Low Superheat Normal Subcooling: 3 Causes + How To Fix AC?

Low superheat normal subcooling is a less frequent AC problem. It happens when some culprit is decreasing superheat, but, very importantly, it doesn’t affect subcooling. Let’s look at what is causing low superheat and normal subcooling issue, and fix it (to avoid compressor flooding).

Namely, we can look at culprits that cause low superheat here, and check the causes that cause either high or low subcooling here. Here is a table with these causes that will help us understand why we get low superheat normal subcooling:

Low Superheat Causes:	Non-Normal Subcooling Causes (High Or Low):
Indoor airflow (CFMs) is too low. Oversized AC unit. Outdoor airflow is too low (or condenser coils heat exchange is impeded). High refrigerant charge (overcharged AC unit). Metering device (TXV, AEV, or piston) is overfeeding.	Low or high refrigerant charge (causing low and high subcooling). Metering device (TXV, AEV, or piston) is underfeeding (high subcooling) or overfeeding (low subcooling) Restriction in the liquid line (high subcooling). Poor compression (low subcooling).

We can clearly see, for example, that both high refrigerant charge and TXV valve overfeeding will cause low superheat. However, at the same time, the high refrigerant charge will cause high subcooling and TXV overfeeding will cause low subcooling.

Hence, these two culprits will not cause low superheat AND normal subcooling. Rather, the high refrigerant charge will cause low superheat high subcooling explained here, and the metering device overfeeding will cause low superheat low subcooling explained here.

We are left with 3 key culprits that can indeed cause low superheat normal subcooling issues, namely:

1. Indoor airflow (CFMs) is too low.
2. Oversized AC unit (accompanied by high humidity).
3. Outdoor airflow is too low (or condenser coils heat exchange is impeded).

Let’s take a structured approach and check each of these causes one-by-one, explain why it’s causing low superheat normal subcooling, and how to fix it. Let’s start with the indoor airflow being too low:

1. Low Superheat Normal Subcool Caused By Low Indoor CFMs (Dirty Filters Or Coils)

We start with the metering device (TXV, piston) feeding the saturated refrigerant into the evaporator coil. Inside the coil, the warm indoor air blown over the evaporator coil, will turn the saturated refrigerant into 100% vapor refrigerant. This vapor will be heated up, and we will measure superheat.

Now, if the indoor airflow over the evaporator coil (measured in CFM or Cubic Feet per Minute) is lower-than-needed, it will not bring enough heat with it to turn the saturated refrigerant into vapor quickly enough. The vapor temperature increase will thus be lower-than-needed, resulting in us measuring low superheat.

Subcooling will not be affected by this; hence, we will get the low superheat normal subcooling problem.

Here is how you fix low superheat normal subcooling caused by low indoor CFMs:

We have to consider 2 possibilities here. We know the evaporator coil is not receiving sufficient heat load from the indoor air. This can be either because:

1. The airflow is lower than needed. Restriction of airflow in the indoor unit is usually caused by dirty filters. Alternatively, the reason can also be a problem with the fan motor; this would reduce the airflow (CFMs it can generate).
2. The heat exchange is lower than needed. This is usually caused by dirty indoor coils. If the coils have dirt on them, the heat exchange (the heat coils get from incoming airflow) will be reduced, resulting in low superheat.

   

If you see low superheat normal subcooling, the first thing you should do is clean or change the filters. If that doesn’t help, we proceeding with cleaning the indoor evaporator coils.

This will normalize the airflow and heat exchange, thereby bringing low superheat back up to normal. This is the No. 1 way how to fix low superheat normal subcooling. If that doesn’t work, it might be that you have an oversized AC or there is a problem with the outdoor coil:

2. Low Superheat Normal Subcooling Caused By Oversized AC Unit

If you have an AC unit that is too big (let’s say 3-ton unit for a 500 sq ft area), you will get low superheat and normal subcooling.

Namely, a bigger AC unit has a lot of refrigerant in the evaporator coil. Vaporizing it and increasing the temperature of vapor refrigerant up to target superheat temperature requires quite a lot of heat load from indoor air. A too-small-space will not be able to provide enough indoor heat load to vaporize and increase the vapor temperature quickly enough, resulting in low superheat.

If you have an oversized AC unit, the subcooling doesn’t charge; it is in line with the target subcooling temperature. Thus we have the low superheat normal subcooling situation. You will likely see

How to fix an oversized AC unit?

The simple but painfully expensive way is just to replace the big AC unit with a smaller air conditioner. That’s something nobody really wants.

There are 2 alternatives, however. You can:

1. Run the AC unit on Low or Medium speed. This will decrease the cooling output (a 3-ton unit will produce 2 tons of cooling output, for example) and can normalize superheat.
2. Use a fan to move more warm air closer to the AC unit. The oversized AC unit just needs more indoor warm air. If you channel warm air towards it (using a fan, for example), you can virtually expand the space it has to cool down from, say, 500 sq ft to 800+ sq ft. Now, it has to cool down a bigger amount of air and is thus not oversized anymore.

Before you go buying a smaller AC unit, it’s useful to try to employ these 2 alternatives. If it doesn’t work, you will unfortunately have to be $1,000s out of pocket for that new smaller unit.

3. Low Superheat Normal Subcooling Caused By Outdoor Airflow Is Too Low

Outdoor unit (condenser coil) can also cause low superheat while maintaining normal subcooling. If the condenser coil cannot expel enough heat, we will see an increase in condensing temperature. In turn, this will increase the high side pressure, cause the metering device to overfeed, and result in low superheat.

This will not affect subcooling.

There are 2 reasons why the condenser coil cannot expel enough heat outdoors:

1. Outdoor airflow over the condenser coil is not sufficient. This is less common and is usually a symptom of outdoor fan motor problems.
2. Condenser coil heat exchange is impeded. This is a more common situation; there is dirt on the evaporator coils, making it more difficult for the coils to dissipate heat. In this case, you have to clean the outdoor condenser coils.

Once the condenser coils are cleaned and you have sufficient airflow over them, the low superheat normal subcooling should go back to normal superheat normal subcooling.

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Understanding these 3 low superheat normal subcooling causes will help you fix your air conditioner. Just be aware that if the superheat falls too much, you are running a risk of flooding the AC compressor, one of the biggest no-no’s for any AC system.