Thermostatic Expansion Valves (TEV or TXV), one of the most popular metering devices for residential, high-efficient air conditioning and heat pumps, have performed almost impeccably for decades.  Unfortunately, some manufacturers in the past few years have identified batches of valves that have had high superheat issues.  Have we just been blind for so long, not realizing these problems existed?  Or have these valves really been so reliable for such a long time?

  If you think you may have a TXV issue, there is a very simple test that can be completed using just crushed ice and a set of accurate gauges:

  1. With the system operating, attach your accurate/calibrated manifold.
  2. Detach your TXV's sensing bulb and submerse completely into crushed ice.  (Caution: not just ice water, must be 32F).  I recommend using an insulated cup!
  3. Your Saturation Temperature of the Evaporator should be (32F - TXV Superheat).  Example:  Your R-410A TXV has a desired Superheat of 8F.  32F - 8F = 24F Saturation Temp
  4. Using your Pressure/Temperature Saturation Chart, convert Temperature to Pressure for the refrigerant used (Fig. 1).  The Suction pressure should be relatively close to this value.  

Saturation Chart

Sporlan Recommendations, Bulletin 10-9

If outside of the acceptable range (Fig.2), adjustment or replacement is recommended. 

When adjusting TXV Superheat, remember that you make a single turn at a time.  Changes to the TXV Superheat can take as much as 30 minutes of system operation to be measured.

To Reduce Superheat: Turn valve stem COUNTER-CLOCKWISE.

To Increase Superheat: Turn valve stem CLOCKWISE.

  There are many possibilities that could cause high superheat, besides a faulty TXV.  By using this method, you could save yourself some serious service time needlessly replacing a TXV during the busiest time of year!

http://excessair.blogspot.com/2014/07/how-do-you-test-txv.html

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Comment by John Proctor on July 28, 2014 at 11:36pm

Correction 32 is the melting point, not the saturation temperature.

Comment by John Proctor on July 24, 2014 at 9:19am

Oh yes one more item. There has to be a combination of water and ice to achieve 32 degrees F. That is the saturation temperature. Ice alone can be much colder than 32 degrees F. ProctorEng.com

Comment by John Proctor on July 24, 2014 at 9:15am

Thank you for the nice post. Remember that the important thing is the actual superheat that exists entering the compressor is what we really want to control. Superheat serves only one positive purpose -- protecting the compressor from slugging. Any superheat added will have to be removed at the condenser. We specify that you measure the "system superheat" the temperature difference between the evaporator saturation temperature and the suction line temperature entering the outdoor unit. If the result is outside your target you then need to look at the reason. It may be the adjustment or accuracy of the TXV as noted above or it may be connection between the suction line sensing bulb and the suction line (orientation, contact, insulation). There are other less likely possibilities as well.

Be very careful in measuring the suction line temperature -- most clamp on sensors have significant errors under certain circumstances. Here is an abstract of a proposed ASHRAE paper concerning this issue.

Causes of Misdiagnoses -- Refrigerant Pipe Clamp Temperature Sensor Errors 

Accurately measuring the refrigerant temperatures in an air conditioning system is of great importance in system diagnostics. Having inconsistent or incorrect temperature readings in the liquid and suction lines of an air conditioning system can lead to misdiagnosis and lower operating efficiencies.

Building energy codes like California Title 24 specify that refrigerant line temperature measuring devices meet certain instrumentation specifications. These specifications apply to the accuracy of the measurement and may also specify a maximum response time for the device to produce readings within the specified accuracy.  

This paper summarizes both accuracy and response time of various commercially available pipe clamp thermocouples, Type K thermocouples and thermistors.  Tests on various refrigerant lines of a package air conditioner and on a refrigerant line test apparatus at the laboratory were conducted to measure the accuracies and response times of these devices.

Accuracy tests conducted on a package unit air conditioner show that temperature differentials of around 100ºF between the refrigerant line and ambient produce errors in line temperature measurements in excess of 10ºF compared to the actual temperature of the refrigerant line.

Response time tests illustrate that a temperature differential of 40 ºF produced thermistor response times of 12 minutes to reach within 1.3ºF of the actual refrigerant line temperature. Pipe clamp thermocouple response times were between 2-4 minutes depending on the model.

This paper recommends that changes be made in the most common refrigerant line temperature sensing devices to alleviate these problems. 

 

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