Bryan gives a masterclass to Kalos Techs on why TXV/TEV valves are misdiagnosed, how to properly diagnose a failed TXV, and how thermostatic expansion valves work.

TXVs are designed to maintain constant superheat, and many people suspect TXV issues when the suction pressure is low. When technicians see low suction pressure, many of them may add charge and then realize that the TXV throttles down to maintain constant superheat. So, they condemn the TXV even though it’s working as it should because they simply don’t understand how the TXV works.

TXVs generally have an external equalizer, which supplies a closing force to the TXV. The spring also supplies a closing force, but the sensing bulb applies an opening force. The size of the TXV’s orifice depends on the combination of the opening and closing forces. There is a fixed amount of refrigerant inside the sensing bulb, so the hotter the bulb, the more the refrigerant expands. The more the refrigerant expands, the more opening force it will exert on the TXV. When the bulb doesn’t have enough (or any) refrigerant, it can’t exert enough force on the valve to open it. However, you may be able to replace the powerhead without condemning the entire TXV.

The TXV needs a full line of liquid coming into the valve for it to work properly, so we need to check the subcooling or look at a sight glass (in the case of refrigeration). We can get vapor in the liquid line from restrictions or excessively long lines.

TXVs may also have an inlet screen, which can become restricted and prevent the proper amount of refrigerant from reaching the valve. The TXV also needs a proper pressure drop (about 100 PSI); without that pressure drop, the TXV can’t do its job properly.

When the suction pressure is low, the technician should take the subcooling and superheat inside and outside. The goal is to look for the temperature differences between the inside and outside measurements. If the differences are drastic, then there could be a restriction somewhere (often in the line drier or an airflow restriction).

If suction pressures are low and the temperatures are close to freezing, the next step is to take the remaining measurements of your 5 Pillars (superheat, subcooling, delta T, suction pressure, and head pressure). Use those to diagnose the problem; the superheat at the evaporator coil should be 6-14 degrees, and it’s usually not productive to condemn the TXV over a couple of degrees due to the possible inaccuracy of our tools. Instead, it’s more likely that you have an airflow problem.

However, if the outdoor superheat is 27 degrees and the indoor superheat is 23 degrees, then you’ll have to check for restrictions. If you don’t have any restrictions (which may cause low head pressure) and have sufficient subcooling and pressure drop, then it would be safe to condemn the TXV.

When taking measurements, make sure your test instruments are calibrated and work properly. Otherwise, you could misdiagnose a TXV problem.

Moisture, carbon, and other forms of contamination can lead to inlet screen restrictions and possibly even premature TXV failure. If the superheat is high, you can temporarily open the valve more by increasing the suction pressure by placing the bulb somewhere else. NEVER open the valve more if you have low superheat! If you have low superheat, try to see how well the bulb is connected to the suction line; for the TXV to work properly, the bulb needs to be strapped in a way that makes it have good contact with the suction line. In cases where the bulb needs to be insulated, make sure it is insulated.

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41 thoughts on “How to Properly Diagnose a Failed TXV

  1. jon240zx1 says:

    Great video. Add a hot gas bypass system with liquid quenching to this and troubleshooting gets even more interesting. Then throw in flooded condenser and it's a party.

  2. sJ air says:

    Has anyone ever noticed subcooling low and super heat low when first firing off an HVAC system when it’s hot as hell outside and inside? Why is that? Seems like after running 24 hours they fall back in line. I tried to get the sub cool correct on a new install and overcharged the crap out of it!

  3. james lee says:

    been doing refrigeration for over thirty years and I disagree with at least fifty percent of what this guy has said, first of all him calling techs not techs, what are his credentials in the field. being book smart means nothing.

  4. The Aleons says:

    Wasn't sure where to ask this question, but I am looking for an electronic expansion valve with a controller that will allow me to run suction pressure below zero PSI. I can't find a controller that won't fault out when trying to run Ultra load temperatures like -80F. I have to put the controller in manual mode then I can dial the electronic expansion valve to exactly what is needed whereas a TXV loses a lot of its ability to meter at these ultra-low temperatures. BTW your channel is in first place tie with the engineering mindset Channel. That's a pretty big compliment

  5. Brett Kelley says:

    You know you learned the trade in the field with old school techs/installers/owners, when the way I test cool mode TXVs is when I have verified airflow and clean coils, switch system into heat mode to check if charge seems normal and not low for conditions. If pressures are normal-(ish) then cool mode txv is bad lol.

    Much easier way to test a cool mode TXV, and it’s vice versa to check heat mode txv

  6. AaA BbB says:

    there was one time I checked superheat/subcool after checked the filter and blower motor (filter was clean, motor was working) then decided to fill refrigerant then shit happened! Frozen suction line kept getting worse the more I dumped in refrigerant, turned out that blower seized in the middle of nowhere/nowhen (99% was right after I walked to outside to work with outdoor unit). Replaced the motor, found out I overcharged 3lb of R410A (goodness not R22 tho, expensive discontinued crap). Such a shame of me, I always checked both inside/outside machines.

    Not related to the topic tho, but I feel like I need to share my exp (my stupidity).

  7. keith glynn says:

    I guess this should go without saying but I would make sure all of the filters are clean, no slipping belt if there is one, make sure the coils are clean, in this case especially the evaporator coil, CHECK to make sure ALL dampers, diffusers, registers are open and confirm airflow. You would be surprised how many people close registers in a room not used or simply because they don't like to feel a cold draft. You can usually tell when there is a restriction or a bad txv because most times your superheat will be high. And contrary to what some people believe…if you have clean filters, coils, good airflow, then I would absolutely try warming up the txv bulb to see if there is any change in pressure at all…it's a lot easier to do than recovering the charge and changing the txv. Usually before you even get to that point you can usually find the problem. Check for a rubbed through txv capillary, look for any significant temperature differences across filter driers and watch for freezing at the txv. Finally, check to make sure the thermostat is not set too low (people have a tendency to bury it down low) and test it to make sure it stops and starts the system with reasonable accuracy. A system that runs too cold can log up oil in the evaporator coil and reduce heat transfer. Try shutting the system off for a half hour to let it stabilize and startup with a good heat load. I hope this helps.

  8. Bret Walley says:

    I have only replaced a few in 30 years, always low pressure or a vacuum, last one was 4 psi. on suction, then it kicked out on high head pressure, they aren't hard to diagnose.

  9. Nycowboy 22 says:

    Your head pressure is not gonna go sky high if you’re pumping down the system because you have the valve clothes so your head pressure is not gonna read anything ? Am I missing something did he say the head pressure is not gonna go up when you have a valve closed .how’s it going to test the pressure if the valves close ?

  10. John Douglas Music says:

    This is awesome. I'm not a tech, but I work with many and their companies, and used to be a residential heating company manager many years ago until I became self-employed and help them with permits and plans. But I have had a few techs try to figure out what is going on with a heat-pump system and the stories seem to vary – there is a constant high-pitch hiss that comes from the THX after the system runs. And they say the pressures are fine and performance is fine. But the hiss will go on for several minutes after the system is shuts off at temp. It wasn't as long a couple years ago. What are they missing?

  11. John Bipper says:

    5 frigen commercials !! 1 of them about supporting abortions, I guess being responsible enough not to get pregnant is asking to much for their leg spreading minds to comprehend.

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