System Operation; Defrost System - Lennox Merit 10HPB Series Installation Instructions Manual

Approach Method
4 − Use the same digital thermometer used to check out-
door ambient temperature to check liquid line tempera-
ture. Verify the unit charge using the approach method.
The difference between the ambient and liquid temper-
atures should match values given in table 7. Add refrig-
erant to lower the approach temperature and remove it
to increase the approach temperature. Loss of charge
results in low capacity and efficiency.
5 − If the values don't agree with the those in table 7, add
refrigerant to lower the approach temperature or re-
cover refrigerant from the system to increase the ap-
proach temperature.
Table 7
Approach Values
Liquid Temp. Minus Ambient Temp.
Model
10HPB18
10HPB24
10HPB30
10HPB36
10HPB42
10HPB48
10HPB60
NOTE − For best results, the same thermometer should be used to check
both outdoor ambient and liquid temperatures.
IMPORTANT
Use table 6 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.
_F (_C)
12 (6.7)
13 (7.2)
14 (7.8)
10 (5.6)
13 (7.2)
14 (7.8)
13 (7.2)
Page 16
Emergency Heat Function
An emergency heat function is designed into some room ther-
mostats. This feature is applicable when isolation of the out-
door 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 unit 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 10HPB 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.
Defrost Control Timing Pins
Each timing pin selection provides a different accumu-
lated compressor run time period during one thermostat
run 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 16. The
defrost timing jumper is factory−installed to provide a
60−minute defrost interval. If the timing selector jumper is
not in place, the control defaults to a 90−minute defrost
interval. The maximum defrost period is 14 minutes and
cannot be adjusted.
A TEST option is provided for troubleshooting. The TEST
mode may be started any time the unit is in the heating