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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211 thoughts on “Guide to Thermostatic Expansion Valves

  1. I had an A/C tech come out and immediately told me that he suspected that I had a big leak and all of the freon would be out of my system. This is same reason he gave me when he called to say that he was running behind at the customer before me. He opened the outside compressor and after 5 minutes asked me to come see what definitely appears to be a clean cut copper pipe, not ruptured but clean cut and completely void of any remaining freon. I very suspicious since the customer before supposedly had absolutely no remaining freon either. How could that pipe get cut when it was screwed down with only professional access.

  2. Hai sir very good information sir .
    My doubt is TXv only flow of liquid refrigerant to evaporater . Troubleshooting of TXv and what is role of BUlb pls reply

  3. Im searching for an expansion valve.. 3-ton r-22 with 15% bleedport and chatleff threaded fittings…

    Any idea where to purchase..?

  4. Scott:
    I have a Trane AC unit that was installed in late 2014. Since then I have had the TXV replaced twice due to debris. The first time 6 months after the unit was installed. The second time this summer – 6 weeks ago. In both situations prior to replacement the delta T had fallen to 10 – 12 degrees. The last time the TXV was replaced the technician replaced the system’s refrigerant with virgin refrigerant. Now the system is showing showing the same effects again – not sufficiently cool air coming from the supply registers and a delta T of 12 degrees. The TXV problem with Trane units seems well documented on the web so my question is there any hope of finding a permanent solution to this reoccurring problem. Trane has been up front in covering the part replacement but the last time I incurred a labor charge which was not insignificant.
    You mentioned damage to the capillary tube from vibration as another area for refrigerant metering to be compromised.
    I need to get a another opinion on this performance issue.
    Thank you

    • I work for a Trane dealer. I’m not a fan of the Danfoss txv’s they have been using. If your txv is a Danfoss, I’d spec and install a Emerson or Sporlan txv if you have another bad Danfoss txv.

  5. I had some notes that were published on Nov. 6, 2017. I could always check on it for ? ? ?
    and of course other views on or about my notes. Is there anyway for that to be seen
    again on “Let us know your thoughts.” It was erased by mistake. I had it on my “Tool Bar”,
    and while trying to remove or change the “Tool Bar”, it was erased. I had it on my Flasher,
    and whatever you remove from the flasher you remove from the Flasher is lost. Obviously
    what I’m getting at is there any way you could display it again ? ? ?

  6. I’m in a process of retro fitting air con to my Vito, I’ve got this expansion value which has a connecter with 3 pins and I want make sure that I am going to attach each one of these pins to the right place can some one shed some light on this please as I’m not sure how to.
    Many thanks in advance

  7. 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

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