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Low Superheat High Subcooling: 7 Causes and Effective Solutions

What is low superheat high subcooling of your AC and does this condition reduce the cooling capacity? If you want to maintain the efficiency and longevity of your cooling device then I invite you to check this article where I will tell you how to fix this issue.

Let’s get started!

An Insight into Low Superheat High Subcooling: Meaning and Major Causes

Let’s briefly go over the basic essentials you need to know about low superheat high subcooling:

  • Low superheat: It takes place when there is too much refrigerant in the evaporator, and it cannot evaporate quickly enough.
  • High subcooling: This happens when there is an excess of liquid refrigerant in the condenser, often due to an obstruction or overcharge. It results in higher-than-saturated-liquid temperatures in the liquid line.

This may be due to high refrigerant charge, limited airflow in the condenser coil, and impaired metering device. In order to fix this, you need to remove liquid so that both of the problems can get back to their original state.

Read Also: High Superheat Low Subcooling: 4 Causes and Troubleshooting

low superheat high subcooling

But there’s a lot more you should know in order to optimize your system. Continue reading below to find out additional causes of this issue and their easy fixes! Let’s dive deep into this informative guide!

4 Reasons Why Your HVAC Has Low Superheat

Low superheat and high subcooling are critical indicators of an imbalance in the coils. This imbalance can be attributed due to either not enough heat or too much refrigerant.

Several issues can lead to low superheat in air conditioning systems, including:



Excess refrigerant in the AC system

Reduce refrigerant levels as needed

Evaporator airflow restriction

Replace air filters and the blower wheel

Metering device overfeeding

Insulate the TXV sensing valve properly

Insufficient heat load on coil

Clean both filters and the coil

Oversized AC system

Consider opting for a properly sized AC unit

Now that we have gone through the table, let’s explore each problem in detail!

1. Excessive Refrigerant in the AC System

Excessive refrigerant can lead to a problem – low superheat absorption in the evaporator coil. This affects your air conditioning system performance.

  • Low superheat is typically absorbed during the refrigerant evaporation process.
  • Too much liquid refrigerant can damage it, and it’s meant for squeezing gas.
  • The presence of excessive refrigerant may be attributed to a malfunctioning TXV valve
This issue can be resolved by giving off some refrigerant from the AC system to the exact level.

2. Limited Air Flow to the Evaporator Coils

When insufficient airflow reaches the evaporator coil, it absorbs less heat, potentially causing low superheat. This airflow restriction can be caused by:

  1. Dirty Coil: A dirty coil or dust on the coil’s surface significantly diminishes its available area for heat exchange. Consequently, there is a reduced interaction between the coil and the incoming air.
  2. Clogged Air Filter: An air filter that is obstructed also hinders the airflow towards the evaporator coil. Since the air must go through the sieve before it gets to the dirty coils, any blockage or reduced airflow space within the filter restricts the amount of air that can reach the coil. As a result, heat absorption as done by the evaporator becomes limited.
A clogged air filter

In both instances, fixing it quite is clear, here’s how to do it:
  • Don’t forget to give your evaporator coil some regular consistent and scheduled maintenance for best performance.
  • Scheduled replacement of the air filter is essential. Failure to adhere to filter cleaning schedules causes coil contamination.

Thus, by adhering to these practices, the issue of low superheat can be effectively rectified.

3. Regulation By the Metering Device

When metering tool insulation is insufficient, it allows the refrigerant to flood the evaporator coil, causing trouble.

To prevent more refrigerant flooding, ensure proper insulation of the metering device.

4. Oversized AC Unit System

When dealing with an oversized AC system, there will not be enough heat load for the vaporization of the refrigerant at the evaporator coils. This situation can result in the occurrence of low superheat.

It is recommended to use the right-sized HVAC equipment to deal with insufficient heat energy.

3 Reasons Why Your HVAC Has High Subcool

Subcool measures how much of the refrigerant surpluses to the condenser unit, resulting in a high subcool. This can be due to several underlying problems



Excessive refrigerant

Remove excess refrigerant

Small/ malfunctioning metering device

Callibirate metering devices

Liquid pipe Blockage like a wavy line

Ensure lines don't have any blockage

The causes of high subcooling can be summarized in the following section:

1. Overcharging the System Will Cause a High Subcool

Normal subcooling refers to the expected level of subcooling in a refrigeration or cooling system. Excess refrigerant in the system hinders the proper condensation and evaporation processes. As a result, the condenser unit experiences a backup.

high subcooling low superheat

Maintain proper refrigerant levels through regular HVAC system maintenance conducted by a certified technician.

