System Operation; Defrost System - Lennox 12HPB Series Installation Instructions Manual

IMPORTANT
Use table 7 as a general guide when performing
maintenance checks. This is not a procedure for
charging the unit (Refer to Charging/Checking
Charge section). Minor variations in these pressures
may be expected due to differences in installations.
Significant differences could mean that the system
is not properly charged or that a problem exists with
some component in the system.

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.
Filter Drier
The unit is equipped with a biflow filter drier. See figure 12.
If replacement is necessary, order another of like design.
Crankcase Heater
IMPORTANT
If unit is equipped with crankcase heater, it should be
energized 24 hours before unit start−up to prevent
compressor damage as a result of slugging.
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 re-
lays 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 shut-
down, but it should also be used following a power outage if
power has been off for over an hour and the outdoor tem-
perature is below 50°F (10°C). System should be left in the
emergency heat mode at least six hours to allow the crank-
case heater sufficient time to prevent compressor slug-
ging.

Defrost System

The 12HPB defrost system includes two components: a
defrost thermostat and a defrost control.
Defrost Thermostat
The defrost thermostat is located on the liquid line between
the check/expansion valve and the distributor. When de-
frost thermostat senses 42°F (5.5°C) or cooler, the thermo-
stat contacts close and send a signal to the defrost control
board to start the defrost timing. It also terminates defrost
when the liquid line warms up to 70°F (21°C).
Defrost Control
The defrost control board includes the combined func-
tions of a time/temperature defrost control, defrost relay,
diagnostic LEDs and terminal strip for field wiring connec-
tions. See figure 16.
The control provides automatic switching from normal
heating operation to defrost mode and back. During com-
pressor cycle (call for defrost), the control accumulates
compressor run times at 30, 60, or 90 minute field−adjust-
able intervals. If the defrost thermostat is closed when the
selected compressor run time interval ends, the defrost
relay is energized and defrost begins.
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