Guide to Thermostatic Expansion Valves (TXV)

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.

Understanding the Function of the TXV

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.

Understanding 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)


Understanding How the TXV Transfers Energy

Here is a closer view of the TXV in operation. The valve pin restricts the flow of the liquid refrigerant. 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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176 thoughts on “Guide to Thermostatic Expansion Valves (TXV)

  1. My thoughts on Michelle’s problem about the DIY.
    1. No need to buy all that equipment. If you think it’s the valve, go out
    and purchase the exact part. Makes no matter whether it’s brazed
    or flared. Just make sure it’s the exact replacement. Have everything
    ready and remove the guts from the old valve. Also remove the guts
    from the new valve and install them on the old unit. allow the
    least amount of freon to be flowing at both ends and also the
    equalizer connection. You won’t even have to replace the drier.
    I like to keep things down to a low roar. Being careful not to be sloppy.
    You should make out like a bandit

  2. The Controling Factor

    If you can’t keep up with a two ( 2 ) car funeral don’t bother reading any further.
    I possess a tremendous imagination, but more important my, approach is different.

    First of all the TXV has only one opening, that being fully opened. In order for the Valve to open any less than that, would require for the compressor’s RPM to be reduced. So, If I’m right that would mean that:
    A. Valve #1 pulling down to 45 PSI.
    B. Valve #2 pulling down to 70 PSI.
    Not only are both openings the same but the feed rate is the same.
    How can that be? ? ?

    Keeping in mind that the only way a valve can open is by pressure from underneath the diaphragm being removed or being pumped out. That being the case, the valve can only close by suction pressure pushing up on the diaphragm. Allow me to clear the air and this deals with A/C Pressures. Some of this might seem like double talk.

    I’ll explain. In A and B above you would be tempted to point out B is pushing up harder than A and Rightfully so, which would mean A is open wider by an eye lash or two also I want to clear up one Very important point. At no time am I attempting to claim that the Valves don’t work. They do work.
    They just don’t work the way it’s been explained over time and beyond.

    Getting back to A and B. Just to be more precise, in example A, there is less pressure pushing against the diaphragm and the valve opening is supposed to be less, and the gauges do show a lower reading instead of the expected higher reading. Keep in mind that supposedly that as the pressure drops underneath the diaphragm more refrigerant should be pumped in. If that were the case it would, but the valve can’t open anymore. I hope that is not confusing.The valve has bottomed out.

    In order to raise the pressure, it’s not done by having the valve open wider or by having it feed more.
    The best way to explain my theory is by having the unit shut down. This will cause the suction pressure to rise very high and the valve will be closed with all that pressure pushing against the diaphragm. So the unit is started and just because the suction pressure is reading such a high
    Reading doesn’t mean the valve is open wide. The valve remains closed at start-up, but the pressure starts dropping very fast. (This is the B reading). WHEN the suction pressure reaches 68 PSI, your needle should flutter momentarily and settle at or about 70 PSI, without the valve having had to open any wider. You notice I capitalized WHEN. Well that is the key . That becomes the controlling factor. Or am I the controlling factor. You decide. In example A the Spring pressure was too high and the valve had to pull down too low befire it started to open so it was controlling at a very low suction pressure. Very low for A/C.

    Now we can assume the following:
    Lowering the spring pressure raises the suction pressure.
    Raising the spring pressure lowers the suction pressure
    The above is not an assumption. It’s a fact.
    That being the case, what really takes place, there is a change in the suction Pressure.
    The degree of opening does not change to a greater or lesser degree.
    Either adjustment affects the WHEN.
    That isn’t ( ain’t ) too shabby.
    Let’s get back to A for a moment. Since you have 70# pushing up on the diaphragm wanting to close the valve, there is movement in that direction. But then just as quickly the unit’s suction stroke goes into action to remove this closing pressure and suction pressure remains at 70# PSI. This action is very rapid and may or may not show up on your gauges. It’s known as the modulating action. Keep in mind that as long as the compressor coninues to operate the higher pressure on the inlet side of the valve continues to enter Into the low side, the valve remaining open as long as the compressor is operating. This pressure that is being forced in is more than is needed. It therefore pushes up on the diaphragm moving it in the closing direction. Then just as quickly the pressure is removed By the pumping action of the compressor.
    Okay I’m closing my CASE.
    Almost forgot. When making adjustments on the valve, you don’t have to wait 15 nor 30 minutes for the new adjustments to get results . All you have to do is place your hand on the bulb and the valve will reset real quick. I’ll trust you can figure that out. . .
    I never raised the pressure, all I did was change it. The Valve opening remained the
    12. I did not mention SUPER HEAT. I DIDN”T FORGET. I still get SUPER RESULTS. .
    13. Increasing the spring will make it easier for the Valve to close. Reducing the spring
    Pressure will make it harder for the Valve to close..
    Insttead of using or not using Super Heat. Let’s go for PSI CONTROLLER

    14. For those of you who have some holes to poke. Well poke away. I have one question for
    Anyone reading this just in case you made it this far. Do you agree or disagree if I make
    Tthe claim that I just might be the only one in town that can rightfully claim to being the
    Only person that can rightfully claim to “Knowing how the Expansion Valve Really Works”.

    Please let me know what you think about my Theory.

    • TXV —Modulating

      Modulating is put into play by the suction pressure. However, to better explain what is actually taking place is, should the suction pressure rise more than what is needed or better yet, to want to flood the evaporator, the excess pressure will push up on the diaphragm in the closing direction performing the closing action of the Valve. Then just as quickly the suction pressure will be reduced underneath the diaghpragm. Causing the Valve to allow for the suction pressure to be removed thereby completing the modulating action. It may or may not be repeated. It”s an automatic operation built into the Valve’s operating feature.
      Most likely I’ll be adding or provide a better explanation…
      I’m not certain that the TXV with the so called Balanced Port would operate in the same
      Way. I’ve never dealt with one yet. However I am eager eager to look into it…

      How (1) Spring, and (2) Pressues play out on the TXV.

      | Bulb Pressure
      | 90 #
      \ /

      ^ ^
      Suction Pressure Spring Pressure

      70 # 20 #
      Valve is in closed position

      Obviously the Total Pressure underneath the
      Diaphragm is a total of 90 #

      If the suction Pressure is changed to 50 #, then it would require for 20 # to be
      Be added to the spring Pressure to close the Valve. Or it can simply be stated
      That by reducing the spring Pressure, you increase the suction Pressure, and, or
      By increasing the spring pressure the suction pressure would would decrease.

      Or better yet, let me come at you a different way.
      1. Increasing the spring pressure would lower the Suction in an oprtating system
      It would not change the opening of the TXV. Nor would it change the feed rate. However
      It would change the pressure in the System. Giving you a lower reading. It’s important that
      You understand what is taking place here.
      2. Lowering the Spring pressure, would not open the Valve more but it produce a
      Higher reading with the Valve opening remaing the same. How could that ever play out
      Like that ? ? ?
      It’s very simple, Simon. By changing the Spring pressure, all you’ve done is change
      The operating pressure of the TXV. . .The opening does not change nor does the feed rate.
      Increasing the spring pressure puts more pressure on the Bulb causing the PSI ln the
      Evaporator to be reduced before it will start to open. Decreasing it would have opposite
      Results. . .

