HVAC superheat & subcooling: calculations & diagnostics for pros
Measuring an air conditioner’s superheat and subcooling is a reliable way to check the unit’s refrigerant charge. These measurements can also provide valuable troubleshooting data. As you gear up for the cooling season, reviewing superheat and subcooling can help you save time on the job and ensure a happy customer.
Superheat and subcooling are two critical measurements within the AC unit’s refrigeration cycle. They help keep the refrigerant at the right temperature and in the correct form (either liquid or vapor). If a unit is not pushing cold air, check both superheat and subcooling before adding refrigerant. This provides the information you need to address the root cause and prevent an unhappy callback. Here’s a refresher on superheat and subcooling–including how to measure them, what the measurements may indicate, and how to explain it to your customer.
How to check subcooling and superheating
Proper subcooling and superheating values ensure air conditioners run efficiently. The subcooling measurement determines when the refrigerant vapor turns to liquid within the refrigeration system. Superheating measures the point at which liquid turns to vapor.
What tools do I need to measure superheat and subcooling?
You’ll need a pipe clamp thermometer or digital thermometer and a manifold pressure gauge with saturation temperatures* to measure superheat and subcooling. When using the pressure gauge, be sure to reference the saturation temperatures that correspond with the unit’s refrigerant type, such as R22.
*If you do not have a pressure gauge with saturation temperatures, you can use a pressure temperature chart, also known as a saturation chart.
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Steps to measure subcooling
Turn off the AC unit.
Locate the service valve just beneath the condenser coils and remove the access caps. Be sure the o-rings remain in the caps.
Tighten all fittings on your pressure gauge.
Attach the blue hose to the suction/low-side line. Low-loss fittings are recommended to prevent refrigerant leaks.
Attach the red hose to the discharge/high-side line.
Check your pressure gauge - when the unit is off, the blue and red pressures should be about equal.
Purge the gauge hoses to prevent air from entering the refrigerant system.
Turn on the air conditioner unit and wait until the readings stabilize. This might take around ten minutes.
If using a pipe clamp thermometer, attach it to the high side a few inches from the service valve.
Once stabilized, read the saturation level on the red gauge - be sure to reference the numbers that correspond with the unit’s refrigerant type.
Then, record the top temperature.
Finally, subtract the line temperature from the saturation number on the gauge. This is your subcooling.
Pro tip: When working with refrigerants, it’s always best to wear gloves for protection.
Steps to measure superheating
*If you just finished measuring subcooling, go to step 9.
Turn off the AC unit.
Locate the service valve just beneath the condenser coils and remove the access caps. Be sure the o-rings remain in the caps.
Tighten all fittings on your pressure gauge.
Attach the blue hose to the suction/low-side line. Low-loss fittings are recommended to prevent refrigerant leaks.
Attach the red hose to the discharge/high-side line.
Check your pressure gauge - when the unit is off, the blue and red pressures should be about equal.
Purge the gauge hoses to prevent air from entering the refrigerant system.
Turn on the air conditioner unit and wait until the readings stabilize. This should take about ten minutes.
If using a pipe clamp thermometer, attach it to the low-side/suction line.
Once stabilized, read the saturation level on the blue gauge - be sure to reference the numbers that correspond with the unit’s refrigerant type.
Then, record the top temperature.
Finally, subtract the saturation number on the gauge from the line temperature reading. This is your superheating.
How to calculate subcooling
Subcooling = Saturation temperature - liquid line temperature
How to calculate superheating
Superheating = Suction line temperature - saturation temperature
Pro tip: Give yourself a cheat sheet! On the back of the red gauge, write G-L=SC, which stands for gauge minus line temp equals subcooling. Likewise, on the blue gauge, write L-G=SH.
How does a thermostatic expansion valve impact superheat?
A thermostatic expansion valve or TXV monitors superheat in an air conditioning system. It adjusts refrigerant flow to maintain a target superheat. Therefore, if the system you're working on has a TXV, then use only the subcooling measurement to determine the refrigerant’s charge.
However, if the system uses a different metering device, such as a capillary tube, then the refrigerant amount entering the evaporator coil will remain consistent. In this case, checking the superheat is important. Generally, there is no service valve at the evaporator coil, so you’ll need to determine the total superheat at the compressor.
What do my superheat and subcooling results mean?
