Freeze Protection Sensors; Thermostatic Expansion Valves; Txv Failure - Carrier Aquazone 50PTH024 Installation, Start-Up And Service Instructions Manual

Lockout Reset
A hard lockout can be reset by turning the unit thermostat off and
then back on when the RESET DIP switch is set to "Y" or by shut-
ting off unit power at the circuit breaker when the RESET DIP
switch is set to "R".
Selectable Alarm Mode
The UPM board can be configured to have either a constant or
pulse signal. If constant (CONT) is selected the UPM will provide
a closed contact at the alarm output until the alarm is cleared. If
pulse (PULSE) is selected the UPM will sequence the alarm con-
tact with the fault LED flashes.
Test Mode (TEST)
In test mode the ASC and Random Start time delays are reduced
(5 seconds and 10 seconds respectively), and serve no function to
the end user equipment. The alarm and display relays also pulse
for both soft and hard lockout conditions, and are both cleared
through a manual reset.
UPM Sequence of Operations
Figure 36 shows the UPM sequence of operations.

Freeze Protection Sensors

The control system employs 2 nominal 10,000 ohm thermistors
(FP1 and FP2) that are used for freeze protection. Be sure FP1 is
located in the discharge fluid and FP2 is located in the air dis-
charge. (See Fig. 43.)

Thermostatic Expansion Valves

Thermostatic expansion valves (TXV) are used as a means of me-
tering the refrigerant through the evaporator to achieve a preset su-
perheat at the TXV sensing bulb. Correct superheat of the refriger-
ant is important for the most efficient operation of the unit and for
the life of the compressor.
Packaged heat pumps typically use one bi-flow TXV to meter re-
frigerant in both modes of operation. When diagnosing possible
TXV problems it may be helpful to reverse the refrigerant flow to
assist with the diagnosis.
Geothermal and water source heat pumps are designed to operate
through a wide range of entering-water temperatures that will
have a direct effect on the unit refrigerant operating pressures.
Therefore, diagnosing TXV problems can be difficult.
Air
Coil
Airflow
(°F)
Condensate
Overflow
(Co)
LEGEND
COAX — Coaxial Heat Exchanger
Airflow
Refrigerant Liquid Line Flow
Airflow
(°F)
Expansion
Valve
Fp 2 Fp 1
Liquid
Line
Water In
Air Coil
Water Coil
Freeze
Freeze
Protection
Protection
Fig. 43 — FP1 and FP2 Thermistor Location

TXV FAILURE

The most common failure mode of a TXV is when the valve fails
while closed. Typically, a TXV uses spring pressure to close the
valve and an opposing pressure, usually from a diaphragm, to
open the valve. The amount of pressure exerted by the diaphragm
will vary, depending on the pressure inside of the sensing bulb. As
the temperature of and pressure within the bulb decreases, the
valve will modulate closed and restrict the refrigerant flow
through the valve. The result is less refrigerant in the evaporator
and an increase in the superheat. As the temperature at the bulb in-
creases the diaphragm pressure will increase, which opens the
valve and allows more refrigerant flow and a reduction in the su-
perheat.
If the sensing bulb, connecting capillary, or diaphragm assembly
are damaged, pressure is lost and the spring will force the valve to
a closed position. Often, the TXV will not close completely so
some refrigerant flow will remain, even if there is inadequate flow
for the heat pump to operate.
The TXV sensing bulb must be properly located, secured, and in-
sulated as it will attempt to control the temperature of the line to
which it is connected. The sensing bulb must be located on a dedi-
cated suction line close to the compressor. On a packaged heat
pump, the bulb may be located almost any place on the tube run-
ning from the compressor suction inlet to the reversing valve. If
the bulb is located on a horizontal section, it should be placed in
the 10:00 or 2:00 position for optimal performance.
Use caution when tightening the strap. The strap must be tight
enough to hold the bulb securely but caution must be taken not
to over-tighten the strap, which could dent, bend, collapse or
otherwise damage the bulb.
The bulb must be secured to the pipe using a copper strap. The use
of heat transfer paste between the bulb and the pipe will also help
ensure optimum performance.
The bulb must also be properly insulated to eliminate any influ-
ence on valve operation by the surrounding conditions. Cork tape
is the recommended insulation as it can be molded tight to the
bulb to prevent air infiltration.
Coax
Water Out
56
CAUTION
Suction
Compressor
Discharge