How to Fix High Superheat Low Subcooling

When you are troubleshooting air conditioning systems or refrigeration systems you might come across a situation where you are facing a high superheat and low subcooling. But what causes a high superheat and low subcooling  that mean and what measures should you take to fix the problem?

High superheat low subcooling on a TXV system means that there is no sufficient amount of refrigerant in the evaporator and  there is low amount of the refrigerant in the condenser unit.  This condition is mainly caused by low charge in an air conditioning system and can be fixed by sealing leaks and adequately charging the system with a refrigerant.


What causes high superheat? 

Superheat determines the amount of refrigerant in the evaporator. A high superheat means there is an insufficient amount of refrigerant in the evaporator. This condition can be causes by a few things that include

  • Excessive heat load
  • Defective metering device
  • Low refrigerant charge on the system
  • Restriction in the liquid line

The above listed issues are the most common causes for high superheat  on a TXV system. We have summarized them for you in table 1 together with their possible fixes

Chart for high superheat causes and possible fixes

High superheat causesHow to fix High superheat
Excessive heat loadCheck AC sizing
Defective metering deviceCheck or replace metering device
Low refrigerant in the systemRecharge with adequate refrigerant
Restriction in the liquid lineCheck or unclog drier filter or kinkled lines
Table 1.

Restricted or Defective metering Device.

A metering device and become defective or restricted. When this happens, the amount of refrigerant that goes to the evaporator will be restricted and this can cause high superheat on a system. A defected or restricted device can by caused by any of the following

  • Foreign material in orifice of the TXV;
  • Manufacturer’s defect in the valve.
  • Oil-logged TXV from refrigerant flooding the compressor;
  • Partial TXV orifice freeze-up from excessive moisture in the system;
  • Clogged inlet screen on TXV;
  • Receiver outlet valve (king valve) partially closed off;
  • TXV adjusted too far closed;
  • Wax build-up in TXV from wrong oil in system; and/or

Restricted liquid Line

The liquid line includes everything between the receiver and the TXV.  This means that even the filter drier is part of the liquid and is one component that can become restricted due to excessive moisture  and debris.. Other factors that can cause restriction in the liquid line include.

  • Kinked liquid line;
  • Restricted liquid line solder joint
  • Sludge from the byproducts of a compressor burnout;
  • Too much oil in the system

All these restrictions can potentially lead to a high superheat

Low refrigerant in the system

When the system is undercharged or there are leaks, the refrigerant will leak out  and become insufficient for the system to work properly.  Mistakes should be made between an undersized system and a restricted line.

Usually where you just have a restricted line, you won’t have a low subcooling in the condenser. For a system with an undercharged refrigerant,, you expect to have both  a high superheat and low subcooling and measuring the subcooling should be the test to confirm that the problem is really with a low refrigerant charge.

If the problem is in fact a low refrigerant charge, be sure to find the leaks before reaching the systems with enough refrigerant.

Also read: AC high pressure line cold or hot?

Excessive heat load

When there is excessive heat load in the evaporator this will cause a high superheat. Excessive heat load in the evaporator can be caused by an unusually high ambient temperature or  an undersized ac system. Check the two variables before going forward with other troubleshooting procedures.

What causes Low subcooling?

Subcooling is the measure of the amount of refrigerant at the condenser. When there is an insufficient or low amount of the refrigerant at the condenser, this is known as low subcooling. Likewise where there is excess amount of the refrigerant in the condenser, this is known as high subcooling.

A low subcooling can be caused by a few problems that include;

  • Restricted air flow in the condenser
  • Defective metering devices
  • Refrigerant undercharge
  • Compressor issues

Let’s look at table 2 which simplify each of the above causes together with the possible fixes

Chart for low subcooling causes and possible fixes

Low subcooling causesHow to fix Low subcooling 
Restricted air flow in the condenserClean condenser unit
Defective metering devicesInspect the metering drive, if possible replace it
Refrigerant underchargeCharge with enough refrigerant
Compressor issuesIf compressor parts are worn out, you’ll need replacements
Table 1.

Restricted air flow in the condenser

Restricted air flow to the condenser will result in high temperatures and build up of pressure. The built high pressures will cause more refrigerant to enter into the flow line and consequently leaking insufficient refrigerant at the condenser. The whole process will rest in low subcooling.

This problem on the other hand can be prevented or reduced by allowing amore air flow to the condenser. This can be achieved by keeping the condenser coils and fins cleansed to allow more surface area for heat exhaustion.

Defective metering devices

A defective or malfunctioning metering device is another possible cause for low subcooling in any HVAC system. A metering device controls the flow of the refrigerant in the system. But this can malfunction and when this happens it could restrict the flow of the refrigerant into the condenser unit.