  3. Hi! So, unfortunately on the hottest day of the year (July) my condenser went! AH! I had to replace the condenser and the coil. When my new unit was put in I was told my old unit was too big, and a smaller one was put in (with the promise that it was a little bigger than I needed due to poor insulation). Well, I haven’t really been satisfied. When I put my old unit to 72 degrees, I froze on both levels of my house. When this new unit reaches 72, its not the same cool, it feels too warm to me. So, I set it to 70, or 69 to keep the air flowing and cool efficiently, and also reach upstairs (*With blankets on my windows and fans on). The only issue is, when I set it to these temperatures it runs FOREVER. My old unit hit 70 degrees no problem, and I’d have to shut it off because I’d be too cold.

    I know this a novel, bare with me, I’ve been losing my mind since July.

    Soooo…. I complained within two weeks of this new unit being put in. The tech came out, checked all the pressures or whatever, said everything is fine and its due to the heatwave/the house not being made for central air. I just can’t buy that… My old, bigger unit worked just fine. A few weeks later, I decided to complain again because my electric bill doubled. The answer I was given this time was “I didnt put a TXV on, I didn’t think you’d need it. I’ll put a TXV on, and it’ll fix the problem.”

    The txv should have been on in the first place. But nonetheless, is this really the answer to my problem? I’m so worried that I’ll be shelling out another $6,000 for a new unit next summer.

    • An oversized system will cool the space near the thermostat quickly after it starts running. It will also overcool many spaces that are close to the vents but away from the t-stat. The problem with oversizing an HVAC system is that it might hit the set point so fast that it does not run long enough to get the humidity out of your space. For a single capacity system (note, there are two speed and variable speed systems that do better here) many contractors will design systems to run almost full time on the hottest days and then on the more moderate temperature days the system will run long enough to keep humidity down by having sufficient run times. One way to make single speed/capacity system work better on extreme days and nights is to set the fan on your system to “run” to keep the air moving throughout the space. You might try this on the hottest days and also on mild but humid nights. I am not sure about the TXV idea and I am also not sure where you live or if humidity is a problem in certain parts of the cooling season but I hope this helps anyway.

  4. I have a unit that is 10 or 11 years old. It stopped cooling about a month ago, prior to this I’ve had no problems with it. I called a company to look at it, he was recommended by a friend. He said I needed an expansion valve, I had him replace it, unit worked fine until yesterday. Unit stopped cooling, I went outside heard loud hissing noise saw a cloud of freon. Called same guy back he confirmed my compressor is bad said it blew out the side. My question , could he have installed the wrong expansion valve or installed it wrong? Thx

  5. My understanding of the compressor is that it cannot take in fluid without damaging it, which explains why the refrigerant from the evaporator comes to the compressor as a gas.

    Where my confusion lies is; once the HVA/C unit reaches the thermostatic set temperature (say 68 degrees) and shuts off for, say 20 minutes or so, does the system between the evaporator and compressor maintain the refrigerant in a gaseous state? If not, how is the refrigerant converted into a gaseous state at start up of the AC unit?
    Thank you.

    • Hi Kevin. The refrigerant upstream from the expansion device is usually a liquid and under high pressure. As the refrigerant passes through the expansion valve and flows through the evaporator, it turns into a gas and takes heat from the air passing over the evap coil to do that, thus cooling the air that passes through it, which then goes out into the space to be cooled. As the refrigerant passes through the evap it should be in a gas state as it returns to the compressor. The compressors takes back to high pressure and the outdoor coil cools it so it returns to a liquid state before going back to the expansion device and the evap. If things are set up properly, the compressor should only see vapor refrigerant. Here is link to another post on this site which has some diagrams that might help explain this. I hope this helps answer your questions. Thanks for using our site!

  6. How can you verify if the TXV is not functioning properly? My 7 year unit has degraded cooling the space. Last summer the evaporator iced up twice. Tech says no problem with charge or air flow. Yesterday a tech determined that the bulb was not installed properly . It was fixed on top of the line with duck tape. He cleaned the surfaces and clamped the unit to the pipe after he tasted the txv functions by applying cold and heat to the bulb. I do not think that the unit is cooling properly. Are there any specific tests to evaluate the proper function.

  7. very interesting blog I may say, yes txv is the most efficient metering device . but no. txv efficiency cant be justified by age of txv….. txv was manufactured to regulate a specific range of temps/pressures. If bulb fails, well the txv fails… only way you can see if the txv is in its “failing” state was actually taking the bulb off during cooling state and do temp test on site,…. which Im sure no one includes that on there maintenance contracts.

  8. My heat pump system is not cooling properly. My repairman indicated that the TMX valve is not functioning properly. The cost will be approximately $800. Would replacing this valve make the system cool properly?

  9. in my office, we had a citec GD40 with 2 compressor PAC, lately its always giving an alarm lan1 disconnected and lan2 disconnected, does anyone know what its mean, because its seems failure on txv valve, the compressor can’t work as long as the alarm still exist, thank in advanced for answer, 🙂

  10. Hello,

    I need some help. Told by my regular HVAC maintenance guy that my TXV needs replacing, total cost being $800. Most times I just pay the money and move on, but this time I am having a problem with the cost of this repair. Is this amount of money appropriate to replace this part when the part appears to be about $60 or so. $740 in labor, really….

    And is it possible I could do it myself?

    • Sure you can do it yourself. You will need a two stage vacuum pump, fluxless brazing wire, a manifold gauge set, an oxy-propylene torch, a nitrogen bottle and nitrogen regulator. Cost of equipment? At least $700, but in the course of learning how to use all of the above, you will learn enough to be able to maintain your own a/c for the rest of your life.

      • Paul, lousy excuse. We, the general public are not that dumb. We may spend the money buy the tools and you will find less and less clients… especially when it comes to almost $1000.

        People like you are the reason people like me, have such a “peachy” feelings toward the Unions.

        BTW, if I’m paying for the tools in full, I’m keeping the tools too.

        • Nick, you’re an idiot. I’d love to watch you properly and legally replace a TXV without any training, union or non.

        • I see where I may have offended you, but I meant no offense. I am a DIY and I did exactly what I wrote when I had to move my condenser to repair the wall of the house. I bought and used the tools for this project only…..and I did learn to repair and maintain my a/c in the process.

    • Well thanks Paul. It looks like either way I go I am screwed. How do you explain the replacement parts selling for $60 or so dollars, online, does this mean these parts are not complete and I would need additional parts? This stinks….

