A thermal expansion valve, more commonly known as a TXV, is a device that meters refrigerant through a cooling coil. The main reason that we use TXV’s is they are more accurate than some of the fixed metering devices such as a capillary system, which is a series of small capillaries that meter refrigerant through multiple passages in a cooling coil, or a fixed metering device known as a piston. A TXV is the most accurate way to meter refrigerant to a cooling coil. It incorporates a sensing bulb that mounts on the outlet of the coil so it can adjust the amount of refrigerant passed through the valve.
The TXV operates on what is referred to as super heat, that is the amount of heat that is absorbed by the refrigerant after it becomes a vapor in the cooling coil. The value of this super heat in an air conditioning coil is between 8 and 10 degrees. This can be checked with a proper set of refrigerant gauges and an accurate thermometer.
Residential air conditioners with TXV’s are fixed and not adjustable. Larger systems and large refrigeration systems can use rebuildable and adjustable TXV’s. Typically, residential air conditioning TXV’s are one of two sizes - the TXV will adjust itself to the load on the coil. A 1.5 ton to a 3.0 ton unit will have the same TXV, and a 3.5 to a 5.0 ton unit will use a bigger size.