High Superheat Normal Subcooling: High CFMs Or Excessive Heat Load

“Subcooling is normal, but I get high superheat. What is causing this? How can I fix high superheat normal subcooling?”

This is quite a common air conditioner issue. Namely, we can find cases of high superheat normal subcooling by:

  1. Looking at every culprit that causes high superheat here.
  2. Removing the culprits that cause high or low superheat here (since we have normal subcooling).
high superheat normal subcooling explained
We have to check for culprits that increase superheat (high superheat) but do not change subcooling (normal subcooling).

Here’s the short summary. High superheat can be caused by (the culprits that at the same time cause low or high subcooling are crossed out):

  1. Low refrigerant charge.
  2. Restriction in the liquid line.
  3. Too high indoor CFMs.
  4. Too high indoor heat load.
  5. Underfeeding metering device (TXV, piston).

Now, some of these culprits will also result in high or low subcooling. Here are the culprits for non-normal subcooling:

  1. Low refrigerant charge (low subcooling) or high refrigerant charge (high subcooling).
  2. Metering device underfeeding (high subcooling) or overfeeding (low subcooling).
  3. Restriction in the liquid line (high subcooling).
  4. Poor compression (low subcooling).

Example: Low refrigerant charge causes high superheat, but it does not cause high superheat normal subcooling. That’s because low charge results in high superheat AND low subcooling (we have covered this in high superheat low subcooling issues here).

We are left with only 2-3 culprits that can actually cause both high superheat and normal subcooling. These are:

  1. Too high indoor CFMs.
  2. Too high indoor load.
  3. Faulty measurement.

Let’s look at all these 3 causes of high superheat normal subcooling in turn (one-by-one), and how to fix this issue:

1. High Superheat Normal Subcooling Caused By Excessive Indoor CFMs (Airflow)

Here’s how normally operating AC should perform:

The metering device feeds saturated refrigerant (liquid + vapor) into the evaporator coil. The warm indoor air is being passed over the evaporator coils (by the indoor air handler fan; unit of airflow is expressed as CFM or Cubic Feet per Minute). The liquid refrigerant absorbs this heat, and is turned into 100% vapor. That vapor then increases in temperature up to target superheat temperature.

Now, if the indoor air handler fan produces an excessively big airflow over evaporator coils, the refrigerant will have to absorb more heat, resulting in high superheat.

excessive airflow over evaporator coils can cause high superheat normal subcooling
Indoor air handler fan spinning too fast can cause high superheat, but it doesn’t have effect on subcooling.

Quite importantly, this does not affect the subcooling. Hence we will get normal subcooling, but high superheat due to indoor airflow excess.

Here’s how to fix high superheat normal subcooling caused by high indoor CFMs:

First of all, you can reduce indoor airflow by setting the indoor air handler on a lower fan speed setting. In most cases, you will have High/Medium/Low setting. Switch from the High setting to the Medium or Low setting, and check if the high superheat normalizes.

If the superheat comes back down to the normal target superheat temperature, you know you have an excessive indoor CFMs problem, and you should check the fan motor.

Why is the indoor fan motor spinning too fast and producing too much airflow?

There are quite a few reasons why this might happen. If you detect excessive indoor airflow, you should call an HVAC technician. He or she will check what is causing the excessive airflow, and maybe change the motor, the capacitors, and so on.

If this doesn’t help, you might be experiencing high superheat normal subcooling due to excessive load (very easy to fix):

2. High Superheat Normal Subcooling Caused By Excessive Indoor Heat Load

This is the situation where the indoor airflow (CFMs) is adequate, but the indoor temperature is too high for the AC to handle. For example, if the indoor temperature is 80°F, we will get normal superheat. If, however, we have a very high 90°F indoor temperature, the superheat can move into high superheat.

Even at very high indoor temperatures, the subcooling should stay normal. That’s how we get high superheat normal subcooling due to excessive indoor heat load.

Here’s how to fix it:

In most cases, you don’t have to do anything. Yes, you might have high superheat normal subcooling now, but when the indoor temperature is decreased (due to AC operation), you will see high superheat slowly transition to normal superheat.

We can get these high indoor heat loads if we have a very high indoor temperature if you are operating several high-wattage appliances (kitchen appliances, washing machine, etc.), or simply if you have a party and all those people are generating heat, the AC unit cannot handle properly.

It is quite normal to see a slight elevation of superheat on very hot days (100°F or more days).

3. Faulty Superheat Measurement

In very limited cases, we can attribute high superheat normal subcooling to human error. Namely, you should redo your superheat measurement. If in doubt, you should follow this 10 step-by-step guide on how to measure superheat, and then use this formula to calculate superheat.

Overall, the high superheat normal subcooling is mostly caused by excessive indoor airflow (CFMs) or excessive indoor heat load (high indoor temperature).

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