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 causes	How to fix High superheat
Excessive heat load	Check AC sizing
Defective metering device	Check or replace metering device
Low refrigerant in the system	Recharge with adequate refrigerant
Restriction in the liquid line	Check 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 causes	How to fix Low subcooling
Restricted air flow in the condenser	Clean condenser unit
Defective metering devices	Inspect the metering drive, if possible replace it
Refrigerant undercharge	Charge with enough refrigerant
Compressor issues	If 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

Probem	Possible causes	How to fix
High Superheat and Low Subcooling	Low refrigerant charge on liquid and suction line	Find leaks, seal and recharge with sufficient refrigerant
Low Superheat and Low Subcooling	Orifice too big, TXV opened too much	Adjust TXV
Low Superheat and High Subcooling	Overcharged system on both liquid and suction line	Remove and recharge to the appropriate level
High Superheat and High Subcooling	Restricted TV or drier valve, blocked blocked/restricted coil	Check TXV tightness and the insulation
Low superheat	Low indoor air flow, overcharged system.	Remove some refrigerant, recharge to appropriate level
High superheat	Low charge,, low outdoor airflow	Add refrigerant. Clean condenser unit
Low subcooling	Low outdoor air flow, low charge	Add refrigerant to the system, clean condenser unit
High subcooling	Weak compressor valves, overcharged systems	Discharge some refrigerant, replace compressor
Low suction pressure	Low charge, low indoor air flow	Recharge freon to appropriate level , replace air filters
Low head pressure	Low charge, weak compressor valves	Recharge with more refrigerant, replace compressor
High suction pressure	Weak compressor valves, low outdoor airflow, overcharge	Remove excess freon, clean condenser, replace compressor
Low indoor TD	Weak compressor valve, low outdoor airflow	Clean condenser or replace defective compressor
High indow TD	Low indoor air flow	Ensure 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)

REQUIREMENTS

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

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

PROCEDURE

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.

EXAMPLE 1. CHECKING SUPERHEAT

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

EXAMPLE 2. CHECKING SUBCOOLING

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.