How Thermostatic Expansion Valves (TXV) Work

Learn how thermostatic expansion valves work in HVAC systems.

Since the minimum efficiency regulation changed to 13 SEER in January 2006, most OEM systems now incorporate a thermostatic expansion valve (TXV) style metering device as the standard for air conditioning systems. It is now extremely important for the HVAC technician to understand the design and operation of this type of valve.

The thermostatic expansion valve (TXV) is a precision device, which is designed to regulate the rate at which liquid refrigerant flows into the evaporator. This controlled flow is necessary to maximize the efficiency of the evaporator while preventing excess liquid refrigerant from returning to the compressor (floodback).

One of the design features of the TXV is to separate the high pressure and low pressure sides of an air conditioning system. Liquid refrigerant enters the valve under high pressure via the system’s liquid line, but its pressure is reduced when the TXV limits the amount of this liquid refrigerant entering the evaporator.

The TXV – What It Does Not Do

The thermostatic expansion valve controls one thing only:  the rate of flow of liquid refrigerant into the evaporator. Contrary to what you may have heard, the TXV is not designed to control:

  • Air Temperature
  • Head Pressure
  • Capacity
  • Suction Pressure
  • Humidity

Trying to use the TXV to control any of these system variables will lead to poor system performance – and possible compressor failure.

How the TXV Controls the System

As the thermostatic expansion valve regulates the rate at which liquid refrigerant flows into the evaporator, it maintains a proper supply of refrigerant by matching this flow rate against how quickly the refrigerant evaporates (boils off) in the evaporator coil. To do this, the TXV responds to two variables: the temperature of the refrigerant vapor as it leaves the evaporator (P1) and the pressure in the evaporator itself (P2). It does this by using a movable valve pin against the spring pressure (P3) to precisely control the flow of liquid refrigerant into the evaporator (P4):

TXV Pressure Balance EquationTXV
P1+P4 = P2+P3
P1 = Bulb Pressure (Opening Force)
P2 = Evaporator Pressure (Closing Force)
P3 = Superheat Spring Pressure (Closing Force)
P4 = Liquid Pressure (Opening Force)

Energy Transfer in the TXV

Here is a closer view of the TXV in operation. The flow of the liquid refrigerant is restricted by the valve pin. As the flow is restricted, several things happen:

  • The pressure on the liquid refrigerant drops
  • A small amount of the liquid refrigerant is converted to gas, in response to the drop in pressure
  • This “flash gas” represents a high degree of energy transfer, as the sensible heat of the refrigerant is converted to latent heat
  • The low pressure liquid and vapor combination moves into the evaporator, where the rest of the liquid refrigerant “boils off” into its gaseous state as it absorbs heat from its surroundings.

The pressure drop that occurs in the thermostatic expansion valve is critical to the operation of the refrigeration system. As it moves through the evaporator, the low pressure liquid and gas combination continues to vaporize, absorbing heat from the system load. In order for the system to operate properly, the TXV must precisely control the flow of liquid refrigerant, in response to system conditions.

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138 thoughts on “How Thermostatic Expansion Valves (TXV) Work

  1. I have a Lennox and when I switch from A/C to heat something happens to the system so that when I turn the A/C back on the unit outside will not kick on (it stays off even though the inside unit is working). I had my home warranty company send someone out and they have been out 4 times saying something different each time. Now they are saying it is the txv valve, and that they will have to replace all of my freon once they replace the valve (total of $500 even with warranty). Is this legit?

    • If it is the TXV, the refrigerant in the system would need to be recovered and replaced. You may want to get additional contractor diagnosis and quote.

  2. May I know if TXV will work properly for bristol compressor H23a383ABCA, this is the set for my chiller which i purchased locally,
    Thank you and kind regards

    Edwin Mahinay

    • The TXV is sized to the evaporator coil (not the compressor) and is selected based on many factors such as refrigerant type, evaporator capacity, condensing and liquid temperatures/pressures, etc.

    • Overcharging is not likely to cause any damage to the TXV as it is a metering device that regulates refrigerant flow to match the heat load. It is either opening or closing to meet demand. If the TXV is allowing too much refrigerant to the evaporator (due to improper size or installation for example) there would be more concern of liquid refrigerant damaging the compressor, and even more so if the system is overcharged.

  3. I recently did a rebuild. All components are new. My suction is 80.1 discharge is 178.3 SH 12.7 SC 10.2. crankcase sight glass is bubbly as well as liquid line sight glass. 5-8 degree across evap, compressor amps low

    I believe the TXV is not responding and flooding back to the compressor.

