Guide to Thermostatic Expansion Valves

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Learn how thermostatic expansion valves work in HVAC systems.

Originally published on June 24, 2013

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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267 thoughts on “Guide to Thermostatic Expansion Valves”

  1. 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?

    Reply ↓

  2. 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

    Reply ↓

    • 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,
      Scott

      Reply ↓

      • 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.

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        • 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.

          Reply ↓

  3. 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

    Reply ↓

    • 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,
      Scott

      Reply ↓

      • 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.

        Reply ↓

    • 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.

      Reply ↓

  4. 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.

    Reply ↓

    • 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)

      Reply ↓

  7. 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.

    Reply ↓

  8. 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

    Reply ↓

    • 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.

      Reply ↓

  9. 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?

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  10. 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 readings.my first thought was a faulty txv. I’m heading back today,but wanted to get your opinion.

    Reply ↓

    • 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,
      Scott

      Reply ↓

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