What Causes High Or Low Subcooling? 4 Culprits + Fixes

We use subcooling for a range of AC diagnostics. Both high subcooling and low subcooling will give us a key insight into what might be wrong with our air conditioner. To fully take advantage of our subcooling measurements, we are going to check these two insights:

  1. Low Subcooling Causes. As we will see in the #1, #2, and #4 causes below, low subcooling can be caused by low refrigerant charge (undercharged lines), oversized TXV or piston, or poor compression (failed compressor, the worst case scenario).
  2. High Subcooling Causes. As outlined in #1, #2, and #3 causes, high subcooling can be caused by high refrigerant charge (overcharged lines), undersized metering device (TXV, piston, etc.), or restriction in liquid line (usually a piece of ice due to filter dryer failure).

Now, first we need to understand what low subcooling and high subcooling actually are:

We have low subcooling when the actual measured subcooling is by 3°F or more degrees lower than the target subcooling. You can check how to measure subcooling in 9 steps here, and you can read how to find target subcooling here. We can put this in an equation like this:

Low Subcooling = Actual Measured Subcooling – Target Subcooling < -3°F

High subcooling is just a reverse situation. We have high subcooling when the actual measured subcooling is by 3°F or more degrees higher than the target subcooling. This is what a high subcooling equation looks like:

High Subcooling = Actual Measured Subcooling – Target Subcooling > 3°F

In most cases, both high and low subcooling have the same root cause (it’s just a reverse situation). We are going to go through 4 high and low subcooling causes one-by-one (as an HVAC technician checking your system would do).

Let’s start with the most common culprit for high or low subcooling. It illustrates well how both of these situations have the same root cause:

1. Low Refrigerant Charge (Low Subcooling) Or High Refrigerant Charge (High Subcooling)

The most common cause for non-normal subcooling is a wrong refrigerant charge. If the system is overcharged (too much freon), we will get high subcooling. In the system is undercharged (low refrigerant charge), we will measure low subcooling.

adding refrigerant at low subcooling and removing refrigerant at high subcooling
Low charge = low subcooling; we have to add refrigerant. High subcooling = high charge; we have to remove refrigerant. On photo: HVAC technician removing R-410A freon from the high subcooling system in order for subcooling to normalize.

When we have a low refrigerant charge, a less-than-needed amount of refrigerant is in the outdoor condenser coils. The heat extraction via the cold outdoor air (relative to saturated refrigerant temperature) will quickly extract enough heat to turn the saturated refrigerant (liquid and vapor mixture) to 100% liquid. That liquid will be cooled below the target subcooling temperature, and we will get low subcooling.

The opposite is also true. When we have overcharged lines, we will have a more-than-needed amount of freon in the condenser coils. The saturated refrigerant cooling process via outdoor airflow over condenser coils will be slower. Once all the refrigerant is turned into liquid, that liquid freon will not be sufficiently cooled, resulting in high subcooling.

Here’s how we can fix high subcooling due to overcharged lines and low subcooling due to undercharged lines:

Simple. We have to add refrigerant in the case of low subcooling, and we have to remove refrigerant in the case of high subcooling.

Obviously, this is easier said than done. For charging or leaking an AC system, you will need a certified HVAC technician. Charging or removing freon from AC lines is not exactly a DIY job; even HVAC technicians usually have to have a certificate for this job (depends on the state regulations).

2. Oversized Metering Device (Low Subcooling) Or Undersized Metering Device (High Subcooling)

For normal subcooling, we need to have a normally-functioning metering device. This can be TXV, piston, or any other metering device.

If our metering device is feeding too much liquid refrigerant into the evaporator coil (overfeeding), we will notice that by measuring low subcooling. If the metering device if not feeding enough liquid refrigerant into the evaporator coil (underfeeding), we will see an increase in subcooling temperature, resulting in high subcooling.

low subcooling caused by txv overfeeding and high subcooling caused by txv underfeeding
This is a TXV or Thermal eXpansion Valve (also abbreviated as TEV). If it’s not feeding correctly, we run into either low subcooling or high subcooling situations. That sensing bulb has to be properly insulated and secured if we want normal subcooling.

Here’s how to fix high or low subcooling caused by a faulty metering device:

You have to check the metering device and check what’s wrong with it. It might be that the sensing bulb in TXV is not adequately insulated or not properly secured on the liquid line. If you have a piston, the problem might be an incorrectly sized piston or even a missing piston.

Once you fix the metering device (if it’s causing high or low subcooling), you should start to see the subcooling temperature going back to normal subcooling (in line with target subcooling with +/- 3°F accuracy).

3. High Subcooling Caused By Restriction In Liquid Line

In too many cases, you see that high subcooling is caused by a restriction in the liquid line. This seems a bit paradoxical; the refrigerant lines are a closed system (nothing should come in, and nothing should go out). However, in many cases, you will see water via humid air enter the refrigerant lines.

That is usually accounted for; that’s why we have a filter dryer that catches that excess water in freon lines. However, if there is too much water for the filter dryer to handle, or if we have a faulty filter dryer, some water might stay inside the lines. Since the temperature inside the lines can be below freezing point (below 32°F), that water will form ice.

The formed ice presents a restriction in the liquid line. That will restrict the normal flow of refrigerant, and we will notice that we have high subcooling.

Note: You will usually see a drop in temperature after the part of the liquid line that is partially blocked. This is how we can diagnose that we indeed have a restriction in the liquid line causing high subcooling.

Here’s how to fix high subcooling caused by a restriction in the liquid line:

Before removing the restriction itself, we have to take care of the root cause first. We check the filter drier and potentially replace it. We also check for places where the water (in the form of humid air) could be entering our lines. If we don’t do this, we run a risk of getting a restriction in the liquid line and thus high subcooling again in the future.

Only after we ensure that the root cause is removed, we remove the restriction in the liquid line. Again, this is not a DIY job anybody can do. If you suspect that you have a high subcooling due to restriction, it’s best to call an HVAC technician, and he or she will take care of this situation for you.

4. Low Subcooling Caused Poor Compression (Potential Compressor Problem)

Low subcooling can be an indication of the worst sort; that our AC compressor has a problem. Everybody hates failed compressors because when you have a faulty compressor, it’s usually financially more viable to buy a new AC unit since replacing a compressor is so expensive.

Namely, if the AC compressor has poor compression, it will result in low subcooling.

Low subcooling caused by poor compression is the worst-case scenario. If you suspect that poor compression is causing the decrease in subcooling far below target subcooling, you should call an HVAC technician and pray that it’s not a faulty compressor problem. The HVAC technician will diagnose the problem and let you know what will be done to solve the low subcooling situation.

Hopefully, these causes of high and low subcooling will give you the needed insight to understand what is happening with your AC unit (why it is misbehaving), and outlay how to fix it.

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