bi−flow filter drier as illustrated
in figure 27 is approved for use with HCFC−22 only. Do not
replace liquid line filter drier with component designed for
use with HFC−410A.
EMERGENCY HEAT FUNCTION (ROOM
THERMOSTAT)
An emergency heat function is designed into some room
thermostats. This feature is applicable when isolation of
outdoor unit is required or when auxiliary electric heat is
staged by outdoor thermostats. When the room thermostat
is placed in the emergency heat position, the outdoor unit
control circuit is isolated from power and field-provided
relays bypass the outdoor thermostats. An amber
indicating light simultaneously comes on to remind the
homeowner that he is operating in the emergency heat
mode.
Emergency heat is usually used during an outdoor
shutdown, but it should also be used following a power
outage if power has been off for over an hour and the
outdoor temperature is below 50°F (10°C). System should
be left in the emergency heat mode at least six hours to
allow the crankcase heater sufficient time to prevent
compressor slugging.
DEFROST
THERMOSTAT
BI−FLOW FILTER
DRIER
LIQUID LINE
SERVICE PORT
Figure 27. Component Locations
CHARGE PORT FOR
FACTORY USE ONLY!
VAPOR LINE SERVICE
PORT
Page 17

Defrost System

The 13HPD defrost system includes two components: a
defrost thermostat and a defrost control board (DCB).
DEFROST THERMOSTAT
The defrost thermostat as illustrated in figure 27 is located
on the liquid line between the CTXV and the distributor.
When defrost thermostat senses 42°F (5.5°C) or cooler,
the thermostat contacts close and send a signal to the DCB
to start the defrost timing. It also terminates defrost when
the liquid line warms up to 70°F (21°C).
DEFROST CONTROL BOARD (DCB)
The DCB includes the combined functions of a
time/temperature defrost control, defrost relay, diagnostic
LEDs and terminal strip for field wiring connections as
illustrated in figure 28.
FIELD SELECT
TIMING PINS
TEST
PINS
COMPRESSOR
DELAY PINS
REVERSING
VALVE
S87
LOW PRESSURE
SWITCH
(OPTIONAL)
DEFROST
THERMOSTAT
S4
HIGH PRESSURE
SWITCH
(OPTIONAL)
Figure 28. Defrost Control Board (DCB)
The control provides automatic switching from normal
heating operation to defrost mode and back. During
compressor cycle (call
accumulates compressor run times at 30-, 60-, or
90-minute field−adjustable intervals. If the defrost
thermostat is closed when the selected compressor run
time interval ends, the defrost relay is energized and
defrost begins.
P1 Defrost Temperature Termination and
Test Jumper
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 DCB selections are: 30, 60, and 90°F (10, 21, 32 and
38°C). The jumper termination pin is factory set at 90°F
(10°C). If the temperature jumper is not installed, the
default termination temperature is 90°F (32°C). The
maximum defrost period is 14 minutes and cannot be
adjusted.
DIAGNOSTIC
LEDS
24V TERMINAL
STRIP
CONNECTIONS
for defrost), the control
13HPD SERIES