Thermostatic Expansion Valves – Part 2


Hopefully, you have read my previous posting and understand how a TXV works?  It is important that you understand how it works so you can use that knowledge to diagnose the TXV.

A few things to check regarding the installation of the TXV before doing the troubleshooting — (1) is the TXV bulb securely attached and insulated? (2) Is it in the correct position on the suction line? Never put the bulb at the 6 o’clock position–it may sense the temperature of the oil instead of the refrigerant. It is recommended it be at the 10:00 or 2:00 o’clock or 4:00 -8:00 o’clock position. Do not install on any fittings. Locate the bulb on a free-draining suction line. If it on a vertical line, is the bulb mounted “tail end UP” to keep the small refrigerant charge in the bulb? (3) Is the equalizer line “open” allowing the pressure to relieve into the suction line?

If the TXV is properly installed and the bulb is well insulated, then we can look at Troubleshooting the TXV.  Subcooling and superheat don’t tell you everything, BUT they are incredibly useful diagnostic tools. Along with other things you learn about the system, like is there too much or too little air flow, they help give a complete picture of how the system is operating. Checking the superheat is the first step in a simple and systematic check of the TXV.

Remember: 

If not enough refrigerant is being fed into the evaporator — superheat will be HIGH!

If too much refrigerant is being fed into the evaporator — the superheat will be LOW!

Next, we need to check the subcoolingSubcooling tells you how much of the condenser coil you are using to make liquid from gas. Ideally, we want to use most of the condenser coil to accomplish this. Once all the gas is condensed to a liquid the temperature begins to drop very quickly. (see my post on subcooling) 

Using these ideas, you can tell a great deal about how the system is operating and if the system is properly charged.  Too much refrigerant increases subcooling and makes the compressor work too hard.  Not enough refrigerant decreases the subcooling and increases the superheat because there is not enough refrigerant and the evaporator coil starves.

Also, keep in mind that other things will affect superheat  and subcooling. Air flow is critical (see the posting on air flow). Restrictions in the refrigerant piping — plugged driers or strainers, lines undersized, long line applications, liquid line vertical lift, and believe it or not — partially closed service valves. :>)

The point I am making is air conditioners are not just  ” BTU’s in a box” – they are systems and all components need to work together for the system to function properly.

I wish to acknowledge both Sporlan and Alco for the information in this posting.  It is from their publications that a lot of this information has been derived so I could present it to you in an understandable fashion.

Attached is TXV troubleshooting directly out of the Sporlan TXV manual. Also attached is a “quick diagnostic chart” you can use. Both addresses different scenarios so you can determine how to check and what to check for when diagnosing the TXV. Just click on either hyperlink below for a printable copy.

TXV Quick Diagnostic Chart

Sporlan TXV troubleshooting guide

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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One Response to Thermostatic Expansion Valves – Part 2

  1. Sergio says:

    Excellent information!

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