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XP21
System Overview
IMPORTANT
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 heat pump control (A175) provides the following func-
tions:
Demand defrost algorithm
Field−selectable defrost termination temperatures
Internal switching of outputs
Compressor anti−short−cycle delay.
Five strikes lockout safety function
High (S4) and low (S87) pressure switches
Ambient (RT13), coil (RT21) and discharge line (RT28)
temperatures monitoring and protection.
COMPRESSOR PROTECTION ANTI−SHORT
CYCLE DELAY
The heat pump control protects the compressor from:
Short cycling (five minutes) when there is initial power
up
Interruption in power to the unit
Pressure or sensor trips
Delay after Y1 demand is removed.
In non−communicating systems the delay is set for 300 sec-
onds (five minutes) and can not be changed. To override
timer when active or inactive, place a jumper on the field
test pins between 1 and 2 seconds.
In communicating system, the icomfort thermostat has a
separate built−in 5−minute non−adjustable short cycle
protection.
Resetting Anti−Short Cycle Delay (101796−XX Only)
The FIELD TEST pins (E33) on the heat pump control can
be jumpered between 1 to 2 seconds to bypass delay.
HIGH (S4) AND LOW (S87) PRESSURE SWITCHES
The unit’s pressure switches (LO PS − S87 and HI PS − S4)
are factory−wired into the control on the LO−PS and HI−PS
terminals, respectively.
Low Pressure Switch (LO−PS) See figure 43 for low
pressure switch sequence of operation.
High Pressure Switch (HI−PS) See figure 44 for high
pressure switch sequence of operation.
Pressure Switch Event Settings
The following pressures are the auto reset event value trig-
gers for low and high pressure thresholds:
High Pressure (auto reset) − trip at 590 psig; reset at
418.
Low Pressure (auto reset) − trip at 25 psig; reset at 40.
COMPRESSOR PROTECTION FIVE−STRIKE
LOCKOUT SAFETY FUNCTION
The five−strike lockout safety function is designed to pro-
tect the unit’s compressor from damage. The five−strike
feature is used for high pressure (S4) and low (S87) pres-
sure switch trips, high discharge temperature (RT28)
sensor input and W input fault or miswire.
Resetting Five−Strike Lockout
Once the condition has been rectified, power to the heat
pump control’s R terminal must be cycled OFF, or a jumper
placed on the FIELD TEST pins between 1− to 2−seconds to
reset the heat pump control.
Defrost System
The heat pump control (A175) measures differential tem-
peratures to detect when the system is performing poorly
because of ice build−up on the outdoor coil. The controller
self−calibrates (see figure 46) when the defrost system
starts and after each system defrost cycle. The heat pump
control monitors ambient temperature, outdoor coil tem-
perature, and total run−time to determine when a defrost
cycle is required. The coil temperature sensor is designed
with a spring clip to allow mounting to the outside coil tub-
ing. The location of the coil sensor is important for proper
defrost operation (see figure 1 for location of coil sensor).
NOTE The heat pump control accurately measures the
performance of the system as frost accumulates on the
outdoor coil. This typically will translate into longer running
time between defrost cycles as more frost accumulates on
the outdoor coil before the heat pump control initiates de-
frost cycles.
DEFROST OPERATING MODES
The heat pump control has three operational modes which
are:
Defrost calibration and operation (see figure 46)
Defrost test (see figure 25)
DEFROST TERMINATION TEMPERATURES
The heat pump control selections are: 50, 70, 90, and
100°F (10, 21, 32 and 38°C). The jumper termination pin is
factory set at 50°F (10°C).
If the temperature jumper is not installed, the termination
temperature is 90°F (32°C). See figure 46 for on how this
settings affects defrost calibration and defrost modes.
NOTE − Colder climates could require a high discharge ter-
mination temperature setting to maintain a clear coil.
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