6 Low Superheat Causes: Low CFMs, Overcharged Or Oversized AC

Low superheat can flood your compressor. That will ruin the compressor, and you will likely have to spend $1000s for a new air conditioner. That’s why it’s very useful to understand what causes low superheat and how to fix it.

We are going to cover all 6 low superheat causes, going one-by-one, detailing why low superheat happens and how to fix it (in much the same way we did for high superheat causes here).

We define low superheat as target superheat being by 2°F or more degrees higher than the actual measured superheat. We can write this is an equation like this:

Low Superheat = Target Superheat – Actual Measured Superheat > 2°F

we detect low superheat with red manifold hvac gauge
We measure the actual superheat with the low side red gauge and clamp-on thermometer (on suction line).

Quick Example: Let’s say that you have used the manifold gauge (red part) and clamp-on thermometer to measure actual superheat using this 10 step procedure. You got an actual measured superheat of 5°F. Now, you consult the target superheat chart here at 82°F outdoor temp (dry bulb temp) and 62°F wet bulb temp; the target superheat is 12°F. The difference between target superheat and actual measured superheat is 7°F; that’s way above 2°F. This is how you diagnose that you have low superheat.

Now, generally, we have these 2 root low superheat causes:

  1. Too much refrigerant in the evaporator coil. This is the case in the #4 and #5 causes in the list below.
  2. Not enough indoor heat to adequately vaporize the refrigerant. Causes #1 and #2 cover this well, and we also have to check the outdoor coils (cause #3).

root causes of low superheat

There are 6 common instances that cause low superheat. Let’s go over them one-by-one as an HVAC technician would regularly do when on low superheat call (starting with the most common cause):

1. Low Airflow Over Evaporator Coils (Low CFMs)

Low airflow indoor airflow is the most common cause of low superheat. Namely, the airflow over the indoor evaporator coils isn’t sufficient (lower-than-needed CFMs). That means that the amount of warm air traveling over the coil is not sufficient. The result is that the liquid refrigerant inside the evaporator coil is not heating up and vaporizing quickly enough, and we will get low superheat.

At low superheat, the refrigerant is slowly turned into vapor, and that vapor gains less temperature than needed. At very low superheat (close to 0°F), it might happen that liquid refrigerant doesn’t turn to 100% vapor in the evaporator coil. That means that liquid refrigerant might enter the suction line (that’s a no-no), and be channeled into the compressor, flooding it and resulting in the AC breakdown.

Here’s how to fix low superheat caused by low indoor airflow:

We have to check for things that might reduce indoor airflow (CFMs). This ranges from something simple like dirty filters (lower CFMs) or dirty indoor coils (lower heat exchange) to something more complex like bad indoor fan motor (lower CFMs) or closed registers.

dirty filters can cause low superheat
Dirty air filters can lead to lower CFMs, and thus lower heat load reaching the indoor evaporator coil.

The first thing you should do it to clean or change the air filters. If that doesn’t normalize the superheat, we have to clean the indoor evaporator coils, and check if we still have low superheat. If that doesn’t solve it, we check the fan motor and registers.

Another reason why the airflow over the indoor AC coils is insufficient is the oversized AC unit. This is another cause we have to break down:

2. Oversized Air Conditioner (Leading To Short Cycling)

Bigger AC units require a lot of warm indoor airflow to operate adequately. If we put a 3-ton unit in a 500 sq ft space (very oversized AC situation), the amount of heat that the AC pulls from warm indoor air is just not enough to adequately warm the liquid refrigerant in the evaporator coil. The result is that the liquid-to-vapor transition will be slower, the vaporized freon will not gain as much temperature as needed, and we will have a low superheat situation to deal with.

In most of these oversized AC unit cases, the air conditioner will start short cycling. Namely, the unit will detect that there is not enough warm indoor air flowing over the evaporator coils, and it will start and stop every 1 minute, 2 minutes, and up to 5 minutes, for example.

The painful result is also that such low superheat AC unit will not pull humidity out of the space. We will be left with a low temperature, high humidity, and low superheat that puts the AC compressor at risk of being flooded.

Here’s how to fix low superheat if we have an oversized AC unit:

Well, we should get a smaller unit for that space. That’s the go-to move. However, we already paid for the big AC unit and don’t really want to downgrade the size at a $1000s additional cost.

