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Understanding the Thermostatic Expansion Valve (TXV)

Understanding the Thermostatic Expansion Valve (and Why It’s Expensive to Replace)


Occasionally we have to give a customer the bad news that the thermostatic expansion valve (TXV) in their heat pump or air conditioner needs to be replaced—and that it won’t be a cheap fix. This often leads to questions like, what does the TXV do? And why is it so costly to replace?

We’re all about educating our customers and making sure they have all the information they need, so let’s talk about what the TXV does and why replacing it is a lengthy and relatively expensive process.


What Does the Thermostatic Expansion Valve Do?


The TXV is a modulating valve that controls the flow of refrigerant into the evaporating coils. This keeps the system running as efficiently as possible and prevents liquid refrigerant from getting into the compressor, potentially causing a breakdown.

When the refrigerant comes to the TXV valve, the refrigerant is under high pressure and in liquid form. The TXV acts sort of like the spray nozzle on a bottle of household cleaner. The valve restricts the flow of the refrigerant passing through, causing the refrigerant to go from high pressure to low pressure.

This change in pressure results in some of the refrigerant shifting immediately from liquid to gas. As you may remember from your high school science classes, water boils at a lower temperature at higher altitudes. That’s because liquids turn into gases at a lower temperature when they’re under less pressure.

So decreasing the pressure of the refrigerant allows some of the liquid to turn into what’s called “flash gas.” The resulting mixture of low-pressure liquid and vapor travels into the evaporator. There, the rest of the liquid turns into gas as it absorbs heat from the air passing over the evaporating coils.

The key here is that the TXV ensures that exactly the right amount of liquid, at exactly the right pressure, gets into the evaporator. If too much liquid goes into the evaporator, then the heat from the air won’t be enough to “boil off” all the liquid. That means some of the refrigerant will still be in liquid state when it gets to the compressor—and that’s bad for the compressor.

On the other hand, if not enough liquid refrigerant gets into the evaporator, then the refrigerant won’t be able to absorb as much heat as possible from the air. That will make your HVAC unit less efficient or even ineffective.

Of course, the amount of liquid that can get “boiled” at any given time depends on how much heat is in the air passing over the coils. That’s why you need a modulating valve that can adjust the refrigerant flow so that it’s always precisely the right amount.

In order to control the flow, the TXV uses a sensing bulb that’s attached near the outlet of the evaporator. This bulb senses what’s called the “superheat” of the refrigerant as it’s about to leave the evaporator.

Superheat is the difference in temperature between the boiling point of the refrigerant and the actual temperature of the gas. In other words, it’s how much extra heat the refrigerant picked up AFTER being converted from a liquid to a gas.

If the amount of liquid refrigerant allowed into the evaporator is too much, then the superheat will be too low. Conversely, if there’s not enough liquid refrigerant allowed in, then the superheat will be too high.

So, when the temperature of the refrigerant at the evaporator outlet goes up, the sensing bulb, which is attached to the valve itself by a capillary line, causes the valve to open wider, allowing more liquid refrigerant through. When the temperature of the refrigerant goes down, the sensing bulb causes the valve to restrict the flow of refrigerant, letting less liquid through.

As you can see, when it’s working properly, the thermostatic expansion valve helps to keep your system in a perfect balance. Unfortunately, that means when it’s not working properly, your HVAC system is in big trouble.


Why is Replacing the TXV a Costly Repair?


Unlike the compressor or a fan motor, the TXV itself is not a particularly expensive part. The problem is that installing a new TXV correctly is a process that takes a lot of time and skill.

First of all, before the TXV replacement can begin, the technician must evacuate all the refrigerant from the system, then clean out any oil or contaminants left behind.

Next, the TXV is a brass valve with a lot of delicate parts inside, including a thin needle, a spring, and nylon o-rings. The problem is that in order to connect the new TXV with your system, it has to be brazed in at 1000 degrees.

As you can imagine, using 1000 degree heat around tiny, delicate parts–like springs, needles, and o-rings–can easily lead to those parts melting, fusing together, and even being destroyed. To keep this from happening, the technician has to use wet wrapping and thermal paste to protect the inner workings of the valve.

Even with those protective coatings, the tech has to work in stages, taking breaks to allow the valve to cool down before brazing further. And even with all these precautions, even the most skilled and experienced technicians will sometimes find that the heat has damaged the valve and they must start all over again with a new one.

Additionally, correct brazing technique requires purging with nitrogen as the brazing occurs. This pushes out any oxygen present, which would otherwise oxidize from the extreme heat and corrode the system.

Once the installation is done, the system must be recharged with refrigerant and then carefully adjusted so that the sensing bulb will control the flow as precisely and efficiently as possible.

Ultimately, replacing a thermostatic expansion valve correctly is a task that requires lots of time and skill, but it is extremely important to the efficiency and lifespan of your HVAC system.