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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206 thoughts on “Guide to Thermostatic Expansion Valves

  1. We had a new heat pump installed about 4 years ago, and had the air handler portion installed about 5 years ago (with an older heat pump). We have been having issue ever since installation of the new heat pump, and have had everyone and the boss look at it. We get cold air and the machine shuts off appropriately once it reaches temperature. However, upon restart the machine starts blowing air from outside that isn’t cool (it’s the same as the outdoor temp). If we flip our breaker off, then back on, and restart the thermostat it starts blowing cold again. We’ve checked out “fan settings” and everything else on the thermostat, and everything is as it should be. Our HVAC told us that we need a new “TXV” due to an issue with Trane installations using smaller diameter tubing in the past, and issues with the rate of coolant being used in the system. Does this sound legit? We don’t want to pay an arm and a leg to fix this valve, if it could be something similar.

    • Paul let’s do this real quick! I’m sure there is an outside control that initiates the outside
      air damper. Okay? Locate the control and disconnect the wires (There could be as many
      as three but at least two). That will eliminate the outside air from coming into play. But
      the problem obviously lies in the control wiring. As far as the other factor that someone
      is trying to sell you, I just don’t see it. Okay, I’ll see you in the Movies! ! !

    • Henry,

      Great question! Emerson is not aware of any code that addresses insulating the TXV. That is not to say or excuse contractors do not have an obligation to understand and follow local codes.

      Your question is about a “TXV”. To properly address your question, it is important to understand that a “TXV” is a two part construction: a valve body and a sensing bulb.

      The “sensing bulb” section should always be insulated for proper TXV operation; good thermal contact and proper placement between the bulb and suction line is the most important factor.

      The “valve body” section need not be insulated, i.e. performance will not be affected under normal conditions. That said, it’s always best practice to insulate the valve body because condensation can occur on the valve body and, over time, lead to damage of surrounding materials and/or personal safety (water pooling).

  2. Can someone please help I know nothing about air conditioners my installer has been to my house 4 times it’s a two year old 3.5 ton unit his last visit he removed the txv valve to the skinny pipe on the advice of another installer he previously had it on the insulted fat pipe now he moved it to skinny non insulated pipe please advise me

    • Brock,

      There’s not enough information here to make an informed and helpful analysis for you. We would recommend that your service contact our technical support team at 888-725-9797 with all of the performance/operational details for the best solution to your issues. Let’s get your system dialed in and cooling the best it can for the remainder of the summer! Thank you for your submission.

  3. I have installed HVAC for a while now. When the txv valve became the standard I have noticed that some of the time when I start a new unit it will run 150# suction pressure and only put out around 64 degree air. I return the next day and I’m running 125# suction and it’s cooling well. What causes this?

    • Your concern over the 150# and the 125# psi readings are somewhat normal. Let’s look
      first at the 150#. Starting out the room is warm (Hot), whatever you choose so the high
      reading is expected. After some time as the room gets colder (especially the evaporator),
      your Back pressure drops. Let’s take another picture at 150 # equals to 52.5 deg. Then at
      125# that equals out to 42.6 deg. You can refer to your PT chart. But 118# would be better. That means that you would have to increase the spring pressure on the valve one
      turn clockwise. That does not mean that one turn will get you there, but then you can kind of tease it until you get the desired PSI. The valve opening will remain the same.

  4. My question is, on a self contained water source heat pump.. There is 1 TXV called a bi flow Txv… It has IN for inlet and the other side is obviously the Out. So being a bi flow txv, does it really matter which direction it is piped? I think it matters because maybe the direction has to do with a certain flow rate during cooling and different flow rate in heating? I know a bi flow drier can be piped either way but what about a bi flow txv? I pipe them the same as the old one was from the factory.
    I’m working on Climate Master or Mammoth wshp Model# D014HHE

  5. I have a Lennox 37L51 Expansion valve that is leaking condensation into my secondary drain pain. Is this normal?

  6. I have a Fujitsu mini split I had installed about four years ago several weeks ago I started getting an intermittent weird noise coming from the indoor unit .This noise only happens on bright sunny days it starts late morning into early evening it never makes noise at night or on cloudy days.Ive even tried turning power to the head off it still made noise you have any ideas.thanks Marc

    • Typical Mechanical TXV’s have a large circular diaphragm cover where the expansion bulb capillary tube is welded. It is industry standard to consider this the “top” of the valve. TXV valves can be installed up to 90° from this position in any orientation.
      It is not recommended to have the orientation where the diaphragm is rotated to be at the bottom position. The reason is that there is oil in the refrigerant. In the “upside down” orientation the oil can collect on the diaphragm leading to issues.

  7. My AC installer cross-threaded the line to the A coil so he removed the TXV and used a orifice to regulate flow.
    1. Will this hurt the supercool effect of removing moisture?
    2. Can this hurt the compressor?
    The system just died and it might cost a Kings ransom to resolve all issues and fears. But I mostly need to remove moisture as my hardwood floors buckle. Thanks for your input and don’t hold back.

    • No the piston just releases a fixed amount of refrigerant. We use them all the time. As long as it’s faced in the right direction and it’s the right size for your system (use a florater chart) you should be good.

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