TXV Troubleshooting Guide
Diagnose and resolve thermostatic expansion valve problems
Quick Symptom Diagnosis
How a TXV Works
A thermostatic expansion valve regulates refrigerant flow by maintaining a constant superheat at the evaporator outlet. It operates through three forces:
- Bulb Pressure (P1): Pushes the valve OPEN. Generated by the sensing bulb measuring suction line temperature.
- Evaporator Pressure (P2): Pushes the valve CLOSED. Internal or external equalizer pressure.
- Spring Pressure (P3): Pushes the valve CLOSED. Provides the superheat setting.
Balance Equation: P1 = P2 + P3
When bulb pressure exceeds evaporator + spring pressure, the valve opens more.
TXV Stuck Closed / Underfeeding
Symptoms
- ● High superheat (>15-20°F)
- ● Low suction pressure
- ● Reduced cooling capacity
- ● Warm evaporator outlet
- ● High discharge temperature
Possible Causes
- ▸ Lost bulb charge
- ▸ Plugged inlet screen/strainer
- ▸ Moisture freezing at seat
- ▸ Wax from oil breakdown
- ▸ Spring set too high
Diagnostic Test
Warm the sensing bulb with your hand (or a heat gun on low). If the TXV is working, suction pressure should rise and superheat should drop within 30-60 seconds. If no response, the valve or bulb charge is defective.
TXV Stuck Open / Overfeeding
Symptoms
- ● Low or zero superheat
- ● High suction pressure
- ● Frosted or sweating suction line
- ● Liquid slugging to compressor
- ● High compressor current
Possible Causes
- ▸ Bulb loose or poorly positioned
- ▸ Bulb not insulated (outdoor)
- ▸ Debris holding valve open
- ▸ Spring broken or weak
- ▸ Superheat set too low
Diagnostic Test
Cool the sensing bulb with ice or spray coolant. The TXV should close, suction pressure should drop, and superheat should increase. If no response, check bulb mounting or replace the TXV.
TXV Hunting / Oscillating
Symptoms
- ● Suction pressure cycles up/down
- ● Superheat swings widely (3-20°F)
- ● Intermittent frost on suction
- ● Clicking sound from TXV
- ● Inconsistent cooling
Possible Causes
- ▸ Oversized TXV for load
- ▸ Bulb not in good contact
- ▸ Bulb affected by air flow
- ▸ Flash gas in liquid line
- ▸ External equalizer plugged
Solutions
- • Ensure bulb is at 10-4 o'clock position on horizontal suction line
- • Insulate the bulb and capillary tube
- • Check for proper subcooling at TXV inlet
- • Verify external equalizer is connected and clear
- • Increase superheat setting 1-2 turns if needed
Partial Restriction / Contamination
Symptoms
- ● Moderately high superheat
- ● Temperature drop across TXV inlet
- ● Frost at TXV inlet (severe)
- ● Normal head pressure
- ● Reduced system capacity
Possible Causes
- ▸ Clogged inlet screen
- ▸ Sludge or debris in orifice
- ▸ Moisture/ice at restriction
- ▸ Brazing flux contamination
- ▸ Acid from burnout
Diagnostic Test
Measure temperature before and after the TXV inlet. More than 2-3°F drop indicates a restriction before the orifice. If restriction is from ice (moisture), system will work normally until ice forms, then starve. Heat the inlet temporarily - if superheat drops, moisture is present.
Target Superheat by Application
Always refer to manufacturer specifications. Superheat is measured at evaporator outlet.
Proper Bulb Mounting
Horizontal Suction Line:
- • Large lines (7/8"+): 4-5 o'clock
- • Medium lines: 10 or 2 o'clock
- • Small lines: 12 o'clock
General Rules:
- • Clean copper surface
- • Good thermal contact
- • Use thermal paste if needed
- • Insulate bulb and tube
- • Mount after oil trap
TXV Adjustment
CW (Clockwise): Increases superheat (less refrigerant flow)
CCW (Counter-clockwise): Decreases superheat (more refrigerant flow)
Caution: Adjust in 1/4 turn increments. Wait 10-15 minutes between adjustments for system to stabilize. Never force adjustment screw.
When to Replace TXV
- Lost bulb charge (no response to temp)
- Seat erosion (can't achieve superheat)
- Internal damage from compressor burnout
- Wrong refrigerant/charge type
- Excessive hunting that can't be fixed