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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216 thoughts on “Guide to Thermostatic Expansion Valves

  1. Is it possible to control the expansion valve for vrv system thru its undividual wire remote control?
    Let say you have 3 package evaporator and the 2 unit is operational, 2 unit is standby but this units expansion valve id maintain open thru wired remote control..
    Thanks

    • In the application that you are describing, and without specific model numbers of equipment and components, the EXVs that are probably being used are controlled by a pulse width modulated signal. Therefore, without knowing the precise amount of time needed and the signal strength to drive the valve to a certain position (usually proprietary OEM data), it would be very difficult to operate the valve. These types of EXVs receive many signals to drive to certain positions over a short amount of time to control superheat in order to achieve precise refrigerant control. There might be a setting from the OEM, if the valve is operational, to drive it to a certain fixed position in an emergency situation to provide refrigeration effect/operation.

      • Hi, thanks for immediate response
        I am refering to Daikin VRV 3 models for wall mounted type of evaporators.
        3 units was operated with separate wired remote controls, 2 units is running 24/7 and 1 unit is on standby, but is it possible in the standby unit to maintain its expansion valve to open even if it is not operating?

        • I’ve scanned the Service Manual (https://www.daikinac.com/content/assets/DOC/SiUS34-801A_b%20VRVIII%20Service%20Manual.pdf) and don’t see a specific reference to the EXV position when the system is in standby mode. I would suggest contacting Daikin Technical Support @ 1 (855) 770-5678.

  2. Charge weighed in and drawed down to 400 microns acid away and new driers installed, the compressor side good, threaded TXV, set at 2 o’clock and tight insulated. It’s a straight cool unit on a trailer that they wanted an upgrade to 410 because of compressor failure 5 years ago. New trailer 4 ton coil with 4 ton goodman condenser 13 seer back then. Never serviced since install found with burnt windings. Coils clean inside and out. Pretty simple question without going to nuclear colledge on this. I have never heard of using a TXV that wasn’t for the sized unit you were putting it on, unless your trying to create Ozone or something. Just a serious 60 psi hunt which makes it impossible to set a good subcool.

  3. Recently I encountered an issue with a extreme hunting of the txv after changing the compressor, never have seen that before and felt maybe that’s why the compressor burnt on this mobil home unit from being slugged. I called it a bad TXV and went to where I purchased the 4 ton AC condenser and matching 4 ton coil. They said it was an aftermarket coil set that was purchased with the set and this valve will work for a 2 ton through 7 ton. I have never heard of such a thing myself but said ok fine. 6 hours later on a friday night I’m fighting with a hunting TXV like the one I removed. looking at the box they gave me a 5 ton TXV. I’m really calling BS at this point and pulling my hair out also. I cant imagine this well known supplier doing the same to me on the compressor too and having my world in an uproar. The instructions have where you can adjust the superheat on the bottom of the valve screwing it in or out. Nothing seemed to work and like my instinct was telling me ……….they didn’t give me the right valve! Should have been for a 4 ton unit then you have a range to adjust your superheat after setting subcool from 8 to 12 degrees. Nothing worked and it’s the weekend that god the person it was for is going to alaska and wont need it repaired for 2 weeks. What do you think is going on.

    • Brands and model #s of original equipment and replacement parts might help. Airflow calculations and static pressure verification. Age of condenser. Position of bulb on suction line. Is TXV brazed in or or threaded?

      • Also, was the charge weighed in after compressor replacement? What was the reading on the micron gauge after vacuum pump?

        • Joe, if I were working on the unit, I would focus on the Power Charge that you are using
          on the Valve. Or to be more correct the charge that came with the unit (Valve). That’s not
          to suggest that you don’t have to adjust the valve’s spring pressure after changing the
          Power Charge. OK, see you in the movies. . .

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