Reply · Sep 11, 2026
This is a classic commercial refrigeration headache, and honestly, the transition from HVAC to refrigeration catches a lot of great techs off guard. In HVAC, subcooling and superheat tell the whole story. In refrigeration, receiver systems and head pressure controls (Headmasters) throw a wrench into those standard diagnostics.
Looking at your numbers, you are 100% correct that your evaporator is severely starved. With a 29°F superheat and a staggering **1°F Evap TD** (20°F return / 19°F supply), the refrigerant is flashing off immediately at the valve inlet, and almost zero boiling is happening through the coil itself.
Here is a breakdown of what is likely staring you in the face.
### Suspect #1: The Non-Removable TXV Screen
You mentioned you rebuilt the TXV but the inlet screen "could not be removed" so you just purged it. **This is your most likely culprit.**
In commercial systems, if a Sporlan or Alco inlet screen is so jammed in the valve body that you cannot pull it out with a pick, it is usually friction-welded in place by a dense wall of copper plating, sludge, and debris.
* Purging with nitrogen or refrigerant will not dislodge the fine particulate packed into that mesh.
* If the screen is restricted, it acts as a primary metering device before the actual TXV orifice. The refrigerant flashes to a vapor before it even hits the new cartridge you installed, completely starving the coil.
* **The fix:** You may need to bite the bullet, sweat out the entire valve body, and replace it.
### Suspect #2: The Sight Glass & The Charge
You have 4°F of subcooling leaving the receiver. In an AC unit, low subcooling means a low charge. But on a refrigeration unit with a receiver, subcooling isn't your primary charge indicator—**the sight glass is.**
A receiver holds liquid and vapor in equilibrium, so liquid leaving the receiver naturally has near-zero subcooling. You only measure subcooling on these systems to verify the liquid line isn't picking up too much ambient heat before the TXV.
* **Is the sight glass perfectly clear?** If there are bubbles, your TXV is being fed vapor.
* You added "fresh R404a," but receiver systems with a Headmaster require a very specific "flooded" charge to operate in low ambients. If the system was just topped off until the pressures looked okay, it might still be short of the volume needed to maintain a full liquid seal at the bottom of the receiver. If the receiver dip tube starts pulling vapor, the evaporator dies.
### Suspect #3: The Liquid Line Solenoid (LLSV) or Distributor Nozzle
Since you replaced the Headmaster and filter drier, the liquid line restriction has to be further down the line.
* **The LLSV:** Often overlooked by HVAC techs. If the solenoid diaphragm is failing or the coil is weak, it won't open fully. Check for a temperature drop across the valve. Even a 2°F drop means it's restricted.
* **The Distributor Nozzle:** Right after the TXV is the distributor orifice. If you had enough debris in the system to permanently lock the TXV screen in place, it's highly possible a piece of debris made it past the screen and plugged the distributor nozzle.
### Suspect #4: The TXV Powerhead
When you rebuilt the TXV (new cartridge, new adjustment screw), did you replace the thermostatic element (powerhead)? If the powerhead bulb lost its charge, the internal spring will push the valve almost completely closed regardless of where you set the adjustment screw.
### Your Action Plan
1. **Check the sight glass** while the box is at 20°F. If it's flashing, weigh out the charge and weigh in the factory nameplate charge plus the calculated winter Headmaster flood-back charge.
2. **Check for a temp drop** across the Liquid Line Solenoid.
3. **Address that stuck screen.** If the sight glass is clear and the LLSV is wide open, that non-removable screen (or a plugged distributor nozzle) is your restriction. The valve body will likely need to be replaced.
Reply · Sep 11, 2026
Sbukosky,
Thank you for the reply. I love the education with the action and not just a check list of items to verify.
In my homework of this issue, I did change the power head on the TXV and maybe I just need to change the valve body if everything else checks out. Also I moved the sensing bulb to the inside of the evaporator to get it off a brazed connection and attached it after the equalizing tube.
I did check temperature across the LLSV but was not aware that 2 degrees is significant to determine if its failed. I'll verify that temperature to be sure.
Now I may ruffle some feathers with this question. I've come across a lot of debate on using a sight glass in troubleshooting. In the sense that you see bubbles you have a problem. I was taught the only item to verify on a sight glass is moisture and that bubbles are not necessarily an indication of any issues. In the refrigeration mindset it sounds like bubbles are a indication of charge issues or maybe a restriction. Would that be correct?
In the unit I'm working on I have no charge on the nameplate so I charged according to suction saturation temperature aiming for about -20. I'll do more homework on charging refrigeration with calculating the flood back charge.
Thank you again. I know there's only so much homework books will tell you and experience can teach you what the books can't. I appreciate sharing your experience. I ran out of book knowledge then this forum came up and I'm glad I asked.
Reply · Sep 17, 2026
Update: I first had to go work on some HVAC to restore my confidence in that I do know what I'm doing.
I did confirm no temperature drop across the LLSV and did have bubbles in the sight glass. I replaced the TXV valve body and purged the evaporator from the outlet of the evaporator to clear the feeder tubes. Reread the manuals, confirmed what the factory charge should be a long with the head master winter charge. System has now been charged with the correct amount of refrigerant. Today my box temperature was 10 degrees, and a supply temperature of 8 degrees. I've got 20 degrees of superheat so far, but the discharge pressure is at a saturation temperature of 116 degrees with a suction saturation of -12 degrees. I still have bubbles in the sight glass so with the discharge pressure so high, I'm unsure of adding more refrigerant. It seems like the freezer is working so I'll continue to monitor it to see if it finally gets to - 5 box and if the sight glass finally clears. Thanks again for the replies.
