How to Determine If a TXV is Faulty

Expansion valves are commonly used in most heat pumps and air conditioners manufactured today. This has been one of the components used to increase the efficiency of equipment over the years. Troubleshooting an expansion valve has become something an HVAC technician needs to be familiar with. You will use it frequently with air conditioners and heat pumps. 

A typical failure of an expansion valve is a valve that is mostly stuck closed. Other failures, though, can be stuck open or a “hunting” valve.

Symptoms of a failed valve vary but will include one or more of these:

  1. Low refrigerant suction pressure
  2. High refrigerant superheat
  3. TXV frosting or frosting after the expansion valve
  4. Equalizer line frosting

Symptoms that may mimic a failed TXV but may be caused by something else entirely are:

  1. Proper air flow/water flow and out of range air or water temp in heating
  2. Proper air temperature in cooling mode
  3. In heating an overly frosted outdoor coil or in geothermal a scaled/dirty water heat exchanger
  4. Under charge of refrigerant

How to Diagnose the TXV

To properly diagnose an expansion valve, you will need a set of gauges, a thermometer, a P-T refrigerant chart if it isn’t on your gauges, and the manufacturer's requirement of subcooling and possibly superheat.

You will need to measure your subcooling and superheat to determine the function of the expansion valve. You will need to connect to the high- and low-pressure sides of the refrigerant circuit. You will need to take 2 temperature measurements. 

Determining Superheat and Subcooling

To determine superheat, you will need to measure the temperature on the suction line near the compressor. Subtract the temperature recorded from the saturation temperature that correlates to the low side pressure reading.

To determine subcooling, you must read the temperature of the pipe feeding the expansion valve before. In cooling mode, this would be between the exit of the outdoor coil and before the cooling expansion valve. In heating mode on a heat pump, this would be on the pipe between the exit of the indoor coil and the heating expansion valve (outdoor valve in an air-to-air heat pump). 

For a geothermal package unit in cooling mode, you would read the temperature between the expansion valve and the water-to-refrigerant heat exchanger. You would read the temperature between the expansion valve and the air coil for geothermal in heating mode. Now subtract that temperature from the saturation temperature that correlates with the high side pressure reading.

Now that you have an accurate full picture of what is going on with the refrigerant circuit, refer to the chart below for diagnosis.

Currently, manufactured geothermal units typically have thermostatic expansion valves. The expansion valves used in packaged geothermal units are very similar in operation to other expansion valves. However, two major differences should be noted. They are designed for an extended range of incoming water temperatures and meter refrigerant flowing either way. Typical air-to-air heat pumps have bi-flow valves that may flow both ways but don’t meter both ways. If you are replacing an expansion valve in a packaged geothermal heat pump with an aftermarket valve, make sure you use a bi-metering expansion valve. The symptom of an incorrect valve will be that it will work in either heating or cooling but then will not meter the other mode.

To learn more, watch this video from Enertech on troubleshooting TXVs as a part of their Geothermal Troubleshooting series.

Contact the B-Y hydronics specialist with any questions about troubleshooting TXVs for your job.