Our Pick HVAC pros answer a lot of FAQs. This is one of the more technical questions we’ve covered. It’s a topic and explanation best suited to HVAC pros, but homeowners like you want an understanding of their equipment - what makes it tick, and what causes issues with its performance. So, here we go.
A TXV valve, or a thermostatic expansion valve, is a very precise valve that regulates the rate of flow of refrigerant to the evaporator on an air conditioning system. It’s purpose is to maintain an efficient flow of refrigerant while also protecting the compressor. These features of the TXV valve are explained below.
Another name for the TXV valve is the thermal expansion valve. By controlling the flow of the liquid refrigerant with the TXV valve, the evaporator on your air conditioning unit or heat pump will run more efficiently.
The TXV valve also serves to protect the mechanical reliability of the unit. It prevents any extra refrigerant from going back to the compressor, a process called “floodback,” “flowback” or “flow-back.” Whichever term you use for this thermodynamic activity, the process isn’t a good one. Flowback can damage the compressor.
What Does a TXV Valve Do?
There are many different design features for the TXV valve. One such feature is the valve’s ability to separate the high and low pressure sides of the air conditioning unit. As the liquid refrigerant enters the thermostatic expansion valve (TXV), it is under very high pressure. The TXV valve then makes sure that the refrigerant’s pressure is lowered as it enters the compressor. All of this sounds very technical and confusing, but basically, the TXV valve makes sure that the liquid refrigerant is evenly distributed throughout the compressor and the compressor isn’t under too much pressure.
What Doesn’t a TXV Valve Do?
Now that we have a basic understanding of what the TXV does for your air conditioning unit, we can rule out what the TXV valve doesn’t do. We mention these things because there is some misunderstanding about the role of the TXV valve in the operation of an air conditioner or heat pump. First of all, the TXV valve does not control the air temperature. It also doesn’t regulate the pressure on the unit’s head or control the suction pressure. Finally, the TXV valve does not impact humidity control, which is an important part of air conditioning.
How Does a TXV Valve Work?
The TXV valve controls the liquid refrigerant from the outdoor condenser and regulates how much of the refrigerant flows to the indoor evaporator coil. The valve does this by lowering the pressure in the AC unit, which causes the refrigerant to boil at a lower temperature. The boiling of the refrigerant is vital to any air conditioning system. By changing the pressure, the boiling point is lowered to a more efficient and safe level, such as 70 degrees compared to 100 degrees Celsius.
The boiling refrigerant will absorb the heat from the air and carry it to the condensing coil, where it is released through the radiator-like fins before returning to the compressor to be cycled again. Without the pressure being changed the boiling would be more difficult and the unit would not work efficiently.
What Are the Parts of a TXV Valve?
Even though there are many different brands and designs for a thermostatic expansion valve, each one has the same basic components. Below is a list of the major parts of a TXV valve and what it does:
- Valve Body - this is the outer structure of the valve and it holds all the other components and has an opening to restrict the flow of the liquid refrigerant.
- Diaphragm - thin membrane, usually made of metal, that can put pressure on the pin.
- Pin - also known as a needle. The pin moves up and down to change the size of the opening in order to control the flow of refrigerant.
- Spring - this works to counteract the force or pressure of the pin.
- Sensing Bulb and Capillary Line - these components determine the amount refrigerant is needed due to the temperature. They either open or close the valve to regulate the pressure.
TXV Valve Troubleshooting
When your air conditioning unit is not functioning properly, you may suspect that the TXV valve has gone bad and may need to be replaced. Before you call an HVAC technician to schedule a service, there’s a way to test the TXV valve to determine if it is the problem.
Try the following steps to test to see if the valve is working or not:
- Make sure that the unit is plugged in and the power is on.
- Remove any necessary covers to access the TXV valve.
- Locate the thermostatic expansion valve by using your air conditioning unit’s owner manual. It has a small bulb, a thin capillary tube, and is usually connected to a T-shaped body.
- Using instructions from the manual, remove and lift the sensing bulb from the suction bulb well.
- Either place the sensing bulb in warm water or in your hand for a few minutes. This ensures that the temperature of the bulb is increased.
- As you do this, the refrigerant should start flowing into the evaporator. You should be able to hear the process. If this doesn’t happen, the TXV bulb has probably failed – not allowing the flow of refrigerant – and will need to be replaced.
There are many different reasons why a TXV valve might not work properly. The major issues that cause the valve to go bad deals with either a dirty evaporator coil or a dirty air filter. Also, if there is restricted airflow or the return is blocked then the TXV valve will not work properly.
Is the outside condensing unit whistling? This could be a sign the TXV valve is clogged with debris, which will hinder proper functioning.
When troubleshooting the TXV valve, look for specific cues or symptoms when your AC unit is running. For example, if there is low evaporator pressure or not much suction occurring, then this is a sign that you may need to replace your TXV valve. Below is a list of other symptoms that might occur if your TXV is not working properly. Most of these are beyond the experience of the average homeowner who doesn’t have the proper testing tools. However, so you’ll know what the HVAC tech is looking for, here are the symptoms.
- Low compressor amp draw;
- Short-cycling on the low-pressure control;
- Somewhat high discharge temperature;
- Low condensing (head) pressure;
- Low condenser split; and
- Normal to a bit high condenser subcooling.
A failed TXV valve isn’t the only cause of an AC or heat pump not doing its job. Here are other potential causes for the unit not cooling properly.
How Much Does It Cost to Replace a TXV Valve?
Purchasing the thermostatic expansion valve can cost anywhere from $75 to around $200 for just the valve. You can find them easily on Amazon.com or on any websites that sell HVAC parts. Most times an HVAC technician will install a universal/third-party replacement TXV valve. Most jobs take 1-3 hours for a professional HVAC technician to replace the valve and test it to make sure everything is working properly. This will cost around $150-$300 for labor. Overall, replacing your TXV valve will cost between $225-$500.
Replacing your TXV Valve
If you feel like your TXV valve is failing or not working properly, you should troubleshoot using the information above. Also, don’t hesitate to call a certified HVAC technician to help through the process. The 800 number on this page will ensure you get written estimates from some of the top air conditioning and heat pump repair technicians in your area.