How to Tell Whether Your TXV is Bad
The thermal expansion valve (abbreviated as TEV, TXV, or TX valve) controls the volume of refrigerant released into the evaporator of refrigeration systems. You’ll find them on air conditioners, heat pumps, chillers, refrigerators, and freezers.
The TEV works by dropping the pressure between the liquid line and the evaporator. This causes the refrigerant to boil at very low temperature – 134a refrigerant boils at about -15° F, for example – and thus transition from a liquid to a vapor throughout most of the evaporator coil. In that phase it absorbs heat, cooling the area.
When the TEV malfunctions:
- The unit may blow warm air
- Frost may form on the evaporator
- The AC compressor may run constantly
To troubleshoot the problem, start by checking the refrigerant pressures and system temperatures and comparing them to the manufacturer’s standard operating values. Take readings on the superheat, subcooling, evaporator coil and condensers, and compare their temperature or pressure with the optimal operating conditions specified by the manufacturer.
To diagnose a bad TXV, look for:
- Low evaporator pressure
- High evaporator and compressor superheats
- Low compressor amp draw
- Short cycling on the low-pressure control
- Higher than normal discharge temperatures
- Low condensing pressure
- Low condenser split
- Normal to high condenser subcooling
These conditions may appear with either a clogged TXV or a clogged liquid line, because the TXV is part of the liquid line.
Most often, cleaning or replacing the inlet screen or replacing the power head to eliminate the clog resolves the issue. But, coolant flow through the TXV also may be restricted by:
- A partially-closed valve
- Wax buildup caused by using the wrong oil in the unit
- Refrigerant flooding into the compressor
- Excessive oil in the system
- Foreign material in the valve’s orifice
- Sludge left over from a burnt-out compressor
- A partial TXV orifice freeze caused by too much moisture in the system
After checking for a clogged filter, ensure there is sufficient liquid pressure to generate the manufacturer’s suggested pressure differential between the liquid line pressure and the evaporator pressure. (That’s typically at least 100 psi.)
Next, check the superheat (the temperature of the vapor above its boiling point when in liquid form) at the evaporator. It should be between 6°F and 14°F of superheat at the evaporate outlet. Superheat below 6° suggests the valve is overfeeding (open too far), while temperatures above14°F superheat suggest underfeeding ( the valve isn’t open enough).
The pressure, and therefore the temperature, should be fairly consistent throughout the system. Thermal imaging can help you identify where the temperature changes
A bad TXV isn’t the only possible cause of those symptoms, even when it contributes to them. For example, an underfed refrigeration system exhibits many of the same symptoms as a bad TXV:
- Frost on the valve or coil
- A low pressure reading at the suction service valve
- High superheat
- High liquid pressure at the liquid line valve
Those are the same symptoms as an undercharged system, except that there is low pressure – rather than high pressure – at the liquid line service valve. The difference between an undercharged system and a clogged TXV is that an undercharged system exhibits low levels of condenser subcooling. If refrigerant is added under those conditions, when the problem is a restricted TXV, the head pressure may increase and the receiver may overfill, creating a potentially dangerous head pressure.
In contrast, an overfed system exhibits:
- High suction pressure
- Low superheat
- Low-to-normal liquid pressure
These symptoms may be caused by an overcharged system, a loose TXV sensing bulb, an open TXV valve, or a clogged external equalizer.
TXV problem are misdiagnosed frequently when technicians focus on pressure. Instead, looking at the broader system may help you address the root of the problem and, sometimes, make a simpler repair.