Guide to Thermostatic Expansion Valves

Learn how thermostatic expansion valves work in HVAC systems.

Understanding TXVs

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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197 thoughts on “Guide to Thermostatic Expansion Valves

  1. 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. . .

  2. 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.

  3. 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

  4. 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!

  5. 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.

  6. 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.

  7. 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?

  8. 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, 🙂

  9. 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.

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