Thermostatic Expansion Valve Troubleshooting Guide

Having heat pumps problems? The reason might be your thermostatic expansion valve (also known as TEV or TXV valve). Here’s what you need to know for proper troubleshooting and efficient and long-lasting heat pump operation.

TXV

All devices with the primary purpose of heating or cooling, generally need a component, a metering device that can control the refrigerant. A thermostatic expansion valve (TXV) has that same purpose – it regulates the rate at which the refrigerant flows into the heat pump evaporator increasing its efficiency.

The importance of a TEV or TXV in heat pumps is enormous. Here we are going to help you understand how a thermostatic expansion valve may be limiting your heat pump, and how to troubleshoot. Keep reading and find out!

What is a Thermostatic Expansion Valve Exactly?

In heat pumps, this TEV or TXV is usually found near the heating coil. Here it has the main task of keeping the right amount of refrigerant going in and out the coil for proper temperature. In addition, this process also helps to avoid superheat which is the name of the boiling point (temperature) of the refrigerant in which an evaporator coil won’t do its work anymore, passing the point of saturation.

The refrigerant flow control is important to maximize the efficiency of the evaporator and at the same time prevent the floodback, or liquid refrigerant going back to the compressor.

How a Thermostatic Expansion Valve Works in Heat Pumps

Simply, a TEV has the main purpose of controlling how much refrigerant passes through the heat pump system’s evaporator before reaching the compressor. Most, especially older type heat pumps and AC systems use a piston or orifice as a metering device, instead of a TXV. However, these thermostatic expansion valves are a lot more effective in metering and controlling refrigerant – so more and more devices nowadays are starting to use them. And the main reason for its implementation was the change in regulations in 2006 when the minimum efficiency change to 13 SEER.

They improve the system efficiency, avoid overheating problems, and often reduce the amount of energy a device uses. But as they offer many benefits, they also have a downside. 

Thermal expansion valves also tend to break more often than metering orifices or pistons. Even though it is not a problem in most heat pumps, sometimes a TEV issue affecting the performance of the device is the reason why you’re experiencing a malfunction.

But to know how a TEV may be damaged, you may need first to understand how it works, exactly.

• A TXV is a pressure-correcting device. When an evaporator, heating or cooling coil starts having too much pressure, a TXV releases that pressure as soon as the refrigerant hits superheat and is converted into gas. 
• The TXV will immediately open a new way for the refrigerant to pass, to get away from the evaporator or coil, and make sure the pressure slows down.
• A part of this refrigerant boils down, while the other, without the same pressure, comes back to the evaporator to help it do its job.
• As soon as the forces are in equilibrium, the TXV will enter a stable position, maintaining pressure and waiting for the proper superheating temperature to do its job.

This is the simple job of a TXV. However, if any of the parts of the device doesn’t work, and any of these steps are overlooked, very likely the heating or cooling device will malfunction.

How to check a TXV for proper work

Make sure that the system has a proper charge. Measure a superheat - it should be between 8 and 12 degrees, but the specs should be checked for your model.

The sensing bulb should be properly connected to the suction line and insulated.

Make sure that the system has a proper refrigerant charge and flow.

Clean the debris from the inlet screen.

Troubleshooting Common Issues with a TEV or TXV

The three main issues with a thermostatic expansion valve are flooding, starving and hunting.

Flooding 

This occurs when the TXV is feeding the evaporator with too much of the refrigerant, more than it can evaporate, and thus harming the compressor eventually as the liquid refrigerant is going back to the compressor. This can cause compressor frosting and noise, higher pressures, and low superheat (less refrigerant evaporation than needed). 

The main for flooding are:

• The compressor is undersized or low efficient.
• The superheat setting is not configured correctly, so the TXV doesn’t work properly.
• Too much moisture in the TXV or frozen TXV – causing superheat to be overlooked.
• Dirt or debris in the TXV or in the refrigerant pathways making the TXV work badly.
• Damage in the TXV. Leaking or improper functioning.
• Refrigerant valve is too large for the TXV – letting too much refrigerant pass through.
• Bulb (TXV head) improperly positioned. 

