What is a TXV – thermostatic expansion valve – in an HVAC system?
A TXV, or thermostatic expansion valve (TEV), is a piece of equipment that meters the flow of liquid refrigerant into the evaporator while measuring the vapor refrigerant leaving the evaporator. It thereby controls the superheating, or superheat limit, at the outlet of the evaporator.
This component is important in an air conditioner or HVAC system because it helps to maintain the correct superheat and prevents liquid droplets from entering the compressor, which could cause damage.
How Does a TXV Valve Work?
The TXV valve is filled with a refrigerant that boils at a lower temperature than the liquid refrigerant passing through the valve. This causes the pressure in the valve to rise, and as it does so, it opens wider to allow more liquid refrigerant to flow in. At the same time, the vapor refrigerant leaving the evaporator is also being measured. If this vapor is too hot, it will cause the pressure in the valve to drop and the TXV will close off until the vapor has cooled down. By doing this, the TXV helps to maintain the correct superheat and prevents any liquid droplets from entering the compressor.
What is the Superheat Limit?
The superheat limit is the temperature above which liquid refrigerant will start to evaporate (turn into a gas). When the TXV valve senses that the vapor leaving the evaporator is too hot, it will close off until the vapor has cooled down. This prevents liquid droplets from entering the compressor, which could cause damage.
Common Problems with a TXV Valve
A TXV valve can sometimes become clogged with dirt or debris, preventing it from performing its function correctly. However, more commonly:
Subcooling
It can also malfunction if the subcooled liquid refrigerant charge is not correct or if there is a restriction in the line. It should be filled all the way to the inlet. On a split system, it is installed on the high-side of the liquid line and you should ensure there is no significant temperature drop along the length to the expansion valve inlet. and on a package system, it is installed in the condensing unit.
Pressure Drop
There must be a substantial pressure difference between the evaporator’s designed pressure and the liquid pressure entering the expansion valve for an expansion valve to operate. A pressure drop that is insufficient – say, less than 100 PSIG – will not cause the valve to open. This may happen at cooler times of the year.
Compressor Surge
If the compressor start up surge is too great, it can cause the expansion valve to close. This will usually result in an automatic reset of the compressor, which should correct the problem.
Bulb Placement
If the bulb is not placed in the correct location, it can cause the expansion valve to not open. The bulb should be located as close to the evaporator as possible, but still in the suction line.
How to Adjust a TXV Valve
If the TXV valve is not performing correctly, it may need to be adjusted.
This is a job for an experienced HVAC technician with proper training.
You will need to measure the superheat at the evaporator outlet, near the equalizer and TXV bulb. The manufacturer’s specifications will inform the HVAC technician of the correct superheat value. You will also need to measure the evaporator inlet and outlet pressure.
Before making any adjustment, the system needs to have been operating for sufficient time to allow the supheat to stabilize.
The adjustment screw can usually be found on the top or side of the valve. Remove the hex cap to expose the screw.
Adjustment can be performed with a screwdriver or by using a special tool called a TXV adjustment wrench.
Turn the screw clockwise to increase superheat or counter-clockwise to decrease. Make only a half-turn at a time, without forcing the screw, then measure the superheat again. This should be repeated until the maximum setting is reached.
Related Links
Guide to Thermostatic Expansion Valves (TXV)
TXV or Thermostatic Expansion Valve – Refrigeration for HVAC
Thermostatic Expansion Valve (TXV)
Thermostatic Expansion Valve (TXV or TEV) – This Is How It Works
How Thermostatic Expansion Valves Work
Thermostatic expansion valve (TXV) transient modeling; TXV hunting
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