      • The parts are complete except for a new filter/dryer that needs to be replaced whenever the system is opened, but in order to replace them you have to discharge the system to basically vacuum, cut out the old TXV, place the new TXV, cut out the old filter/dryer, place the new filter/dryer, charge the system with nitrogen, braze in the TXV, braze in the filter/dryer, pressure test, pull a hard vacuum / vacuum test, then appropriately recharge the system. You’ll need your 608 certification, a torch, brazing rods or rings, a vacuum pump, a refrigerant recovery system and tank, gauges, nitrogen bottle, some scotch bright pads, and likely a bottle of fresh refrigerant to top off the system. It may be possible to use the service valve on the condenser to trap most of the refrigerant there as a time saver, but it doesn’t change the procedure much.

      • Michelle – the best advice I could give would be to get a second opinion from another qualified technician.

        • Not all TXV are brazed in. You can get a bolt in TXV. Let’s tell both sides in here people, not just the worst case scenario. Trying to scare people out of DIY is BS.

  11. if you go to a job site to install a two ton condenser with a tow ton coil. but when you get to the site you realize that the coil is good for three ton. what we should do? can we use txv valve with three ton coil?

    • If the valve is sized at 3 tons, but you have a 2 ton system, this is usually not a major concern, but you may see a little more hunting than you would with a 2 ton valve at some lower load conditions.

      There are many installations out there that have much more oversized valves than one ton. Certainly, it’s optimal to have a perfectly sized valve for the tightest superheat control and efficiency, but you also want to make sure you have enough capacity for the worst case scenario as well. Some tend to favor a little oversizing to make sure the unit will provide cooling in high load scenarios. It’s always a good idea to run the system for a while and monitor the system pressures, temps, & superheat to make sure you are not starving or flooding, and make any needed adjustments.

  12. Need more info and explanation. When the system is off is the valve open or closed? Now the compressor comes on because of a call for cooling. Does the valve open or close? As the system runs what happens to make the valve maintain a constant superheat? What happens in the system to make the supoerheat change that it has to be maintained?

    • Flow control, or metering, of the refrigerant is accomplished by use of a temperature sensing bulb, filled with a similar gas as in the system, that causes the valve to open against the spring pressure in the valve body as the temperature on the bulb increases. As the suction line temperature decreases, so does the pressure in the bulb and therefore on the spring causing the valve to close.

    • The TXV controls superheat by utilizing the system pressure and temperature at the exit of the coil (suction) to regulate how much refrigerant is fed at the entrance of the coil. If the superheat is above the setpoint, the valve will open wider. If the superheat is below the setpoint, the valve will start to close. It does this through a balance of different pressures acting on the valve.

      The two controlling pressures are the equalizer and bulb pressures. The equalizer line is a direct line from the valve into the suction line of the coil (on an externally equalized valve), so the system suction pressure is transmitted directly to the valve as a closing force (under the diaphragm). The bulb pressure is actually transmitted as a function of the suction line temperature at the same general location as the equalizer line (exit of the coil). As the bulb temperature increases, the pressure inside increases and vice versa, and this pressure is transmitted to the top side of the diaphragm as an opening force. The bulb is filled with refrigerant/gas that mimics the properties of the system refrigerant, and in some cases, it is simply filled with the same refrigerant as the system. Because of this, it has the same pressure/temperature profile as the system refrigerant, and the resulting valve movement will have a constant relationship to the boiling point of that refrigerant.

      The third pressure acting on the valve is the spring pressure, which can be adjusted (in an adjustable model). The spring pressure is applied as a constant closing force on the valve (like a thumb on the scale toward closing the valve) and is adjusted to increase or decrease the superheat setting. As more spring pressure is applied, more force is applied in the closing direction of the valve, which increases the superheat.

      Typically, when a system shuts off, the suction line pressure immediately increases because the compressor is no longer pulling. This increase in suction pressure results in a decrease in superheat, which causes the valve to close off. Functionally inside the valve, the equalizer pressure will increase (which is the same as suction pressure). The equalizer pressure is a closing force.

      When the system kicks back on, the opposite happens, but even more so. Not only does the suction pressure immediately drop when the compressor starts pulling, but the suction line temperature has been slowly rising as the system has been off, and the load or space the system is cooling has also been absorbing heat. All of these factors result in a high superheat, and the valve immediately opens to lower the superheat level. This is typically known as the “pulldown”. The valve will typically be wide open until the system superheat drops to near the desired level, or setpoint, then the valve will begin closing off to try to maintain that setpoint.

      After a running for a while, the system will become more stable, and thus the TXV becomes more stable because it moves when system temperatures and pressures move. The valve will settle around the balances of the system pressures, temperatures, and spring pressure being applied, which is your superheat setting.

      Other than start-up and shut-down, there are other system changes that can disrupt this balance, such as a blocked coil/fan, loss of refrigerant, increase/decrease in heat load, and changes in the ambient/outdoor conditions.

  13. Is it possible to determine if valve is “getting old” and should be replaced as a preventative maintenance measure?

      • It can take out a compressor if you don’t have or have a malfunctioning high pressure cut out switch

        • I understand that if not enough refrigerant is allowed through, superheat rises and there is no cold refrigerant to cool the compressor motor. But if too much refrigerant is allowed to flow, will liquid get to the compressor and slug it?

          • HI Paul – you are correct that having too much refrigerant in the system can cause liquid refrigerant to enter the suction line of the compressor and this might cause problems. Also, as you indicate, not having enough refrigerant could result in low or no cooling and very long run times and low pressures causing the system to shut off due to sensors designed to prevent continued operation at low charge levels. It is important to get the proper refrigerant charge in the system before running it. Both too much charge and not enough charge can cause problems.

  14. We recently purchased a second home in northern PA (winter temps are about 10 degrees colder than NYC). The heating bill for Dec.-Jan. billing cycle shows a 4030 kwh usage as compared to 1089 kwh for same period a year ago. The thermostat is set at 56 degrees (as a second home/investment property it got very little usage in that period). The home and heat pump are only 6 years old. Because of the drastic increase in kwh energy consumption we had an HVAC contractor check the system. He ultimately determined that the txv valve is causing the problem and needs to be replaced. I don’t understand the technical explanation he provided. My question: if this valve is malfunctioning could it be causing such a high level of kwh usage?

    • For a heat pump, the expansion valve regulates the flow of refrigerant to the coil. If that valve is not working then the heat pump might not be working efficiently, if it is even working at all. When this happens, your system begins to use auxiliary electric resistance heaters to provide heat energy that is not being provided by the heat pump system (AC in reverse). This type of resistance heating is not very energy efficient and can cause high energy bills. Auxiliary resistance heaters are needed for heat pumps in areas where it gets really cold because when the outdoor temperature gets below about 15F there is not enough heat in the atmosphere to “pump” back into your house. If the heat pump is not working, the aux heaters kick on. This could have been happening in your situation but you can ask your contractor to make sure. There are a lot of articles on this site about heat pumps so you can use the search tool to learn more. I hope this helps to answer some of your questions.

  15. 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.

    • a contractor can isolate refrigerant in system, then “pump down” the area near the TXV. At that point it can be safely replaced. If system is already low, it may need to be charged after new TXV is installed. Aprox. half of mechanical problems in split a/c systems is the result of metering device and the TXV is only one type. Often the super heat spring may be replaced and not change out the complete unit. Get second opinion.

