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Low superheat and low subcooling are not uncommon HVAC problems. Once in a while, your HVAC system may experience these two problems. So, what is low superheat low subcooling and how can you fix it?
Low superheat and low subcooling are an indication of limited heat load in the evaporator coil and limited refrigerant in the condenser. Low superheat low subcooling in a TVX system can indicate a malfunctioning metering device but this problem can be fixed by properly adjusting the meter or replacing the meter power head.
What is superheat and what does low/high superheat mean?
Superheat simply indicates the amount of refrigerant in the evaporator. High superheat means there isn’t enough refrigerant in the evaporator whereas low superheat means there is an excess amount of refrigerant in the evaporator.
Chart for low superheat causes and possible fixes
| Low Superheat CAUSES | Low Superheat FIXES |
| REDUCED AIRFLOW TO THE EVAPORATOR | CLEAN AIR FILTERS AND BLOWER WHEELS |
| MALFUNCTIONING METERING SYSTEM | REPLACE METER POWER HEAD |
| INSUFFICIENT HEAT LOAD TO THE COIL | CLEAN FILTERS AND COILS |
| EXCESSIVE REFRIGERANT | DISCHARGE REFRIGERANT |
| DIRTY / OUT OF ADJUSTMENT METER | CLEAN METER AND PROPERLY ADJUST IT |
| CLOGGED EVAPORATOR COIL | DISCHARGE/BLEED THE REFRIGERANT |
| OVERSIZED HVAC EQUIPMENT | USE WELL-SIZED EQUIPMENT |
A low Superheat can be caused by low heat getting to the evaporator coil. If you have high superheat and while you are adding the refrigerant into the system, the superheat doesn’t change but subcooling increases, this is a good sign that the problem has to do with the metering device.
In a TVX (thermal expansion valve) system you’ll most likely need to replace the meter power head to resolve the issue.
What causes Low Superheat?
1. Defective metering device
A defective metering device will either overfeed or underfeed the refrigerant to the evaporator. Usually, when it overfeeds the evaporator with excess refrigerant, this will result in low superheat.
Before you do anything take a look at the meter and ensure it’s not dirty and that it’s properly adjusted.
That said, if the sensing bulb of the (thermal expansion valve) is not properly insulated, it’s highly possible that the expansion valve will be overfed or flooded. It should also be noted that a low heat load to the evaporator could also cause low suction pressure.
The fix
If the metering device is dirty, clean it and adjust it accordingly so that it feeds the evaporator with the right quantity of refrigerant.
Otherwise, if the meter is defective, you’ll need to replace the meter power head to resolve the issue.
2. Reduced airflow to the evaporator
When there is restricted airflow to the evaporator, there won’t be enough warm air to vaporize the refrigerant running in the coil and this could be the reason for the excess liquid refrigerant in the evaporator.
An insufficient heat load to the evaporator coils can be due to any of these issues below
- Dirty filter
- Slipping belt
- Undersized or restricted ductwork
- Dust and dirt buildup on the blower wheel
And if indeed there is excess liquid refrigerant in the evaporator, there is a good chance that this can get into the compressor and potentially damage it. (That is especially the case if the system doesn’t have a separate liquid filter receiver)
The fix
Reduced airflow to the evaporator can be fixed by replacing or cleaning the filters and cleaning up any build-up of dirt on the blower wheel/motors. This is all to enhance airflow to the evaporator.
3. Oversized HVAC equipment
Another possible reason for low superheat is oversized HVAC equipment. When the system is oversized, there won’t be enough heat load to vaporize the refrigerant in the evaporator.
The fix
The best way to solve these oversized HVAC issues is to install properly sized equipment. An oversized system has got more downsides when it comes to dehumidifying the air. It will make your home or office feel warmer than it is.
How to Check For Superheat
To check superheat, use a thermometer designed to take pipe temperature and attach it to the suction line. Then take the suction pressure and the temperature.
To get the superheat we need to find the difference between our temperature reading at the suction line and the temperature after converting the suction pressure (Psi to temperature). Let’s look at this example.
Assuming that you are dealing with an R-22 system
Take the suction pressure and use the pressure/temperature chart to convert it to temperature.
Let’s say the suction pressure is 60(Psi) which converts to 34F. And assuming that the suction line temperature is 44F. Then to get superheat we just get the difference between the two temperature readings. In this case, we have 10F of superheat.
Remember that for TVX systems, the normal superheat range is 10F when measured near the evaporator and 20F to 25F when measured near the condenser. Meaning that anything below that is low superheat and anything above is high superheat.
What is subcooling and what does low subcooling mean?
