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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171 thoughts on “Guide to Thermostatic Expansion Valves (TXV)

  1. 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.

  2. 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

  3. 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. http://www.ac-heatingconnect.com/homeowner-behind-the-scenes-how-your-air-conditioner-works/ I hope this helps answer your questions. Thanks for using our site!

  4. 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.

  5. 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.

  6. 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?

  7. 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, 🙂

  8. 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.

  9. 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.

  10. 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.

  11. 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.

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