One way how you can still use an oversized AC unit is to let it cool a bigger space. We can open doors to additional rooms, for example. Another option is to run the oversized AC unit on a Low or Medium fan setting.

In both cases, we should monitor the superheat and confirm that we don’t still have low superheat (due to compressor flooding risks). If we can’t increase the superheat to normal superheat with these 2 measures, we do need to downgrade to a smaller AC unit.

3. Low Airflow Over Condenser Coils (Dirty Coil)

The outdoor unit can also cause low superheat. If we have insufficient airflow over the outdoor condenser coils, we will see an increase in condensing temperature. That will automatically increase the pressure inside the condenser coil. This higher-than-normal pressure liquid will reach the metering device, and the metering device will thus feed more-than-needed liquid refrigerant into the indoor evaporator coil.

Now, the indoor airflow will have to heat up more liquid refrigerant; it usually can’t do that. Thus we will see a slower transition from the liquid refrigerant to vapor, resulting in low superheat. If the metering device lets through too much liquid refrigerant (that cannot be 100% turned into vapor in the evaporator coil), we will get liquid freon in the suction line and run the risk of flooding the compressor.

Here is how you fix low superheat due to low condenser coil airflow:

The root cause, in this case, is usually not sufficient heat exchange between the outdoor air and condenser coil due to dirt on the condenser coil. The first order of business is to clean the condenser coils. If that was the real cause, the superheat should go back from low to normal again.

Another reason might indeed be low CFMs over the condenser coil. This is usually a bad outdoor motor problem (not producing enough CFMs). We also might have a defective capacitor (the outdoor fan motor will turn on and off repeatedly), or there is some kind of obstruction in the outdoor unit (leaves, twigs, even dead animals).

bad fan motor leading to low superheat
Bad outdoor fan motor (or capacitors) will decrease airflow and the refrigerants will not give away sufficient heat outdoors.

We have to check for all of these culprits, clean the outdoor unit, and check the motor and capacitors. After the true culprit is fixed, we should see an increase in superheat from low to normal again.

4. High Refrigerant Charge (Remove Freon)

Overcharged lines is another very common culprit that results in low superheat. If we have too much refrigerant in the lines, the metering device will feed more-than-needed liquid refrigerant into the evaporator coils. The existing heat flow over the indoor evaporator coils will not be sufficient to turn the liquid refrigerant to vapor and increase the temperature of that vapor quickly enough. The result is low superheat.

Here is how we fix low superheat due to overcharged AC unit:

We have to remove the refrigerant (R-22, R-410A, R-134A, etc.). This is simple to say but hard to do. We have to leak the AC unit, and that job is best left to licensed HVAC technicians (with a license to drain freon). This is not a DIY low superheat fix.

low superheat caused by overcharged ac unit
We use the yellow pipe on the gauge to remove refrigerant. In this example, we are removing R-22 freon.

After enough freon is removed from the lines, we should see superheat normalize.

5. Metering Device Feeding Too Much Refrigerant Into Evaporator Coil

The metering device, located on the liquid discharge line, has only one job: Feed an adequate amount of liquid refrigerant into the evaporator coil. If there is something wrong with the metering device (be it TXV or piston), we can see overfeeding.

That means that too much liquid freon is fed into the evaporator coil. Turning that amount of liquid freon to vapor and then vapor increasing in temperature is slower, resulting in low superheat. We should also see suction pressure and discharge pressure increase.

Here’s how you can fix low superheat due to metering device overfeeding:

If you have a TXV, check if the sensing bulb is adequately insulated and secured to the suction line. We usually see low superheat if the TXV is not properly secured to the suction line. Once that is secured,  you should see the normalized superheat.

If you have a piston for a metering device, check if everything is OK with it. In too many instances, you will see an incorrect piston or piston missing completely. Most people will have to call an HVAC technician to check their metering device.

6. Wrong Superheat Measurement

This is the human error that we should always think of. It might be that you don’t have low superheat; it might just be that you have measured and calculated superheat incorrectly. You should redo your calculation using these superheat calculation instructions.

These 6 points cover all common low superheat causes. It is important to understand that we have to fix low superheat; if we run an AC unit in low superheat state, we will eventually flood the AC compressor, which is pretty much the most expensive mistake you can make with an AC unit.

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