Subcooling is the measure of how much refrigerant is packed up in the condenser. Therefore, Low Subcooling is an indication of insufficient refrigerant packed in the condenser. Normal subcooling on the TXV system is usually between 10° measured at the evaporator.
Some reasons for low subcooling include poor compression, malfunctioning metering device, undercharge, overfeeding.
Chart for low subcooling causes and possible fixes
| LOW SUBCOOLING CAUSES | LOW SUBCOOLING FIXES |
| DEFECTIVE METERING DEVICE | CLEAN METER OR REPLACE PARTS |
| POOR COMPRESSION | CHECK CAPACITOR OR OTHER COMPRESSOR PARTS |
| REFRIGERANT UNDERCHARGE | REFILL WITH ADEQUATE REFRIGERANT |
| RESTRICTED AIR FLOW TO THE CONDENSER | REMOVE POSSIBLE OBSTRUCTIONS OR REPLACE MOTOR BEARINGS |
1) Defective metering device
Metering systems control the flow of the refrigerant in the liquid line. If this device is malfunctioning or it is not properly sized, it could be restricting the flow of the refrigerant into the condenser and could cause a limited amount of the refrigerant in the condenser unit eventually leading to low subcooling.
The fix
Try cleaning up the buildup on the meter. If this doesn’t work replace the meter power heard.
2) Poor compression
Now, when the compressor is not working as it should, there will be poor compression of air. This can be due to any part of the compressor, but a compressor capacitor is among the common reasons for this.
The fix
If you suspect your compressor is not doing its job, remove the capacitor, discharge it and test it for continuity. Compressor capacitors are easily and cheaply replaceable.
3) Refrigerant Undercharge
Did you just refill your system with the refrigerant? If that’s the case, there is a possibility that you undercharged your system or there is a leak somewhere. Here are a few tips to take note of before recharging your system
· Find the leaks and seal them off
· Take note of the superheat and subcooling to avoid undercharging or overcharging your unit.
4) Restricted airflow to the condenser
When there is reduced airflow to the condenser, this can lead to a higher temperature in the condenser causing a high pressure at the metering device. Because of that high pressure, more refrigerant will enter into the flow line and consequently leave little or no refrigerant in the condenser. This is what will cause low subcooling.
The fix
If there is an obstruction at the condenser coil, clear it away. Obstruction can be caused by any objects including leaves or nearby shrubs. You can use a hose to spray away the dirt from your condenser unit or a leaf blower to blower to clear away the leaves.
How to Check Subcooling
Attach the thermometer to the liquid line. This time, near the condenser. Then take the head pressure in Psi and convert it to the temperature on the pressure/temperature chart.
Assuming you are dealing with a R22 system, let’s say the head pressure is 260Psi. This will convert to 120F. And if your liquid line temperature reading was 86F, then the difference between the two readings is 34F meaning that’s a high Subcooling.
What is high superheat low subcooling?
High superheat low subcooling is when there is a limited flow of refrigerant in the evaporator and a limited flow of the refrigerant in the condenser.
It is worth noting that in systems with liquid line receivers, subcooling will not increase because the liquid will go to the receiver instead of the condenser although the receive line filter is common in refrigeration and walk-in coolers. In such a case, how would you know whether or not high superheat is due to low refrigerant? This is how to do it…
1) Check the liquid line sight glass and if it is full of liquid and free of bubbles, that in many cases indicates the refrigerant level is fine.
2) If the liquid line sight glass has bubbles through the glass, that’s a good sign that the sight might be low on the refrigerant, or the filter/dryer is plugged.
High superheat low subcooling causes
The common cause of high superheat (high heat load in the evaporator) and low subcooling (low refrigerant in the condenser, is simply low refrigerant in the whole system.) But always ensure to find the leaks and seal them before adding any refrigerant.
And whilst recharging your system ensure to keep an eye on both the superheat and the subcooling to avoid overcharging your system.
However High Superheat high subcooling is usually an indication of a defective metering device.
Signs of low refrigerant in your HVAC system
Low or no refrigerant can cause high superheat and low subcooling and these are a few common signs to indicate that your system is running low on the refrigerant or if there is a leaking refrigerant.
- Air conditioner blowing warm air (not cool)
- Strange sounds coming from the condenser unit
- The set temperature is never reached
- High electricity bills
- Ice building up on the refrigerant lines
Hopefully, this answers for you how to fix the low superheat and low subcool problems you have in your AC unit. If anything isn’t clear please use the comment section below with your specific question.
HVAC tech with over 30 years of experience. Retired and doing repair work on the side around Madison County, AL.