The best thing to do to fix this problem is to clean the metering device. In cases where this does not fix the problem , you might want to replace the meter power head.

Leaking/ Low refrigerant

Low refrigerant charge on any system will also result in low subcooling.  Low refrigerant level can be caused by a system that has leaks.. If there is a low charge of the refrigerant on the system, this will result in a high superheat and a low subcooling.

As mentioned earlier in this guide,  start by  finding leaks before recharging the system with any more refrigerant.

 Defective Compressor 

A malfunctioning compressor or one that is not compressing the refrigerant properly will  deprive  the condenser of the refrigerant. A detective compressor can be a result of  a defective capacitor or any other compressor component.

Based on your test findings, and if the problem is any of the compressor components, most likely they can easily and cheaply be replaced  but when the compressor itself is dead, you will need to replace the whole compressor.

Superheat and subcooling troubleshooting chart

ProbemPossible causesHow to fix
High Superheat and Low SubcoolingLow refrigerant charge on liquid and suction lineFind leaks, seal and recharge with sufficient refrigerant
Low Superheat and Low SubcoolingOrifice too big, TXV opened too muchAdjust TXV
Low Superheat and High SubcoolingOvercharged system on both liquid and suction lineRemove and recharge to  the appropriate level
High Superheat and  High SubcoolingRestricted TV or drier valve, blocked blocked/restricted coilCheck TXV tightness and the insulation
Low superheatLow indoor air flow, overcharged system.Remove some refrigerant, recharge to appropriate level
High superheatLow charge,, low outdoor airflowAdd refrigerant. Clean condenser unit
Low subcoolingLow outdoor air flow, low chargeAdd refrigerant to the system, clean condenser unit
High subcoolingWeak compressor valves, overcharged systemsDischarge some refrigerant, replace compressor
Low suction pressureLow charge, low indoor air flowRecharge freon to appropriate level , replace air filters
Low head pressureLow charge, weak compressor valvesRecharge with more  refrigerant, replace compressor
High suction pressureWeak compressor valves, low outdoor airflow, overchargeRemove excess freon, clean condenser, replace compressor
Low indoor TDWeak compressor valve, low outdoor airflowClean condenser or replace defective compressor
High indow TDLow indoor air flowEnsure coil and air filters are clean

We have put this chart in pdf below:

What is high superheat and normal subcooling?

High superheat occurs when there is an insufficient amount of the refrigerant in the coil and normal subcooling is when there is just enough refrigerant in the condenser.

So a condition where you have excess refrigerant in the coil and enough refrigerant in the condenser is referred to as high superheat normal subcooling.

Usually when you have  High superheat  you expect to have a low subcooling or  high subcooling so generally speaking high superheat normal subcooling is a rare condition on any system.

How to determine superheat and subcooling  (Practical Examples)


To check for superheat on a system follow  you will need:

  1. Special thermometer for taking pipe temperature
  2. Temperature/ pressure chart  for converting the taking pressure to temperature


  • Take pipe temperature to the suction line
  • Take the pressure at the suction line and convert it to temperature using the temperature/ pressure chart
  • To get the superheat get the difference between the two temperature points
  • Superheat is about 10 degrees for many systems( taken at the evaporator coil) and  20°F to 25°F near the compressor.


If I wanted to check superheat on a R-22 system,I would start with taking the pipe temperature and let’s say the suction line temperature is 51 degrees. I would then take the suction pressure as well.

Let’s say the suction pressure is 70 psi . Converting this on a R-22 temperature pressure chart converts to 41.

The difference between the two temperature point is:

51 F-41 F= 10 F degrees.

10 F superheat is normal when taken at  the evaporator. Anything above is considered high superheat and anything lower is low superheat. Now let’s look at another example for checking subcool


Subcooling on systems that use thermostatic  expansion valve (TXV)  is approximately 10°F to 18°F

To check subcooling on a R-22  system, I will need to attach a thermometer near the liquid line at the condenser. Suppose my temperature reading is 90°F. I would then take the head pressure. Let’s say the head pressure is  278 psi. Converting my head pressure to temperature on the T/P chart will give me 125 F. 

Therefore the difference between the two temperature points is:

125 F-90 F= 35 F. 

Because Subcooling is above normal range, this means that we have a high subcooling . If the temp difference was below the normal range it would be a low subcooling and if it was within range, it would have been normal subcooling.

Final Thoughts

To wrap up, we have seen in this guide how you can fix high superheat low subcooling.  We believe you have  been able to  find the refrigerant leaks and sealed them. If you have any questions, you can contact me directly on [email protected] and I will personally help you regarding this. 

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