    ?

  4. Our system has intermittently stopped blowing cold air (blows lukewarm). On occasion, coils near air handler in attic freeze up. If it’s not blowing cold air, we flip the breaker switch and press reset button in circuit box in attic. It starts blowing cold. Could this be due to the TXV?

    • Hi Tesha. Based from the explanation you have provided, it sounds like your system might have a leak and may have lost some of the refrigerant charge. In general, if the system has lost charge and is tripping on internal overload protection and it is starting to enter a “loss of charge” failure mode. You might want to hire a HVAC technician to check on your HVAC system since we can never tell for sure without being there.

  5. Hello,
    I just changed the two condenser units. 2nd floor 4 ton unit and down stair 5 ton unit. He also changed the valve that goes with condensers. However, after everything is done he realized that he used wrong valve. He used 5 ton valve to 4 ton condenser unit and 4 ton valve to 5 ton condense unit. He mix matched the valve. He said it not a big deal and everything works fine. Is it really OK to mix match the valve?

    • James,

      Operationally you will probably be ok, but understand that a TXV typically has a maximum output limited to 10% over rated capacity. With that being said, your 5-ton system will most likely never achieve full capacity if the demand is present. Depending on where you live, that may or may not be an issue.

      Thanks,
      Scott

  6. I have an American Standard AC. The Compressor was just replaced under warranty, thankfully!! Now I am having a problem with not being able to cool the house quick enough. What used to take an hour to cool take much longer. The system can’t keep up. My AC guy said the expansion valve is bad or has dirt in the line. Question for this blog, could they have induced this problem? They put new freon in the system after the replacement of compressor.

  7. Just had a new Goodman installed by a local ac company. I noticed when they were brazing the lines they were not running nitrogen thru. I said I thought with these new system that it was important to do just that and they told that was not the case, that with the filtering in the new systems it was no longer necessary. Is this true or was he just covering up for laziness?

    • Jim,

      Typically, when brazing a line set, an inert gas should be used to remove the oxygen from the line set. This is done to prevent oxidizing carbon from forming on the inside of the line set. Filter driers can catch it, but it depends on where the brazing occurs. If the brazing takes place downstream from the Filter drier, then there is a chance for the carbon to get caught by the valve, accumulator (HP), or compressor.

      Thanks,
      Scott

  8. Hi Scott
    My heat pump when cooling delivers air at about 50 degrees to start. Over the next hour the temperature rises until the heat pump is just circulating room temperature air. Apparently the pump cycles on and off, possibly due to excess pressure. The technician believes the 50 year-old TXV is the cause. As a second opinion before system surgery does this sound to you like the correct diagnosis?
    Thank you.

    • Hi Norm,

      The best way to determine if a TXV is not operating properly is to measure superheat. Check superheat and subcooling to make sure the valve is operating correctly and compare these values to the manufacturers specifications. Since the TXV is designed to control superheat it must be known when evaluating the TXV’s performance. Checking subcooling will indicate if the system has the proper charge. Also check to see that the coils are clean and receiving the proper air flow and check the filter/drier to make sure it is not clogged. A clogged liquid line filter/drier will show a frost line or ice buildup on half of the drier canister.

      A pin whole leak in the diaphragm would cause symptoms like the one you are describing, but if there is a leak in the power head diaphragm, the system will have very high superheat due to the valve not opening.

      Thanks,
      Scott

  9. I’ve got a stuck TXV in a more open than closed position (as evidenced by a low superheat and high Psi suction). Tech wants to try some type of fluid cleaner to see if it unsticks the valve — it’s a new product. My refrigerant is R22 on a 10 year old Goodman unit. My question is will this damage anything within the refrigerant lines, including and most importantly the compressor?
    Thanks!

  10. I’ve had a tech do a preventative maintenance on my 11 year old Lennox 2-1/2 ton A/C.

    After he left, the TXV pops up every other day or so, which I have to reset to get the A/C to work. He said the charge for replacing the TXV would be around $600, but I wonder if:
    1. there is something in the system that is now causing the valve to pop or,
    2. if the valve itself is bad and,
    3. if $600 is a reasonable price.

  11. I have a small home at 1,120 sq ft. Recently the HVAC system will not shut off and runs constantly. The fan mode is not ON. It has been a warm summer.
    My electric bill has increase significantly as a result. What could be causing this system to run constantly?

  12. Just bought a new unit, the old unit was just about to die (22yrs old) Went from a 3.5 ton to 4 ton. The new unit can’t keep up on a 93 degree day, live in TX and there will be a lot hotter days to come. The installer put in a capillary coil in the house, and didn’t put a TXV on it. When I read the manual it says DO NOT use capillary inside coil. Did I just get screwed?

