TXV

 

 1. What is a TXV?

A TXV (Thermostatic Expansion Valve) controls the amount of refrigerant entering the evaporator coil while maintaining a constant superheat. In simple terms, the TXV ensures that refrigerant is fully evaporated by the time it reaches the end of the evaporator coil.

2. Common Symptoms of a Bad TXV

• Low suction pressure: Many technicians assume that when suction pressure is low, they need to add more refrigerant. However, a properly functioning TXV may throttle down refrigerant flow as more refrigerant is added, leading to the mistaken belief that the TXV is faulty.

3. Key Components of a TXV

• Bulb: Senses the temperature at the evaporator outlet and adjusts refrigerant flow. When the bulb gets warmer, it allows more refrigerant to flow; when cooler, it restricts the flow.

• External Equalizer: Measures pressure at the evaporator outlet to control refrigerant flow.

• Spring: Another force controlling refrigerant flow, which can be adjusted in some valves.

4. Reasons for TXV Malfunction

1. Bulb Issues: If the bulb is damaged and loses its internal refrigerant, the valve may fail to open, restricting refrigerant flow.

• How to Diagnose: Warm the bulb by hand or place it in warm water and observe if suction pressure increases. If there is no change, the bulb might be faulty.

2. Insufficient Refrigerant Supply: If the TXV doesn't receive a full supply of liquid refrigerant, it can't function properly.

• How to Diagnose: Measure subcooling to check if enough liquid refrigerant is reaching the valve.

3. Airflow Problems: Inadequate airflow can reduce the effectiveness of the evaporator, leading to low pressure that might be mistaken for a TXV problem.

• How to Diagnose: Check air filters, ducts, and airflow conditions to ensure proper ventilation.

4. Blockages or Restrictions: If the TXV's inlet screen or other parts are clogged, it can limit refrigerant flow.

• How to Diagnose: Disassemble the valve and clean or replace any blocked screens.

5. Diagnostic Procedure

1. Measure Superheat and Subcooling: If suction pressure is low, check superheat and subcooling inside and outside. For a functioning TXV, superheat should be in the 8-14°F range.

2. Bulb Test: Warm the bulb by hand or with warm water to see if suction pressure rises. If it does, the valve is functioning. If there's no change, the bulb may be faulty.

3. Check Refrigerant Flow: Ensure the TXV is receiving a full flow of liquid refrigerant. Subcooling measurements or listening for abnormal sounds in the refrigerant lines can help identify if there's vapor instead of liquid entering the valve.

4. Check Airflow: Inspect air filters, ducts, and overall airflow. Poor airflow could be the root cause of low suction pressure, not the TXV.

6. Common Mistakes

• Adding Refrigerant: If superheat is low, adding refrigerant is the wrong approach. Low superheat indicates the evaporator coil is already receiving enough refrigerant, and adding more will only worsen the situation.

• Incorrect Measurements: Using improperly calibrated tools or measuring inaccurately can lead to incorrect diagnoses. Always ensure tools are properly set before making any conclusions.

7. Exercising the Valve

In some cases, manually “exercising” the valve (opening and closing it) can temporarily fix the issue, but replacing the valve is usually required due to internal contamination or damage.