  16. 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.

  17. 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.


  18. 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.

  19. 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.


  20. 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.

  21. 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.


  22. 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.


  23. 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?

  24. 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.

  25. 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?

  26. 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?

  27. 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.

  28. 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.


  29. 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?

  30. 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…

  31. 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,


  32. 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.


  33. Using R410A, Is there a maximum distance between the TXV and the distributor?

    Are there guidelines for routing connecting pipe between the TXV & distributor, e.g., horizontally, vertically, maximum bends or elbows?


    • 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,


  34. I realize the following will send everyone back. But I have to do it. There’s one very
    important operation about the Valve that is not explained correctly. How can it be
    explained if it’s operation is unknown or it just wasn’t understood ??? In a normal
    operating system, there”s only one way that the valve can open. The pressure
    underneath the diaphragm has to be removed in order for the Valve to open…
    Super heat is not going to open it when the unit first starts. There is no super
    heat when the unit is not running. The Valve can only open by the pumping action
    of the compressor…I hope that’s not too difficult to understand ? ? ?

    • I realize the following will send you back to square one But I have to do it. There’s one very
      important operation about the Valve that is ignored or it’s just not recognized or lt’s just not understood…In a normally operating system, there”s only one way that the Valve can open. The pressure/refrigerant underneath the diaphragm must be removed. There is no super heat when thee unit first starts. That leaves only the pumping action o the compressor to pump out pressure from underneath the diaphragm. I trust that’s not
      too difficult to understand ? ? ?

  35. hy hav a question can a expansion valve bkock the flow of gas.
    i am working on a coldroom which gad a leak in its evaporator coil after closing the leak it ran for oneday and raised the temoerature i thought it leaked agaig to my suprised i realised the gas is blocked n the only valve on the evaporator side is the TEV thats y i am asking if its posible that the TEV is blocking the flow of refrigerant.thnx in advance

  36. are you suggesting that the smaller the expansion valve aperture (ie between p2 & p4 controlled by the spring) the better the performance?

    • Raph,

      A correctly sized valve will most likely respond better to any change in system load. Most systems are designed to operate at a sustained load of about 80%, meaning the system is not truly design to operate at 100% capacity, 100% of the time. The valve aperture size is dictated by the nominal capacity. Therefore, the larger the valve aperture the higher the valve’s nominal capacity. Oversizing the valve capacity could cause the valve to hunt more as refrigerant surges through the larger aperture. This can be amplified when the system load reaches set point. Please keep in mind, that most valves are capable of reaching capacities 10-20% above their rated nominal sizing.

      Hope this helps,

  37. Thank You Sir for a simple explanation. I understand it so much better. I stumbled upon your page when googling Tev Valves. Do you have a page ot YouTube channel that you talk about other parts of the refrigeration process, especially those used in a commercial setting? Thank You in advance.

  38. Hello Sir,
    It was nice to read all the above, that push me to ask Question. In air cooled chiller the Screw & gaters got damage of Single screw compressor. is there any possibility that due to TXV malfunctioning, compresses got damage ?

    • Hi Muhammad,

      Yes, it could be possible but it is difficult to determine without being on site. Generally, some component malfunctions or failures could result in compressor/system damage depending on what is going on in the system at the time.

      Hope this helps,

  39. I was trying to get across to you that formula P1=P2+P3 never comes into play. Even if it did the valve would be completly
    closed. I brought that up about 15 (Fifteen) years ago…TRC, Technical Reporting Corp. out of Chicago. Anyway I don’t
    think they’re still in print. I’m on to something else, Industry shattering. My version of how the TXV really functions. For now
    I’m going to put it on the back-burner.

  40. Some years back I pointed out that P1=P2 +P3 never come into play. However should they come into play all that would happen would be that
    the valve would close. But in reality this could ever happen when the valve
    is in operation

  41. I have a question if you have the time.I installed a replacement indoor coil for a home warranty company because it had developed several leaks(I am a licensed mechanical contractor by the way)in the summer and also a TXV with an external equalizer line.I recharged the unit for 10 degrees subcooling and superheat stayed constant at I think 12 degrees.Anyway the customer called me back out at the very beginning of the heating season.I had to remove the TXV and install a piston because it was not allowing refrigerant to flow in the other direction.Question:aren’t most TXV’s made for heat pumps or could this one just be for air conditioners.I called Lennox and had to give up b/c I was on hold for 30 minutes.By the way,I can through the valve fairly easy after I removed it.Thank you

    • Hi Tom,

      Yes, you are correct, TXV’s designed for use in heatpump applications should contain an “internal Check” valve to allow the refrigerant to bypass the metering port during heatpump operation. But, not all TXV’s sold contain this bypass, as some OEM heat pump designs bypass the TXV externally. When selecting a TXV for use in a heatpump application, be sure to cross reference the valve’s OEM nomenclature to ensure the selected TXV contains an internal check if needed.

      Hope this helps,

  42. Dear scott ,
    During servicing of panel ac in textile ,i have found that due to chemical flup evaporator getting chock and due to chemical inside flup it errosions or eaten up the coil,i have replace it,but during this process expension valves capilary tube sensing bulb capalery broken and gas releases how can i repair it or refill gasR135a
    Please sugest

    • Hi Pyare,

      Unfortunately your only option is to replace the TXV, or replace the power element (if removable). The “gas” inside the dome/bulb/capillary tube is typically a combination of substances (proprietary mixtures) based on the valve manufacturer, and is not field serviceable.

      Hope this helps,

  43. My original valve does indeed have an arrow showing direction but the replacement one does not. The valve is a 90 degree design with the flow control in the opposite leg than the original. Unfortunately I can’t find one just like the original. Thanks for your advice.

    • Doug,

      TXVs are direction specific and typically will have a directional arrow stamped on one of the valve ports. Others types of metering devices, particularly the piston in a fixed orifice, may be directionally reversible.

      Hope this help,

  44. Hello Scott,

    Could you help answer me a few questions? Your advice will give me a lot confidence on the heap pump system in my house before moving forward.

    1. Current situation is
    Original split heat pump system has been more than 30 years old. 3 ton with R22 freon. While the system had never been working good, inside air handler was replaced with new one 3 years ago due to down. The installer said the outside unit would run 5 more years. The whole system works but still questionable. The outside unit completely down a few weeks ago.

    2. What I found recently.
    The new air handler including TXV, Evaporater, Blower and Heat Strip was manufactured for R410A, which I did not know about. The system has been running with R22A since the air handler replaced.

    3. What I like to do
    I like to replace the outside unit for R410A. I know some knowledge about HVAC as a mechanical engineer but not an expert. I will hire HVAC contractor.

    4. My questions are
    4.1. Can I replace the outside unit while using the air handler as is if the whole system would be flushed, evacuated and charged with R410A?
    4.2. Can I replace a different brand outside unit other than the current air handler brand as long as tonnage matches to?
    4.3. Should the TXV in the air handler be replaced?

    Thanks in advance for reading the long comment and questions.

