Piston vs. TXV Metering Devices

The piston (fixed orifice) and TXV (thermostatic expansion valve) are the two most common metering devices in use today, though some modern systems utilize an electronically-controlled metering device called an EEV (electronic expansion valve). It should be noted that there are other types of fixed-orifice metering devices, like capillary tubes; however, their use is not common in most modern A/C systems. (You will more likely see them in refrigeration.)

HVAC/R systems move refrigerant by creating a pressure differential. We think of the compressor as the part that creates that differential; it decreases the pressure on the suction side and increases the pressure on the discharge side.

So, the purpose of the metering device is to create a pressure drop between the liquid line and the evaporator coil or expansion line (the line between the metering device and the evaporator, though it's not present on all systems). When the high-pressure liquid refrigerant is fed into the metering device at the inlet, the refrigerant flows out the other side with an immediate pressure drop. That quick pressure drop results in an expansion of a portion of the liquid directly to vapor. We call that “flashing.” The amount of refrigerant that “flashes” depends on the difference in temperature between the liquid entering the metering device and the boiling temperature of the refrigerant in the evaporator. If the difference is greater, more refrigerant will be “flashed” immediately. If the difference is less, then less refrigerant will be flashed.

Piston

A piston is a replaceable metering device with a fixed “bore.” It is essentially a piece of brass with a hole in the center; the smaller the bore, the less refrigerant flows through the piston, and vice versa. The advantage of a piston is that it is simple and can still be removed. You can also change the bore size and cleaned it if need be.

piston_flow

As you can see in the diagram above, some piston systems also allow reverse refrigerant flow. When the reversing valve is energized (cool mode) in a heat pump system, the refrigerant will follow the path indicated on the bottom. This process seats the piston so that refrigerant must pass through the orifice. With the reversing valve de-energized, the flow reverses. That unseats the piston and allows the free flow of refrigerant. In this case, there is a metering device in the condensing unit (outside unit) that meters the flow of refrigerant in heat mode, and there is one inside that meters in cooling mode.

TXV

The TXV can vary the amount of refrigerant flow through the evaporator by opening and closing in response to the evaporator heat load. Compared to a fixed orifice, a TXV operates more efficiently in varying environmental conditions (theoretically, at least).

To operate, the TXV has a needle and seat that restricts the flow of refrigerant and acts as the orifice. This needle, when opened, allows more refrigerant to flow, and it restricts refrigerant flow when closed. There are three factors that affect refrigerant flow through a TXV.

The first of those is a sensing bulb filled with refrigerant that exerts force to open the TXV. Since gas pressure increases with a rise in temperature, the bulb attached to the suction line after the evaporator coil “senses” the temperature of the suction line. If the suction line becomes too warm, the additional pressure created by the heated refrigerant opens the TXV more to allow additional refrigerant flow. There is also a spring inside the bottom of the TXV that exerts pressure to close the valve. The final factor is an external equalizer, which senses pressure in the suction line after the evaporator, and it also works to close the valve. In essence, the TXV is a constant superheat device; it sets a (relatively) constant superheat at the evaporator outlet by balancing bulb, spring, and equalizer pressures.

 

The primary method of charging a system changes based on the type of metering device. A piston system uses the superheat method of charging, and a TXV uses the subcooling method of charging. The illustration below shows how to add charge to a TXV system using probes. Monitoring temperature and pressure is critical to setting a proper charge.

No matter what primary method of charging you use, it is still important to monitor suction pressure (evap temperature), head pressure (condensing temperature), superheat, subcool, and delta T (or some other method of airflow verification).

While a TXV and a piston function differently, the end result is a pressure drop and boiling refrigerant in the evaporator.

—Bryan

7 responses to “Piston vs. TXV Metering Devices”

  1. Bryan, in your opinion, overall which is the better metering device, piston of TXV? How about during seasonal changes when going from summer to winter and visa versa?

  2. I just had new ac unit installed that came with TXV . My unit kicked on and off every 10 mins at the same temperature it was set at with a new Honeywell thermostat installed . If i lowered the temperature setting it would run for 2 1/2 hrs or more unless I shut it off. I tried another Honeywell thermostat and the same thing happened. The AC tech thought it was due to the TXV not working right so he changed it to a piston. Same thing happened with it running every 10 min or running nonstop depending on where the thermostat was set. After he left, I reinstalled my old thermostat which he said no longer worked and BINGO….the temp in my house lowered by 6 degrees in about 20 mins , shut off and didn’t turn on again for another hour and a half. Since I changed back to my old thermostat this ac unit is working like it should. The tech was blaming everything from my sheer curtains over the registers to heat coming from the TV to me using the wrong filter in my furnace because I buy the good ones. According to him, it was constricting airflow. My question is, should the piston be replaced with the original TXV part.?? Tech stated piston would only lower my SEER rating. Now I wonder if he’s telling me the truth. Why does my old thermostat make it run properly and the new ones Won’t?

    • Without knowing superheat at the compressor, subcooling at the metering device and delta T it is hard to answer your question. First thing that comes to mind is tstat was not installed correctly (wiring, position, etc.) also heat load as a conditioned space cools down the unit will run less. Maybe your space was warm so unit “short cycled” which is common. As interior space cools unit will operate less frequently.

  3. We have a 4-Ton Heat Pump. The air handler has a matched coil with a TXV. The installer took the piston metering device out of the Outdoor the Outdoor unit. With a Heat Pump, don’t we need both ends with a metering device?

  4. My unit is not working and the tech wants to replace TXV with piston. Should it be replaced with Another TXV or piston?

  5. educational, exact technical info needed for the tech’s

    about the TXV selection. what required data to find what best suit the system?

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