Superheat and subcooling can reveal key insights regarding the AC unit’s operation, refrigerant charges, and issues. Let’s break down what high and low superheat may indicate, as well as high and low subcooling.
| Superheat Measurement | Low Subcooling | High Subcooling | Probable Issue |
|---|---|---|---|
| Low Superheat | X | Metering device overfeeding | |
| Low Superheat | X | Too much refrigerant | |
| High Superheat | X | Low refrigerant charge | |
| High Superheat | X | High-side restriction |
What is a good subcooling measurement?
Generally, the subcooling should range between 10ºF and 12ºF. However, reference the unit manufacturer's OEM manual.
What is a good superheat measurement?
Similar to the subcool measurement, it’s important to reference the unit’s operating manual to confirm the correct superheat range. Often, 10ºF to 15ºF is acceptable.
What does high superheat indicate?
Generally, high superheat indicates there is not enough refrigerant in the evaporator. However, it’s important to take both superheat and subcooling measurements into account.
High superheat and low subcooling
This is likely the most common superheat/subcooling combination. As mentioned above, high superheat means the evaporator is undercharged. Likewise, low subcooling means there is not enough liquid refrigerant in the condenser. Rather than immediately adding refrigerant to the system, it is important to first find the leak. Otherwise, you’ll end up with a second service call and an unhappy customer. Once the leak is addressed, recharge the system.
High superheat and high subcooling
High superheat paired with high subcooling perfectly exemplifies the importance of checking both values. Otherwise, you could easily misdiagnose the issue. High subcooling means there is too much liquid refrigerant in the condenser. So, if there is too much refrigerant in the condenser and not enough in the evaporator, then there must be a high-side blockage. This can happen, for example, when the metering device is restricted or if the dryer filter is plugged.
What does low superheat indicate?
If the superheat measurement is low, there is too much refrigerant in the evaporator coil. This is a big deal. The job of the evaporator coil is to transform the refrigerant to complete vapor. When there is too much refrigerant, this change of state cannot happen properly. As a result, liquid could enter the compressor, causing severe issues.
Low superheat and low subcooling
Low subcooling means there is not enough refrigerant in the condenser. So, how can the condenser be undercharged while the evaporator is overcharged? Check the metering device. It’s overfeeding the evaporator. This can happen when the TXV gets stuck open or if the piston is oversized. The metering device is responsible for keeping proper pressures within the refrigerant system and flashing the liquid into a partially gaseous state as it enters the evaporator coil.
Low superheat and high subcooling
Is the suction line sweating and cold? Perhaps the liquid line feels a bit hot? This system has too much refrigerant.
How to explain superheat and subcooling to a customer
AC units operate using a refrigeration system. There are 4 main components within this system: the compressor, condenser coil, metering device, and evaporator coil. In order to understand the role of superheating and subcooling, it’s helpful to have a basic understanding of the system.
Compressor: The compressor acts as the refrigeration system’s pump, circulating refrigerant throughout. In addition, it adds pressure, and therefore heat, by compressing the refrigerant vapor.
Condenser coil: After the compressor, the refrigerant travels through the discharge line as high-temperature and high-pressure vapor on its way to the condenser. Here, the vapor condenses into a liquid, and additional heat removal produces subcooling. This is important because, as the refrigerant moves down the line and through the metering device, there must be no vapor present. Subcooling ensures the refrigerant is in liquid form.
Metering device: In a refrigerant system, the metering device (such as a thermostatic expansion valve or capillary tube) governs the system’s flow, similarly to a garden hose nozzle. In this application, the metering device reduces refrigerant pressure. As a result, the high-temperature/high-pressure subcooled liquid undergoes a drastic change as it passes through the device, turning the liquid into a low-temperature/low-pressure flash gas/vapor mixture before entering the evaporator coil.
Evaporator coil: At this point, the system superheats the refrigerant to completely transform it into vapor. In a sense, the evaporator coil’s job is to evaporate any remaining liquid. This is vital because if any liquid molecules enter the compressor, the system would not operate properly. Superheating ensures the refrigerant is in a low-pressure vapor form as it travels into the compressor, restarting the cycle.
What is subcooling?
Subcooling (or heat removal) transforms high-temp/high-pressure vapor into high-pressure liquid in the condenser. Specifically, it is the temperature decrease of liquid refrigerant below its saturation temperature.
What is superheat?
In an AC unit refrigerant system, superheating takes place in the evaporator coil. It is the temperature increase of vapor above its saturation temperature. In other words, the value of superheat is the difference between the refrigerant gas temperature and its saturation temperature (or boiling point). It is the heating of vapor above its boiling point.
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