    • Hi Jason,

      Since your current Evaporator and TXV were apparently designed for use with R-410A, you should not have any issues with using R-410A in a new outdoor unit. I would discuss the use of these components with your contractor just to be safe, as they can look up the component information for any compatibility issues.

      The expansion device (e.g. TXV) should not be a problem either if, as you stated it is working properly and designed for the new R-410A refrigerant. But I would discuss with your contractor the need to adjust the TXV, to make sure the expansion device on the coil is adjusted for R410A.

      The old R-22 system’s oil would be the concern and flushing this oil would be required from the coil and the refrigerant lines going from the outdoor to the indoor. You might even want to replace the refrigerant lines if is not too much trouble as sometimes the braze joints become “work hardened”, and might develop leaks over time (30 years is a long time). Remember to remove all restrictions before attempting to flush the components

      The outdoor and indoor systems do not have to be the same brand, but sometimes some electrical control features can be difficult to retrofit when using mismatched manufacturers. Again, talk with your contractor about this. We usually recommend getting 2-3 quotes from different contractors before making these HVAC investments.

      Hope this helps,

      • Hello Scott,

        I really appreciate your answers which help me a lot to understand. Again, thank you very much.
        I would strongly recommend them to consider Emerson products including TXV as long as I could.

  45. Scott,

    I have several units where I work that the TXV is not operating properly. The TXv’s are externally equalized and while trying to test them with ice water and holding it in my hand the compressor current never changed nor did the discharge pressure. We have one unit that the compressors are out so we took the valve apart on it and found oil in the equalizer port. Could this cause the valves to not move? The equalizer line is connected on top of the evaporator discharge however it has a loop in it prior to entering the valve. Any help would be appreciated.

    Thanks Gene

    • Gene,

      Any obstruction in the TXV equalizer line could render the valve inoperable. The purpose of the external equalizer line is to transfer the suction pressure to the under-side of the valve diaphragm. The suction pressure from the external equalizer line is the only closing force for the valve. Therefore, an obstruction from debris, oil, or a pinched line will cause the valve to lose control. It would be advisable to have a qualified HVAC Contractor replace any TXV with a blocked external equalizer line.

      Hope this helps,

  46. Very informative section. Your answer to Scott, above, pretty much answers my question. We have been struggling getting a unit online after losing a compressor/condenser fan dual value run capacitor. The line heading into the building would frost up, first at the flow view port. I have noticed that right after start up the small lines on the bottom of the TXV would frost up and the TXV was very cold. From what I read above I am concluding that the TXV is acting as too much of a restriction and that it may be clogged. Is there an adjustment for the TXV?

    • Hi Tony,

      Yes TXV’s are adjustable, and should be adjusted to a particular system once installed. Adjustment of a TXV is handled by the stem located under a cap on the bottom portion of the valve. But, once a TXV is initially adjusted, it should never need future adjustments.

  47. I have a question from an apprenticeship paper of mine, if anyone can help it would be greatly appreciated!

    During routine maintenance you find ice forming on the inlet port of the txv.a higher than normal discharge pressure and lower than normal suction pressure.

    What is causing this and how do you remedy it?

    If anyone could shed some light on this it would be greatly appreciated

    • Hi David,

      Without knowing all the particulars of the system, I would start with the TXV. Inside the inlet of the TXV is a very fine screen made to catch debris before they travel through the valve seat. My guess is that the inlet screen is clogged and acting as a restriction to flow. The restriction would cause a pressure/temperature drop at that point, along with the other system pressures you described.

      Hope this helps,

  48. I have a thermal zone unit that keeps freezing up inside and outside changed the evaporator thought it was a blockage in the metering device but it is still freezing up compressor is working. I charge the system with R-22 but still freeze up. suction pressure drops to 50 psi and stays the head pressure stays at 175 psi what do you think is causing this. thank you

    • Hi Gilbert,

      Without actually being on site, it is very difficult to fully understand some of the system challenges you are experiencing. So I put together two listings of areas to check, that could potentially help with troubleshooting the system.

      Causes of low evaporator loads:

      A) Inadequate air flow – Blockages (dirty filters, closed grilles)
      B) Restricted air passages Dirty fan blades Fan motor failures
      C) Poor heat transfer – Dirty coils, excessive frost or ice build-up, oil logging
      D) Recirculation of air – Furniture location, improper supply and return duct location

      Causes of low suction pressure:

      A) Wrong sizing of components
      B) TXV adjustment
      C) Loss of refrigerant charge
      D) Plugged Driers and strainers
      E) Suction line sizing/ piping – pressure drop

      I hope these help,

  49. Can R22 work with a valve that is for A12 (I think it is called). I have had 9 years of an ac not working correctly. I had a friend check it and he said that is the problem. Intake air is 73, exit air is 60. AC runs continuous during the day and finally shuts off about 11pm. Its a 3 ton unit that con not keep the house cool during the day or if cooking.


    • Hi Bob,

      The TXV sensing bulb charge is typically matched to a particular refrigerant’s pressure / temperature correlation. If the bulb charge does not match the refrigerant type or application, then the metered refrigerant flow will not match the evaporator load and the unit will not be able to deliver the expected system capacity. Any drop in system capacity will effect system performance and run time.

      Hope this helps,

  50. Hi Scott

    I am working with a Tandem compressor, 38 KW Condensing unit on 410A Refrigerant, Unit nameplate says that 57 lbs of refrigerant is needed. I charged it with 25lbs while the compressor is not running. Before starting the compressor, suction port pressure is 70 psi. after starting the it drops to 25psi in less than 30 seconds Then the compressor stopped due to low suction pressure. Thinking that its just undercharge, I added 25lbs more suction pressure is now 110 lbs. But after running it still drops to 25 increasing the discharge pressure to 380 . Could it be the TExV which is failing? I saw very less flow on the sight glass. I w ill try to dip the sensing bulb to iced water , Will it help?
    There are 4 units on site which are not running since 2010 with flat refrigerant charge, 2 units starts running well with 50 lbs of refrigerant but the other two has the same problem

    Your reply will be greatly Appreciated

      • Hi Michael,

        It is very difficult to troubleshoot a system without actually being on the job site, but it sounds like the system is still low on charge. Looking over the information you provided, you seem to be headed that way. One tool you might find useful with charging of a new system is a free mobile app named “HVAC Check & Charge”.

        Hope this helps,

        • Thanks Scott..

          What do you mean I am headed that way?

          Ill try to add some more 410 and see if I will get a better result.

          Thanks a lot

  51. Hi, in a much hotter more humid environment, if heat exchange on the condenser side is not optimal, I.e. ambient air is high or lacking airflow or dirty condenser, how does that affect the expansion value or the overall ac conditioner performance? Came across a product call bn6 thermal conductive coating where it improvee heat exchange on the condenser side increasing, so by having better more efficient heat exchange on the condenser side, what kind of improvement effect can I expect? Will that lead to more energy saving? Thanks

    • Hi Mike,

      Maintaining proper air flow over the outdoor coil surface is essential to achieving optimal performance. Periodically keeping the outdoor coil clean of dirt, debris, lint, trees, flowers, leaves, etc., is something we frequently include on our homeowner maintenance items.

