High Superheat Low Subcooling: Only 1 Major Cause + How To Fix It?

High superheat low subcooling is the most common combination we find when diagnosing AC units. In order to normalize both high superheat and low subcool, we have to understand why these 2 key parameters are abnormal.

measuring high superheat and low subcooling
We use high side red gauge to detect low subcooling, and low side blue gauge to detect high superheat.

Namely, we have written about:

  • High Superheat Causes here. We have high superheat when the actual measured superheat is by 2°F or more degrees higher than the target superheat. As you can see, high superheat can be caused by low refrigerant charge, restriction in the liquid line, high indoor CFMs, excessive heat load, or metering device overfeeding (TXV or piston underfeeding).
  • Low Subcooling Causes here. We diagnose subcooling when actual measured subcooling is by 3°F or more degrees lower than target subcooling. In short, low subcooling can be caused by low refrigerant charge, TXV or piston overfeeding, or poor compressor compression.

When we have a high superheat low subcooling problem, we just need to check the common causes for both high superheat and low subcooling. It turns out there is only 1 common cause for high superheat low subcooling:

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What Causes High Superheat And Low Subcooling?

From the causes for both of these individual states above, we can see that high superheat low subcooling is caused by low refrigerant charge. Here is why we get both high superheat and low subcooling:

  • The undercharged unit will feed less refrigerant into the indoor evaporator coil, resulting in high superheat.
  • Similarly, if we have a low refrigerant charge, the outdoor condenser coil will have less-than-needed refrigerant flow, resulting in low subcooling.
we fix high superheat low subcooling by adding freon into the refrigerant lines
Example of how to fix a 3 ton 16 SEER AC unit running on R-22 freon that has high superheat and low subcooling: Add R-22 refrigerant (but only after you fix the leak, more below).

To simplify this, we can say that:

  • High Superheat = Amount of refrigerant in the evaporator coil is too low.
  • Low Subcooling = Amount of refrigerant in the condenser coil is too low.

causes-of-high-superheat-and-low-subcooling-in-the-same-AC-unit-scaled

 

From this, it’s pretty clear that we have an undercharged air conditioner.

Of course, causes for high superheat low subcooling don’t need to have the same origin. You can have any of these combinations that will also cause superheat to rise and subcooling to fall:

  • High superheat due to too high indoor CFMs and low subcooling due to poor compression.
  • High superheat due to excessive heat load and low subcooling due to poor compression.

In practice, more than 95% of issues, when we see high superheat and low subcooling, are caused by a lack of refrigerant (undercharged refrigerant lines).

Here’s how to fix that:

How To Fix High Superheat Low Subcooling In AC Unit?

The solution seems pretty obvious: Just add refrigerant to the refrigerant lines, right? If the unit is undercharged, you have to charge it.

While that is true, we don’t start with charging an undercharged AC unit.

Very importantly, we have to start by eliminating the culprit of why we got undercharged lines in the first place. Namely, all AC lines are a closed system; nothing goes in, and nothing goes out. That’s the normal operation.

If we have a high superheat low subcooling problem due to low refrigerant lines, there are only 2 possible explanations for that:

  1. Refrigerant lines were undercharged from the beginning. If you have a new air conditioner and you measure high superheat low subcooling, the most likely explanation is that the lines were not sufficiently charged to begin with.
  2. Leak in refrigerant lines. This is the most common reason why we see low freon levels. The leak can happen anywhere – in the suction line, liquid line, evaporator coil, condenser coil, or even at the TXV valve.
high superheat low subcooling causes by leak in evaporator coils
It’s quite difficult to locate the leak. That’s why we use the nitrogen test.

If we have a leak – which is very likely – we have to first seal the leak. Usually, you have to call an HVAC technician to find the leak with the nitrogen test. The leak is then sealed with a leak seal.

soldering condenser coil leak before adding freon
In some cases, we can also solder the leak (these are copper lines, quite easy to solder).

Only then can you charge the appropriate amount of refrigerant (R-22, R-410A, R-134A, and so on) into the lines while measuring the superheat and subcooling.

Once the high superheat falls down to normal superheat (in line with target superheat +/- 2°F) and the low subcooling increases to normal subcooling (in line with target subcooling +/- 3°F), we can stop charging the refrigerant lines.


We hope that this illustrates what to do in the event of high superheat low subcooling. Immediately think about low refrigerant charge and, even more importantly, the leak. Fix the leak and adequately charge the lines to get back to normal superheat normal subcooling.

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