Defrost System - Lennox HP21 Series Installation Instructions Manual

High Pressure Switch
HP21 units are equipped with a high pressure switch that is
located in the discharge line of the compressor. The switch
(SPST, manual reset, which is normally closed) removes
power from the compressor when the discharge pressure
rises above the factory setting at 410 + 10 psig.
Low Pressure Switch
HP21 units are also equipped with a low pressure switch
that is located in the vapor line of the compressor. The
switch (SPST, auto−reset, normally closed) removes power
from the compressor when vapor line pressure drops be-
low the factory setting at 25 + 5 psig.
Crankcase Temperature Limit Switch
HP21 units are equipped with a crankcase temperature
limit switch which is strapped to the outside bottom of the
compressor shell. This limit switch prevents compressor
operation at exceedingly high oil temperature by opening
the control circuit at 190°F (87.8°C). It automatically resets
at 110°F (43.3°C).
Crankcase Heater
IMPORTANT
Crankcase heater (when provided) should be ener-
gized 24 hours before unit start−up. This heater must
always be energized before start−up to prevent com-
pressor damage as a result of slugging.
Filter Drier
The HP21 drier is equipped with an internal check valve for
correct refrigerant flow (Refer to figure 13). If replacement
is necessary, order another of like design and capacity. A
liquid line strainer gives additional compressor protection.
Thermostat Operation
The electromechanical room thermostat is not included
with the unit and must be purchased separately. The ther-
mostat incorporates isolating contacts and an emergency
heat function (which includes an orange indicating light).
The thermostat displays additional indicator lights and
uses an ambient compensating thermistor. Each feature is
explained below.
1 − Emergency Heat (Orange Light) − An emergency
heat function is designed into the thermostat. This fea-
ture is applicable only to those systems with auxiliary
electric heat staged by outdoor thermostats. When the
thermostat is placed in the emergency heat position,
the outdoor unit control circuit is isolated from power
and field−provided relays bypass the outdoor thermo-
stats. An orange indicating light simultaneously comes
on to remind the homeowner that the unit is operating
in the emergency heat mode.
Emergency heat is usually used during a outdoor unit
shutdown. 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.
2 − Service Light (Red) − The service light is wired in series
with the second−stage heat bulb of the room thermostat
and a thermal sensor that is strapped to the compressor
discharge line. The sensor monitors discharge temper-
ature and closes on a temperature fall which indicates
the compressor is not operating properly and that ser-
vice is required. A red light represents an outdoor unit
malfunction during the heating mode. It warns the
homeowner that the electric elements are providing all
the heat.
If the red light comes on, place the room thermostat
into emergency heat to bypass any outdoor thermo-
stat(s) making all the auxiliary heat available. Under
these conditions, the orange light will be energized
continuously and the red light will cycle with the indoor
unit.
3 − When the system is operating on low speed the green
light will come on.
4 − When the system is operating on high speed the amber
light comes on.
5 − The ambient compensating thermistor (located in the
outdoor unit) cuts down thermostat droop to improve the
operating characteristics of the outdoor unit system.

Defrost System

HP21 units are equipped with a demand defrost system
which initiates a defrost cycle based on temperature differ-
ential and compressor run time. The control board includes
two permanently attached sensors which monitor coil and
outdoor ambient temperatures. The coil temperature sen-
sor is equipped with a spring clip which allows proper posi-
tioning on the outdoor coil return bend. The ambient tem-
perature sensor is installed in a sampling tube at the base
of the unit. These sensors must not be detached from the
control board and must be replaced as part of the control
board. Do not attempt to cut or splice the temperature sen-
sor wires.
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Components