      The charter for this site does not allow us to comment on particular products or service providers but in the general use of “coatings” for the outdoor coil it is very important to insure that the designed air flow is not reduced. Decreasing the affective heat exchange area of the outdoor coil and not removing heat from the refrigerant, greatly reduces the density (Subcooling) of the refrigerant on the outlet of the coil. TXV’s are designed so that at “wide open throttle” (full load) they expect to meter 100% liquid refrigerant through the valve body for full capacity. Any reduction in subcooling or metered liquid refrigerant would reduce the capacity of the valve and could potentially cause problems under high load system conditions, possibly resulting in compressor overheat.

      Think of the system refrigerant as a sponge, if the sponge is fully saturated with water (heat) and I don’t squeeze the sponge (condenser coil), I can’t absorb more water (heat).

      Hope this helps,

  52. Dear Sir.
    I am engineer on cargo ship and have one question for you regarding evaporator pressure regulating valve on dairy and vegetable chamber.Have read on internet all about it and still don’t understand why the same don’t existing on meet chamber .As I have understand meat chamber evaporating pressure is the lowest and how it is possible that suction pressure on compressor is higher than outlet pressure from meet evaporating unit.If the for example pressure in dairy evaporator outlet is 6 bars,vegetable evaporator outlet 4 bars and meet evap. out 2 bars what would be the suction pressure on compresor and if it is higher than meet evap pressure how is counterpressure solved.please reply to my email adress
    Thank you for your advice

    • Hi Sasa,

      Typically, when you have multiple evap temps operating on the same system, you use an EPR valve to keep the pressures higher on those cases that you want to maintain at a warmer temperature. The pressure set and maintain by the EPR (at Saturation) is the temperature setting. The case that runs the coldest is the one you set the compressor suction pressure for; hence, that case does not require an EPR.

      The pressure at the evaporator outlet of the coldest case then, should be the same as the compressor suction pressure, assuming no pressure drop thru the lines. In practice, I’d expect the compressor suction pressure to be 1-2 psig LOWER than the pressure at the evap outlet…..not higher.

      It is not possible to have a lower pressure at an evaporator outlet than the suction pressure at the compressor. This would against the laws of nature, as pressure differential is needed in order have refrigerant flow in the system.


  53. I have one free dryer (Lyophilizer)machine. This machine I’m not achieved -45°c temperature in shelf side, I have change expansion valve the model is TES2 ,-60 To -25 range,make danfoss.
    Compressor capacity is 1.5 hp , discharge pressure is 180psi. Gas:Freon R404a, expansion valve not getting ice foarmi running time when I touch expansion valve its normal temperature, I have adjust also but not find result,

    • Arun, This appears to be an application issue. You might try contacting technical support from either the OEM system provider or the (TXV) component supplier.

  54. P1+P4 = P2+P3
    In many other literatures (including wiki & Danfoos) parameter P4 is simply absent …& the equation is simply P1 = P2+P3
    Is this eq. universal or is design dependent

    • Thanks for the question. Regarding the valve type featured in this article, P4 pressure is an opening force against the valve pin due to liquid line pressure. An increase in outdoor condensing pressure effects the liquid line pressure, which influences the pressure against the valve pin seat. This results in an increase to the opening force of the valve, and is the main reason why “balanced port” TXV valves were developed. A “Balanced Port” valve looks to eliminate this P4 opening force.


  55. In March our lennox heat pump was a block of ice. It was not defrosting. the technician told my husband to find a way to defrost it, which he did, and the tech checked it and said everything was fine. the first time this spring that we turned on the air conditioner nothing happened. The tech came and determined that we need a new TXV valve. Are these two issues related?

  56. Hi I have a doubt and i need clarifications

    I have a dehumidifier with a precooling coil capacity of 8.5TR and for this we have used 7.5TR TXV- Danfoss make . We had charged the R407c properly in the copper pipe line ( after vaccuming with the nitrogen gas ) . Compressor is running well . But we are not getting suffiicient cooling after the evaporator coil . We got 25.6deg C . It seems like things are doing good but we are not getting the req.temp..The same issues in post coil also . Kly explain .

  57. I need some help

    I have replaced TXV in an undercounter fridge with three doors, the systems is working with R507, but the txv replaced is for r 134A, the consecuence is that the fridge not reach the set point 40F, only have a fluctuation betwen 45f and 55F .
    How I can solve the problem
    thanks in advance

    • Hi Jesus,

      TXV’s are designed for operation with a specific system refrigerant as listed on the part. Many TXV’s are designed for use with a single system refrigerant, while others are designed to be interchangeable for multiple refrigerants. For example, TXV’s rated for R-507 are many times interchangeable with R-404A. In addition, TXV’s rated for R-134a are typically interchangeable with R-12. However, the TXV should be selected based upon the type of refrigerant or refrigerants listed on the part only. In this situation, the valve is likely undersized for the application and should be replaced with one specifically rated for R-507 service.


      • Thanks for the answer
        My problem is that I need to run this fridge for 20 days at list before I will get the right TXV,any tips to improve the cooling?

  58. Hey this is awesome……love the breakdown on txv operations……Been in the field a long time and still love the shop talk.

  59. Hi, Trying to diagnose my sister’s furnace problem. Electric furnace, blows air but doesn’t get hot. Does run okay in emergency heat, but expensive to operate in that mode. Wondering if the TVX should be replaced.

    • Hi Tony,

      This is a common situation that can occur during the extreme low temperatures of Winter. If a problem arises with the HeatPump unit, the furnace will produce heat by using the “back up” stage of the furnace. In your case this is the electric resistance coils in the furnace. I would recommend having a qualified HVAC Technician service the unit.

      Additional information on this topic can be found:

      Hope this helps,


  60. Scott,

    I’m working on a newer car with a TXV (really thought they all moved to CCOT, but not the Chevy Aveo). Anyhow, it was about 45 F outside and the engine was running “nominally” (1500 rpm, compressor continuously engaged). The low side pressure was about 47 psi and the high side was only about 85 psi. So, one might reason that the TXV was “stuck open” (albeit, maybe only slightly). I say this because i’m used to seeing more like 40 psi maybe even 30 psi (but then again, that’s on a CCOT system). On the high side, i’m used to seeing at least 150 psi, but on hot days, more like 250 psi. I’ve never worked on an AC while it’s 45 F outside, and maybe that’s why the head pressures were low. But could it be that the TXV was “stuck open”? I also read the post above on bubbles getting to the TXV and how that would reduce the TXV capacity. In that case, it might make sense that it’s not operating properly. It would also make sense that the situation might be remedied by adding refrigerant. Any ideas/comments?

