There has been much written and many jokes made about the misdiagnosis of TXV (Thermostatic expansion valves) and rightly so. This article will cut straight to the point to help those of you who may still need a bit of clarification and hopefully, we will save the lives of a few TXVs and the pocketbooks of some customers.

Q: What is a TXV?

A: A TXV (TEV) is a type of metering device. The metering device’s job is to create a pressure drop from the liquid line into the evaporator which will result in refrigerant boiling (changing from liquid to vapor) through the majority of the evaporator coil. This low temperature “boiling” absorbs heat from the space or product being cooled.


Q: How does a TXV Function?

A: A TXV “measures” the temperature and (usually) the pressure at the end of the evaporator coil with a bulb and a tube called an external equalizer. The bulb measures temperature and provides an opening force, the equalizer measures pressure and provides a closing force. There is also a spring that may have an adjustable tension that provides additional closing force. When working properly these forces achieve a balance and maintain the evaporator superheat to the designed of set levels at the end of the evaporator.  The TXV’s job is to maintain superheat within certain operational ranges and conditions. 


Q: How do they fail?

A: A TXV may fail either too far open or too far closed. Too far open is also called “overfeeding” and it means that boiling refrigerant is being fed too far through the evaporator coil, this would show up in low superheat. If the TXV fails closed it can be said to be “underfeeding” which means not enough boiling refrigerant is fed through the evaporator coil and superheat will be too high at the evaporator outlet. 

These failures can and do occur, but they are usually caused by contaminants or moisture in the system that have worked their way to the valve and caused it to stick or become restricted. Another cause of valve failure is a rub out on bulb tube and an external equalizer without a core depressor installed on a port that has a Schrader core in place.  

When a valve is overfeeding the first thing to check is bulb insulation, placement and strapping. If the numbing isn’t properly sensing the suction line it can lead to the valve remaining too far open.


Q: Why are they misdiagnosed so often? 

A: TXV’s are often incorrectly condemned in cases of low evaporator airflow or load. This happens because techs will find a system with low suction pressure and assume that means it is low on refrigerant. They will then start to add refrigerant and the TXV will respond by closing further the more refrigerant is added. The tech will see that the suction isn’t increasing and they will conclude that the TXV is failed. 

This occurs because the tech is paying too much attention to suction pressure without considering the other readings.


Q: What is the correct way to diagnose a TXV? 

A: First take all of your refrigerant readings as well as your liquid line and suction temperature at both ends (on a split system). This means superheat, subcooling, suction saturation (evaporator coil temp) and liquid saturation (condensing temp). For a TXV to do what it is supposed to you need a full line of liquid before the TXV, this means you need at least 1° of subcooling in theory but in reality, you will want to make sure that you have the factory specified subcooling which is usually around 10°. In refrigeration, we do this same thing by looking for a clear sight glass. On a split system checking the subcool outside and then confirming there is no big temperature difference inside to out is a great way to ensure that kinked lines or plugged line driers aren’t an issue. 

The next thing that a TXV needs is enough liquid pressure to have the required pressure differential. This amount of required pressure differential will vary a bit based on the valve but usually, we want to see a 100 PSI minimum difference between the liquid line pressure and the desired evaporator pressure. If the head pressure drops too low due to low ambient conditions this can come into play and impact the ability of the valve to do its job. 

Once this is all confirmed then it is simply a matter of checking the superheat at the end of the evaporator. Most A/C systems will be maintaining 6-14° of superheat at the evaporator outlet. If it is in that range then the valve isn’t bad, it’s doing its job. 

If it is lower than 6° of superheat at the evap outlet then it could be overfeeding (double check your thermometer and gauges) and if the superheat is well above 14° at the evaporator outlet, with the proper subcool and liquid pressure entering… then you have a failed closed (underfeeding valve). Keep in mind that some valves will have a screen right before the valve and this can be the cause of the restriction rather than the valve. You can intentionally freeze the coil and try to see the freezing point or use thermal imaging to help spot if it’s the valve or the screen. When you find the point of temperature you find the point of pressure drop, just remember that the TXV is DESIGNED to provide pressure to maintain a fairly fixed superheat. 