  13. I have a related question, is the TXV clogged if only half of the evaporator coils are cold or do you think it the coils are clogged or both? Also, we have not had to add Freon either.

  14. Hi Scott,
    Thanks for the article. I have a 410 system and the high pressure is too high, 400. The suction pressure is only 100. I thought there was a blockage so I changed the dryer and the tx valve. On start up the high was 200 and the suction 100 and I was not getting enough sub cooling. I added some refrigerant and the high went up to 400 and the low stayed at 100. Is there a blockage in the evaporater.

    • Hi Trevor,

      It is very difficult to “real-time” diagnose a system without being at the jobsite. Without knowing the indoor load, along with outdoor temperatures and unit location, I cannot offer a recommendation based on stated pressures alone. However, I would suggest checking the outdoor coil for anything that might be disrupting air flow across the coil surface (e.g. grass clippings, dust, leaves, pollen, etc.). Cleaning the coil might help in this case as these might be reducing heat transfer. If it is not something simple like that then you might want to call a local qualified contractor for support.

      Thanks,
      Scott

  15. My unit is giving a high pressure reading which my technician thinks is due to a bad txv. He said that this is also causing the short cycling of my unit. Can you advise and tell me if he is correct?

  16. I can’t pass this up any longer. “The TXV is not designed to control the suction.”
    I’ve had very good (great), results controlling the suction pressure by first
    understanding how the valve works. Then by me controlling the suction pressure.
    I’ve had to follow behind where other technicians have bailed out because they
    could not TROUBLE SHOOT.
    That’s where I come in. I don’t have a journey man’s license. In fact I don’t have
    any kind of license. I have IMAGINATION. More Important my approach is
    different. So different that I can’t see how a valve’s opening can be any less
    than it”s rated opening. To put differently. The valve has only one opening and
    that is it’s rated opening. If the ambient is high the valve will move in the closing
    direction, but it won’t close. If the ambient is low, there’s problems. You almost
    want me to point out that a lower suction pressure underneath the diaphragm
    would open the valve. Your best bet would be to look for other means of
    controlling your head pressure. I get a lot of those problems, but I prevail.

    Well I have to leave something for the back burner. See you in the movies…

  17. Hi David,

    A few recommendations, care must be taken to assure that each distributor tube has the same length and that there is no difference in pressure drop between the tubes. Bend the tubes carefully so as not to kink the tubes or diminish the internal cross-section. Try to keep all the bends with a large, smooth radius and ideally have similar bends on every tube. Ideally, mount the distributor right at the TXV outlet. If this is not possible, use a straight piece of tubing between the TXV outlet and distributor inlet. Try to keep this as short as possible. When possible, mount the distributor in a vertical position. Downward flow is recommended but upward is also acceptable. Vertical mounting helps to assure even distribution of liquid to all the tubes. If vertical positioning is not possible, horizontal is acceptable recognizing that tubes at the top may not feed solid liquid. Distributor capacities are based on 30″ long tubes. If longer lengths are used make sure you de-rate per the distributor manufacturer.

    Hope this helps,

    Thanks,
    Scott

  18. I’ve had an issue with my compressor short cycling, (every five to ten seconds) The AC tech diagnosed the problem as needing a new compressor unit. The existing unit was a trane 4 ton, and has been replaced with a Guardian 4 ton unit. The txv was also replaced and the unit ran and produced cool air from the ducts on the day of the installation (outside temp 60F). One week later on the first hot day (86F) the new unit is also experiencing short cycling every 10 seconds or so. The indoor coil was not replaced. After the tech left on the installation day I was checking out the work when the unit was operating and noticed a hissing sound around the TXV and was able to feel a slight breeze near the brazing connection. Is there any chance this isn’t a leak? I called the tech 30 minutes after he had left and described what I was seeing and he indicated that this was normal. I’m wondering if their is an air port, or relief port on TXV’s, but haven’t seen anything like that in your description and pictures above.

    • Hi Steve,

      Most Air Conditioning systems operate above normal atmospheric pressure. The system needs to remain completely sealed in order to contain the refrigerant gas within the unit. If a leak is suspected, one method to determine the location is using a bubble solution. An inexpensive bubble solution can be made from simple dishwashing soap and water. Spread this solution on the suspect area and look carefully for “bubbles’ to be created from the difference of pressure between the system and atmospheric.

      Thanks,
      Scott

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