  61. Hi Scott,

    Is txv the same with exv(electronic xv)? Our temp and humidity controller is not working well. We suspect it is the saginomiya pkv-18bs is the faulty parts. There are times that the acummalator thanks is freezing or icing but i cannot meet the 22deg setpoint. We are using R407c gas. I suspect that the exv or the rotor of the exv is not responding accordingly

    • Although TXV’s and EXV’s are trying to do the same job of metering refrigerant, they operate differently. TXV would operate due to the line temperature being converted into an opening (or closing) pressure force on the value. EXV operate on the temperature being converted into a digital signal to control the value. Both are metering devices but internally operate differently. Assuming your 22 degrees set-point is being expressed in Celsius (72F), “freezing” or “icing” of the accumulator, might be a floodback scenario taking place due to the metering device over feeding. Other possible causes might be loss of load on the Evaporator, or Evaporator Fan failure.

      Hope this helps,

      • i think it is due to floodback. I noticed that the cooling coil is not chilling at all this means that gas did flow to the cooling coil. Am i Correct? Thanks a lot.

        • Glenn,

          This does not sound right to me. A floodback condition would cause a very low or no evaporator superheat condition. The opposite of that, would be a very high superheat condition, caused by a starved coil. You might want to call another HVAC contractor to diagnose the problems.

  62. why do I get a loud Tea Pot type Whistle sound in the Expansion valve before the heat kicks in–and does this hurt the unit

    • Hi Gary,

      It is difficult to diagnose a potential problem without knowing exactly where the sound is coming from or how often it makes this noise. If it is just the refrigerant gas passing through the check valve within a bi-directional TXV this typically can happen after the heat pump has come out of defrost and the refrigerant gas flow changes direction and this is probably not a problem. If it is happening every time the system cycles on or off then it might be something you should have an experienced certified contractor investigate – especially if it seems to be getting worse. If your system has not had a check up recently this might also be a good time to have a contractor do some preventative checks on it before you get into the peak heating season. You can ask the contractor about this noise when they do the maintenance checks.

      Hope this helps,

      • I have the same a/c issue as Gary, Last Co. called Trane expert #. My contractor said he was told to remove and replace the check valve. Unit amps are perfect, pressures perfect, delta T, perfect, etc. Only problem is the tea pot like whistle coming from a 3/8 copper tube that connects to the rev. valve. The contractor notice the noise then a rattle from the ball in the check valve.

    • I have the same a/c issue as Gary, called in several a/c co. Last Co. called Trane expert #. My contractor said he was told to remove and replace the check valve. Unit amps are perfect, pressures perfect, delta T, perfect, etc. Only problem is the tea pot like whistle coming from a 3/8 copper tube that connects to the rev. valve. The contractor notice the noise then a rattle from the ball in the check valve.

  63. Could you explain in general terms what would happen if the bulb on a txv were not insulated at all, just properly clamped to the line? Assume attic temps range 75 to 110 degrees. Thx.

    • Hi Randy R,

      In the case of a TXV not being properly attached to the suction line, the bulb would be warmer than the actual suction line temperature. Since the bulb simply converts this suction line temperature over to an opening force, the valve would be overfeeding due to the increase in temperature resulting in an increase in pressure (P1)

  64. Dear Scott Lanzer please send me your mail address. I need help to design some water chilling evaporator using 2 ton ac out door unit. Plz..

    PS Nestle Pakistan.

  65. I am currently troubleshooting a package R410a 4 ton capacity. The TXV looks like it has been replace just looking how it was brazed. I do not know if the installed txv match the capacity of the unit, The no. on the txv is 1712 – G-EA 36YD199. This is for a Payne Model 3GNAA48090NA Serial No 3312C43502.
    Do you know if the installed txv matches the capacity of the unit.

    • Hi Zebbs,

      Not knowing the specifics about each OEM, your best bet would be to contact the Payne help center.

      1-888-41PAYNE (1-888-417-2963)


  66. Sir ,what would be the changes in the system can happen, if a TXV is removed from a package unit and fixed with a capilllary type expansion of a 3.5 tr compressor

    • Hi Damian,

      Changing an expansion valve (TXV/TEV) to capillary type metering device will reduce the overall system efficiency, along with the system’s ability to response to a change in load. In a larger 3.5 ton system, the large refrigerant charge amount and required capacity/load changes could cause the system to become more susceptible to refrigerant flood back. If you are set on changing the system metering device, you might want to consider a selectable piston fixed orifice device as another alternative, because it might be difficult to find a true capillary tube selection chart for A/C systems above 2 tons.

  67. I replaced my residential ac/h in late 2011. Unit functioned well until cooling was turned on. It was diagnosed as having a faulty expansion valve. Now after two years the unit is not cooling properly and is again diagnosed as having a faulty expansion valve. While the expansion valve is covered under waranty, labor and refrigerant aren’t which costs me about $300 each time. My question is: is this characteristic of the TXV. Are they all the same, or is there one more reliable than others? Or is this the price we pay for energy efficiency?

  68. I am a service tech in tampa fl. Every ten to fifteen seconds the comp. Makes loud grumbling noise like its slugging, I was there late last night aand didn’t get any superheat or subcool first thought was a faulty txv. I’m heading back today,but wanted to get your opinion.

    • Erik,

      The only way to know for sure if it is slugging, is to take a superheat measurement at the compressor suction line. Slugging in a refrigerant cooled compressor (most common) is cause from Floodback, meaning liquid refrigerant returning to the compressor while the compressor is operating. A TXV over feeding, or a coil under a low load could be the cause of a low (or no) superheat scenario. But without a suction line superheat measurement, it is tough to know for sure.

      Hope this helps,

  69. I have an AC unit that shuts itself off before reaching the set temperature in the home (set at 73 degrees – only reaching 79-80). I have had the compressor replaced, coils cleaned, thermostat replaced, and even the control board in the actual furnace unit replaced. I had another company come out to go over the whole system, which they said was fine but they thought it might be the TXV line which was dripping – quoting me $542 to replace the line. Could the TXV line been responsible for the whole system shutting off?

    • Based on the information you provided, it sounds like your system was shutting down on the compressor internal overload protection, before meeting the setpoint temperature. If the dripping from the TXV line is oil, then this is a good indication that the 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 overload protection, it is starting to enter a “loss of charge” failure mode. This event could be what is causing your whole system to shut off. My advice would be to get a clear understanding of the work being suggested to your system, and be sure to explain the system history and past experiences with a Certified Contractor.

      Hope this helps,

  70. I am a service tech at the HVAC company I work for. I have to go back to school once more for my refrigeration apprenticeship.
    What I would like to know is in relation to improper positioning of the Bulb. A couple of our installers were not properly shown how to position the bulb other than to make sure it is on top if the line is horizontal. In school they taught us to ensure that the cap tube coming off the bulb is positioned to ensure only refrigerant vapour will be used as the opening force.
    Now these installers have installed quite a few of these bulbs upside down and if my understanding is correct then the diaphragm is always getting liquid pressure from the bulb and being forced open. I have had to replace 3 TXV so far since even after repositioning them I can not get proper superheat.
    So finally the question, does having the bulb positioned this way end up breaking the TXV? Or is it just that these just happen to be bad TXV? I am leaning towards it being the cause of the failure, however I can’t find any documentation to back that up.
    Thank you for your time.