Q: Do TXVs Ever Fail

A: They can fail internally but most often they fail because of a blocked inlet screen (if they have one), contaminants entering the valve, loss of charge from the power head, bulb location and positioning issues and overheating of the valve. In commercial and refrigeration applications you can often replace or clean the screen and replace the power head rather than replacing the entire valve. 


As I have said many times before diagnosis make sure your tools are well calibrated and working and that you are ACTUALLY reading the pressure correctly. I’ve seen many misdiagnoses just because a Schrader wasn’t pushing in or a multi-position valve cracked properly.

As we talked about in an earlier podcast, a TXV is designed to maintain a specified and constant superheat at the outlet of the evaporator coil. It does this through a balance of forces between the bulb pressure (opening force), Equalizer pressure (Closing force) and spring (Closing force). It is the spring pressure that can be adjusted on some valves, but why and when would this be done?

For the quick, cut to the chase version, turning the adjustment on the bottom of an adjustable valve clockwise = higher superheat and counterclockwise = lower superheat. However, before you start messing with the adjustment, I suggest you read on.

First, the valve must be an adjustable type, many valves on small equipment are not adjustable and have no hex cap at the base.

Here are some other items you need to consider first-

Proper Subcool

Before an expansion valve can function properly and do its job, it must have a full line of properly subcooled liquid refrigerant all the way to the inlet. On a split system checking the subcooling at the condensing unit is a good start but you also need to make sure there isn’t a significant temperature drop all the way up the expansion valve inlet. Keep in mind that some valves have a screen right at the valve inlet, so a restriction even at that point will cause operational issues.

Required Pressure Drop 

For an expansion valve to function there needs to be a significant pressure differential between the evaporator` design pressure and the liquid pressure entering the expansion valve (In many cases 100 PSIG +). During cooler times of the year the outdoor condensing pressure/temperature may drop to the point that the required difference in pressure may not exist and in these cases, the valve may no longer be able to maintain the target superheat. While low ambient controls may be employed to rectify the issue in some cases, in many cases, you must simply be aware that the valve will not function as expected.

Improper Bulb Placement 

Ensure that the bulb is mounted on the suction line flat and tight with a proper strap. It is never a bad idea to insulate the bulb, and anytime it is exposed to ambient air it is a necessity.

When to Consider Adjustment 

Now you are at the point where you can consider whether that valve could use some adjustment. First, measure the superheat right at the evaporator outlet in the same general location as the TXV bulb and equalizer in most cases the superheat at that point should be 5-10 degrees but refer to manufacturers specs when in doubt. In some cases, you will not have a pressure port at the evaporator so you must rely on a pressure reading outside. Use common sense when assessing the situation and realize that there may be some pressure drop on a 100′ line set and there should be very little in a 10′ line set. Make some allowance according to the situation.

If the system is running VERY low or VERY high suction pressure and /or superheat readings that are way out of range, it is very unlikely that adjusting the valve will remedy it. Usually, valve adjustments are only for small superheat changes up or down.

The Forces at Play

The bulb pressure is the opening force of the valve, so when the bulb is warmer it exerts more opening force resulting in a more “open” orifice, and when it’s cooler it exerts less opening force resulting in a more “closed” orifice.

The equalizer is a closing force so the higher the suction line pressure, the more the valve is forced closed and the lower the suction line pressure the more the valve is forced open.

The spring is also a closing force and on an adjustable valve increasing the spring tension/force results in lower flow and higher superheat, decreasing the spring tension/force results in more flow and lower superheat. In short, counterclockwise = lower superheat, clockwise = higher superheat.

