31
Causes of TXV Failure — The most common causes of TXV
failure are:
1. A cracked, broken, or damaged sensing bulb or capillary
can be caused by excessive vibration of the capillary dur-
ing shipping or unit operation.
If the sensing bulb is damaged or if the capillary is
cracked or broken, the valve will be considered failed and
must be replaced. Replacement of the TXV “power head”
or sensing bulb, capillary, diaphragm assembly is possi-
ble on some TXVs. The power head assembly screws
onto most valves, but not all are intended to be replace-
able. If the assembly is not replaceable, replace the entire
valve.
2. Particulate debris within the system can be caused by sev-
eral sources including contaminated components, tubing,
and service tools, or improper techniques used during
brazing operations and component replacement.
Problems associated with particulate debris can be com-
pounded by refrigerant systems that use POE (polyol es-
ter oil). POE oil has solvent-like properties that will clean
the interior surfaces of tubing and components. Particu-
lates can be released from interior surfaces and may mi-
grate to the TXV strainer, which can lead to plugging of
the strainer.
3. Corrosive debris within the system may happen after a
failure, such as a compressor burn out, if system was not
properly cleaned.
4. Noncondensables may be present in the system. Non-
condensables includes any substance other than the
refrigerant or oil such as air, nitrogen, or water. Contami-
nation can be the result of improper service techniques,
use of contaminated components, and/or improper evacu-
ation of the system.
Symptoms — The symptoms of a failed TXV can be varied
and will include one or more of the following:
• Low refrigerant suction pressure
• High refrigerant superheat
• High refrigerant subcooling
• TXV and/or low pressure tubing frosting
• Equalizer line condensing and at a lower temperature
than the suction line or the equalizer line frosting
• FP1 faults in the heating mode in combination with any
of the symptoms listed above
• FP2 faults in the cooling mode in combination with any
of the symptoms listed above. Some symptoms can
mimic a failed TXV but may actually be caused be
another problem.
Before conducting an analysis for a failed TXV the follow-
ing must be verified:
• Confirm that there is proper water flow and water tem-
perature in the heating mode.
• Confirm that there is proper airflow and temperature in
the cooling mode.
• Ensure coaxial water coil is clean on the inside; this
applies to the heating mode and may require a scale
check.
• Refrigerant may be undercharged. To verify, subcooling
and superheat calculations may be required.
Diagnostics — Several tests may be required to determine if
a TXV has failed. The following tools may be required for
testing:
1. Refrigerant gage manifold compatible with the refriger-
ant in the system.
2. Digital thermometer, preferably insulated, with wire leads
that can be connected directly to the tubing.
3. Refrigerant pressure-temperature chart for the refrigerant
used.
To determine that a TXV has failed, verify the following:
• The suction pressure is low and the valve is non-respon-
sive. The TXV sensing bulb can be removed from the
suction line and warmed by holding the bulb in your
hand. This action should result in an increase in the suc-
tion pressure while the compressor is operating. The
sensing bulb can also be chilled by immersion in ice
water, which should result in a decrease in the suction
pressure while the compressor is operating. No change in
the suction pressure would indicate a nonresponsive
valve.
• Simultaneous LOW suction pressure, HIGH refrigerant
subcooling and HIGH superheat.
• LOW suction pressure, LOW subcooling and HIGH
superheat may indicate an undercharge of refrigerant.
HIGH subcooling and LOW superheat may indicate an
overcharge of refrigerant. The suction pressure will usu-
ally be normal or high if there is an overcharge of refrig-
erant.
SUCTION
COMPRESSOR
DISCHARGE
COAX
EXPANSION
VALVE
FP2
FP1
LIQUID
LINE
WATER IN
WATER OUT
CONDENSATE
OVERFLOW
(CO)
AIR COIL
FREEZE
PROTECTION
WATER
COIL
PROTECTION
THERMISTOR
(
°
F)
(
°
F)
AIR
COIL
AIRFLOW
AIRFLOW
LEGEND
Fig. 25 — FP1 and FP2 Thermistor Location
COAX
—
Coaxial Heat Exchanger
Airflow
Refrigerant Liquid Line Flow
