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The question we are looking to answer is, should we install the TXV bulb before (upstream) or after (downstream of) the external equalization port?
The first thing to know is that the TXV controls the amount of refrigerant flowing into the evaporator coil. The TXV is a metering device that restricts the flow of refrigerant. The TXV measures and adjusts to maintain a specific superheat. It does this by using three pressures.
TVX Bulb Pressure (opening force)
External Equalization Pressure (closing force)
Spring Pressure (closing force)
If you want to know more on exactly how the TXV works before getting into the Bulb and equalizer locations, check out this video- https://youtu.be/WkgshvMOx00
Let's explore how the position of the bulb, in comparison with the external equalization line, affects the amount of refrigerant the TXV allows into the evaporator coil. The concern really comes from a thought that maybe the TXV will allow liquid refrigerant from the outlet of the TXV to back feed around the internal pin(s) and allow liquid refrigerant into the vapor line. This small amount of liquid entering the vapor line right before the bulb could then cause a change in temperature where the bulb is sensing the temperature at.
TXV Bulb Upstream of the External Equalization Line Port
This is the location where most manufacturers install the TXV bulb. No matter if the TXV internally leaks, the external equalization tube does not affect the bulb measuring the temperature of the line because it is downstream of the bulb.
TXV Bulb Downstream of the External Equalization Port
If the TXV internally leaks, the refrigerant from the external equalization tube will affect the bulb measuring the temperature of the line because the bulb is downstream of the equalizer.
Here is the long and short of this long debated issue. The reality is that any leaking around the pin(s) internally within the TXV would be so minimal that it would really not affect the temperature of the vapor line tube at this point where the external equalizer is connected. I have seen manufacturers install the bulb and equalizer in both ways and actually not even wrap insulation around the bulb. If you are going to do something, add insulation around the bulb so that the bulb is only measuring the temperature of the vapor line and not the air surrounding the bulb.
You can install the bulb upstream of the external equalization line to keep everyone happy because this seems to be the most accepted location by all teachers and manufacturers. In this way, the TXV would be reading and adjusting the true Superheat across the evaporator coil.
The bulb should be fastened to the suction line with a copper or stainless steel strap or clamp for good surface contact but make sure that if you use a stainless steel hose clamp in lieu of a copper one, do not overtighten the bulb or tube. Usually 8-10 or 2-4 o'clock are the best positions for a bulb mounted on the horizontal suction tube. If you are mounting the bulb on a vertical tube, make sure that the small capillary lines coming off of the bulb are faced upwards. Make sure to insulate the bulb, as failure to do this can drastically affect the ability of the TXV to calculate the evaporator coil superheat especially if the bulb is mounted outside of the evaporator coil box.
To learn more about Superheat, read this article!
If you want to learn about refrigerants and how they work in a system, check out our “Refrigerant Charging and Service Procedures for Air Conditioning” book . Test your knowledge with our 1,000 question workbook along with the answer key! We also have quick reference cards for use out in the field! Bundle Packs are a great way to save and get faster shipping! Check out www.acservicetech.com/store
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Published: 05/26/2021 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 16 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel & https://www.facebook.com/acservicetech/