Making an Adjustment

Before you adjust anything, the system must have been running for a good long while, and you have observed that the superheat has stabilized. You then must check the entire system and surmise that everything else is functional, the valve is being provided with a fully liquid, properly subcooled, high enough pressure feed of refrigerant. If at that point you find it is out of range then you can make adjustments.

  • CAREFULLY remove the hex cap from the base of the valve with a properly sized wrench and a backing wrench exposing the adjustment screw.
  • Turn 1/2 turn at a time clockwise (with a refrigeration wrench) to increase superheat or counter-clockwise to decrease superheat.
  • After a 1/2 turn adjustment, replace the panels and allow the system to run and stabilize.
  • Recheck the superheat and not the change.
  • Repeat as needed until the maximum setting is reached. NEVER force the adjustment screw too far, it should require minimal force to turn other than possibly initially to “unstick” the screw.

Adjusting a TXV / TEV is an advanced skill for a technician who has a good grasp on their readings and the forces at play. Tread carefully.

— Bryan

P.S. – Here is a great resource from Parker / Sporlan

I walked into a supply house the other day and I was looking at “universal” expansion valve on the shelf. The friendly guy behind the counter saw me and walked over, after saying hello he offered

“That’s a great valve, it’s even balanced port”.

Now I am a bit of a trouble maker, I should have just nodded and said “uh huh” but instead I asked, “what does balanced port mean?”. The counter guy sort of half shrugged and said “I guess it means it works on a lot of different systems?”

I would bet that most people in the industry have heard the term “balanced port” and figure it sounds like a good thing but don’t really know what it does. Not long ago, I would have been one of them.

We have all been taught that there are three forces that act on an expansion valve –

  1. Bulb Pressure is an opening force
  2. Evaporator Pressure (external equalizer) is a closing force
  3. The Spring is a closing force

while the system is within its design operating conditions these forces are the primary forces at work that allow the valve to “set” the evaporator outlet superheat.

There is a fourth force and that is the opening force applied by the refrigerant passing through the needle. When the inlet (liquid line) pressure is within the normal operating range this force is accounted for in a normal TXV. In cases where the liquid pressure is higher than usual the force will be greater allowing more flow through the coil and when it is less it will allow less flow.

The result of this effect is fluctuating superheat based on liquid pressure which may be acceptable in small amounts but can become unacceptable quickly on systems that require accurate evaporator feeding or systems that have a wide swing in condensing temperatures and pressures.

Sporlan largely solved this particular issue in the 40’s when they brought the “balanced port” valve to market. While the technology is nothing new it has been improved on over time.

Balanced port TXVs can vary in design but they solve this problem by allowing the inlet pressure to effect the top and bottom of the needle (orifice) equally. This eliminates (or reduces) the liquid pressure as an opening force and instead turns it into a “balanced” force that neither opens or closes the valve.

If you have an application where the head pressure is allowed to change or “float” over a wide range, the balanced port TXV is a great choice.

— Bryan

 

 


When mounting a TXV bulb or checking bulb placement there are a few important considerations (listed in order of importance)

  1. Mount the bulb on the suction line. Flapping in the breeze is no good.
  2. Mount TIGHTLY it with a proper metallic strap (usually copper or brass). Not zip ties, not tape.
  3. Position it on a flat, clean, smooth, portion of the horizontal suction line. Not on a coupling or an elbow.
  4. Mount it before the equalizer tube (closer to the evaporator than the EQ tube)
  5. When possible mount it at 8 or 4 o’clock on the suction line (or according to manufacturers specs) . This becomes more important the larger the suction line.
  6. When possible, insulate the bulb so that it is not influenced by ambient air temperature. It never hurts to insulate the bulb even inside the cabinet though not all manufacturers require it.
  7. If you do need to mount it vertically, make sure the tube points up not down

Poor bulb contact will result in a bulb that is warmer than desired, resulting in overfeeding and lower than desired superheat.

Finally… be gentle with the bulb and tube. They break easily.

You can read a more detailed description HERE

— Bryan

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