    • Jeese,

      The best place to mount the bulb is on a horizontal run of the suction line. You are correct that the preferred location for suction lines less than 7/8″ tube is at the 12 o’clock position. The preferred location for suction lines greater than 7/8″ tube is at the 8 or 4 o’clock position. The reasoning for location on the larger size tube is that this location is close enough to the bottom of the pipe to detect any liquid refrigerant but far enough off the bottom so as not to be effected by any oil laying on the bottom of the tube.

      In some cases it is not possible to clamp the valve to a horizontal run of pipe. The conventional wisdom use to be that if the bulb was mounted in the vertical position with the capillary tube at the 6 o’clock potion, that the charge in the bulb would drain out of the bulb and into the powerhead leaving the valve unable to sense the suction line temperature causing the valve not to perform properly. However, as long as the bulb is colder than the powerhead the charge will not migrate and will stay in the bulb regardless of the bulb orientation. In addition, bulb orientation will not impact the reliability or life expectancy of the valve.

      Regardless of the position of the bulb it should be clamped securely to the tube and well insulated. The insulation should be wrapped completely around the bulb and the section of tube that the bulb is clamped to. The purpose of the insulation is to insure that the bulb senses the pipe temperature and is not influenced by any other temperatures.

      Thanks, and I hope this helps,

  71. What would cause a TX valve to go bad? What is the normal time and cost of a repair? My AC unit is only 4 years old and it seems to me that the TX valve should have lasted longer. Is this something that I am going to have to fix every 4 years?

    • There are several primary causes for a TXV failure. One possibility is debris within the system traveling downstream and restricting refrigerant flow at the valve port. Another common failure mode is damage to the remote sensing bulb or capillary tube caused by system vibration. Both of these failures will starve the evaporator and increase system superheat.

      As for the expected useful, system components like TXV’s are designed to an OEM specification for useful life expectancy so these should be the same as for the whole system and the other components.

      As for the costs to change out a TXV, these costs range from system to system and from contractor to contractor. We usually suggest securing several competitive bids from reputable contractors also and getting a second, independent opinion on recurring problems.

  72. We installed MOP TXV as desuperheating valve and sub-cooler/economizer in the 2nd stage of compressor and mistakenly bent 1/4in copper tube w/c was causing restricted/reduced pressure in the equalizer. The liquid line coming from sub-cooler was still warm resulted to high head pressure. When we corrected its equalizer pipeline, upon testing the unit the liquid line become wet in less than 20 minutes.

    If P1+P4 = P2+P3 (opening force = closing force), why refrigerant flow was restricted resulted to high head pressure and inefficient subcooling if P2 was lost?

    Thank you very much.

    • The equalizer line senses the suction pressure at the outlet of the evaporator while the TXV bulb senses the temperature at the same point. As a result, the valve “knows” what the superheat is and can open/close to maintain a relatively constant superheat. But, if there is a restriction in the equalizer line, then the pressure sensed at the valve side of the restriction will be higher than the pressure downstream of the restriction. This higher pressure will tend to make the TXV “close” resulting in an increase in superheat.

      The P2 pressure you refer to is not “lost”. It is in fact higher than it should be, resulting in an imbalance in the closing direction.

      The severity of the restriction also needs to be considered. If it is just a minor restriction than as the suction pressure drops, you may see a small delta P across the restriction causing some increase in superheat. But when the system is operating with stable suction pressures, then the valve should operate normally.

      If this is a Balance Ported valve, then it is also possible that you have a leak across the internal seal allowing liquid refrigerant into the equalizer area. This too will cause in increase in pressure at the underside of the diaphragm (P2) causing the valve to move in a closing direction.

      In summary then, any restriction in the equalizer line will result in higher superheat and that is why your sub-cooler is not operating at its design condition.

      Hope this helps,

  73. Thank you for your information. I have a question which i am not sure of if u can help.

    when fitted into a refrigeration system the TEV operates by
    a. variations in refrigeration flow due to superheat control
    b. the system pressure being greater than spring pressure
    c. reacting to flow measurement
    d. reacting to changes in the liquid line durring operation

  74. All metering devices have a rated capacity range. This capacity rating is based back on the device receiving 100% liquid refrigerant to the valve, (at least 1°F Subcooling). Any “vapor” refrigerant passing through the valve, de-rates the valve’s capacity and thus reduces the rated capacity of the metering device.

    If the load on the Evaporator requires the metering device to operate at 100%, and due to “vapor” the valve cannot supply enough liquid refrigerant to the evaporator coil, this could cause an increase in system Superheat and damage to the refrigerant cooled Compressor.

  75. HI dear
    if the flow to expansion valve have some gas what effect on valve performance?
    since we don’t have sub-cooling

    • There will not be any effect on the valve…effect will with the evaporating coil..its cooling rate falls down due to entry of vapour refrigerant

        • Hi Shyam,

          A capillary tube style metering device acts much like a “fixed orifice”, as the refrigerant is metered by three parameters. The three parameters in which the refrigerant is metered are, internal dimension (ID) of the capillary tube, length of the capillary tube, and differential pressure between the tube inlet and outlet. Capillary tubes cannot respond to any changes in system load and evaporator superheat, unlike a TXV/TEV.


        • Hi Al Oma,

          Generally I would say no, as the system needs a metering device and placing another metering device in parallel with the first could lead to problems. Not knowing how the “bypassed” valve will respond or how the valve fail (open/Closed) could cause a loss of control with the liquid refrigerant being fed into the evaporator. These metering devices control the expansion and evaporation of the refrigerant in the coil to provide cooling. If the liquid refrigerant does not vaporize before returning to the compressor, this could lead to a floodback scenario for the compressor.

          Hope this helps,

    • Have a question,,, my home ac/ heat worked fine,,, heat pump,,,, everything was working fine one night and next day I found the connector on side of pump burnt off and all freon gone,, r410a,,,, ok bought new pump and installed and vaccumed and recharged and a min or few mins system line freezes up inside before expansion valve and on suction line before pump outside and what looks as the dryer freezes up also,,, coils r clean,, is this due to the expansion valve

      • Hi Tommy,

        It is very difficult to troubleshoot a system without actually being on the job site. But by your explanation, freezing of the liquid line after the dryer means there is a drop in pressure, causing a drop in temperature at that portion of the line. If the line (liquid line) freezes up at the dryer, then the dryer could be clogged and is acting as a metering device. After that point the entire system acts as the evaporator.

        Hope this helps,

        • Hey man I have a problem my 6 yr old trane heat pump condensor with horizontal lef t airhandler they fliped the evap coil to make it drain correct but thesubcool is 10 degree and goes up when I add freon but the suction pressure stays at 38 degree inside coil temp I replaced txv 6 months ago and still doing the same thing

          • When the suction line on a A/C does not sweat, but the compressor runs and the amperage is okay, what could be the cause?

          • Hi Seymour,

            This could be normal operation as sweat or condensation will only form when humid air is in contact with a cold surface, in this case the suction line. A qualified service technician would use refrigerant service gauges that connect to the unit that read pressure and temperatures to determine correct operation.

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