To fix this, simply check the TXV if any of the previous symptoms are showing. Then you will need to check if the TXV is good for the device (not too small, efficient enough, or if it fits the refrigerant valve). Then, check for dirt or debris inside the TXV, any leak, or moisture that could be affecting it. And finally, make sure it is configured at the proper superheat temperature, and that its bulb (head), is in proper position to avoid further flooding problems.

Starving 

Is the opposite to flooding, when the refrigerant is incapable of reaching the evaporator, so causing the evaporator to overload, operate at unstable high temperatures, or induce low suction pressure. Here’s what is probably happening if there’s any starving happening to your TEV:

• Moisture on the TEV causing increased condensation or evaporation.
• Dirt and debris may also prevent the TEV to let refrigerant pass through.
• Improper Delta P valve, too low, too small, or too inefficient for the TEV.
• Low refrigerant charge, making it impossible to reach the evaporator on time.
• Pressure drop due to leaks, frost, or broken refrigerant line.
• Pressure instability due to equalizer line doing a wrong job.
• Small valve harming the way refrigerant passes through it.
• Superheat adjusted at a really high temperature.
• Problem with power or charge, making it impossible for the device to produce proper pressure.
• Limited charge due to maximum pressure limiter blocking the valve. 
• Clogged strainer, with too much dirt or debris, harming the refrigerant line pressure.

You can fix starving problems by first checking the overall state of the refrigerant line in search of leaks, too much moisture, blockages, or improper pressure. Then, check the TEV in search of the same symptoms, remembering that the Delta P valve may be culpable. Then, make sure the superheat is configured at the proper temperature, as it may be the reason of starving. And finally, make sure the device has the adequate power to function correctly and that it is isn’t configured to stop pressure to go in the TEV.

Hunting

When the thermostatic expansion valve opens and closes too fast, malfunctioning, letting too much or too little refrigerant pass through, and causing superheat to fluctuate, you have hunting. Here you can experience both starving and flooding symptoms, but also load problems, varying pressure inside the device, and inappropriate amounts of refrigerant into the evaporator. Hunting may be caused by:

• Oversized valve, causing hunting due to a larger size than expected.
• Bulb (thermostatic expansion valve head) improperly located inside the heat pump.
• Distribution problems with the refrigerant, overfeeding, starving, or sometimes making it impossible to reach superheat. 
• Wrongly adjustable superheat temperature.
• Too much moisture inside the thermostatic expansion valve making it induce instability.

To fix hunting, you just simply need to look for problems in the thermostatic expansion valve, such as oversizing, wrong location of the head, or moisture. Also, make sure the refrigerant is properly distributed in the device and that the superheat temperature is correctly configured.

Installation Guide: Replacing a TEV or TXV 

Replacing a TEV or TXV can feel like an immense trouble, especially if you are not an expert on heat pumps or AC systems for that matter. However, doing it is entirely easy, as long as you know how to open the device and look for the thermostatic expansion valve. Here are a few tips that will help you replace and install this component:

• When you install the TEV, you must ensure that it is connected to the refrigerator line. If you are trying to replace it, and you find it is not – very likely that may be your problem. Make sure it is tightened up to the refrigerator line and that the bulb (TEV head) is close to the evaporator. If there’s a line equalizer, instead, the suction line must come before the equalizer, and then the TEV before the evaporator.
• Most TXVs work better in a horizontal position than in a vertical position. It should be located according to the tube’s diameter. The bulb, in addition, should be far from the suction line to avoid false signals from oil.
• The TXV should be in the perfect position to obtain heating or cooling data directly from the evaporator. It should never receive heat or cold from outside the device or any other part of it.
• Heat exchangers should be far away from TXV, as much as possible. Many heat exchangers can cause problems with heating and cooling, misleading the temperature the TXV can detect.
• All TXVs should be directly and without any interruption, connected to the evaporator. Even if there are liquid locks, a collection tube, or a riser for an oil pocket, the TXV should be connected first.

Want to Fix Your Thermostatic Expansion Valve Problems?

As hard as it is to spot inside a heat pump, a TEV is also a complex component to fix. However, with the proper instructions and recommendations, very likely you will repair and possibly replace that faulty TXV.