2. Faulty Metering Device

Device problems, such as Metering device faults being undersized, head pressure control system glitches, or issues with the thermostatic expansion valve can lead to elevated subcooling.

To resolve the issue, calibrate the metering device, inspect the pressure control system for glitches, and use the manufacturer-recommended metering device size.

3. Restriction of the Liquid Line

Water line blockage disrupts refrigerant systems, reducing cooling efficiency and increasing energy use.

The solution involves discharging excess refrigerant, but it’s vital to confirm its presence, as impairment in the metering device may also cause high subcooling.

For a better understanding, I would recommend you check out the following helpful video.

Low or High Superheat on an AC: How To Know Which One Plagues Your System

Discovering superheat’s secrets involves a straightforward method: evaluating heat charge and refrigerant levels.

For precise readings, you will need to check:

  • Specialized pipe thermometer;
  • Temperature-to-pressure conversion chart.


In a standard system, at the evaporator, it’s about 10°F, and a warmer 20°F to 25°F in proximity to the compressor. In the HVAC system, if the temperature difference at the evaporator is less than 10°F and <20°F overall, then superheat is considered low.

For nonoptimized system performance with high superheat, it’s high when readings exceed 10°F at the evaporator and are also greater than 10°F near the compressor.

  • To check the suction line temperature, it’s essential to use a thermometer to employ a reliable method for diagnostics.
  • Measure the suction pressure.
  • Applying the temperature-pressure chart for conversion.
  • Calculate the temperature difference.

If the pressure recorded at the suction line registers 65 psi in an R-22 system, consulting the temperature-pressure chart reveals a corresponding temperature of 38°F. However, if the suction line temperature measures 68°F, a substantial difference of 30°F emerges between the two readings.

This significant temperature of the AC system differential, exceeding the typical 10°F range, indicates a condition of high superheats. Conversely, a difference under 10°F would signify low superheat.

Why Does My System Have Low Superheat with High Subcool

Now that you understand what leads to low superheat and high subcool, let’s see when they happen together. This condition results exclusively from the excessive refrigerant in the AC system, leading to both low superheat and high subcooling.

Low Superheat or High Subcooling: Which One Is It?

For a thorough subcooling assessment, begin by measuring the liquid line temperature in proximity to the condenser. Subsequently, employ the pressure-temperature chart for precise conversion of top pressure into temperature


Start by measuring head pressure, reading 260 psi, equivalent to 120°F. Attach a thermostat to the liquid line, recording 85°F. The verdict? High subcooling, as TXV systems usually range from 10°F-18°F; lower means low subcooling

  • So, when you check the pressure it’s at an impressive 260 psi. When this is converted, the corresponding temperature registers at a steady 120°F.
  • Following the previous steps, you’ll need to attach a thermostat securely to the liquid line and then document when the temperature reaches 85°F., then;
  • This case showcases high subcooling as indicated by a significant 35°F difference (120°F – 85°F).
  • In the case of TXV-equipped systems, it’s crucial to adhere to the recommended subcooling range which should fall between 10°F and 18°F. Anything lower than that? Well, that’s a clear sign of low subcooling.

An Insight Into Service Standards

Maintenance and service practices for air conditioning and refrigeration systems are often guided by industry standards like ASHRAE or ACCA.


These standards may outline best practices for diagnosing and correcting issues related to low superheat or high subcooling. Technicians are trained to follow these standards to maintain system efficiency and safety.


What can cause low superheat?

Low superheat can be caused by low heat load, high levels of refrigerant charge, and low evaporator coil airflow.

What if my subcooling is too high?

If your subcooling is too high, then this means there may be an overcharge or a malfunctioning metering device. In order to fix this, you need to remove excess refrigerant from the system and replace the metering device.

How do you fix high subcooling?

You can fix high subcooling by getting rid of the extra liquid refrigerant charge from the system. Alternatively, you can also check for any blockage in the liquid line and cater it for proper flow. Lastly, you may need to replace your metering device.


The final say is that low superheat high subcooling can harm system optimization. Low superheat and high subcooling occur mainly when the refrigerant charge is high or the faulty metering TVX or AEV device.

You can overcome the issue by removing the excess refrigerant charge, repairing the metering device, and cleaning up the condenser coil to avoid any restrictions in the line. Also, consider cleaning dirty filters and motors, to allow for increased airflow into the coil.

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