9 steps to calculate superheat and why it matters for HVAC.
The definition of superheat already gives you some insights into how to calculate it. Even so, it is wise for HVAC professionals to take the time to understand the detailed steps in the calculation process, as well as why it is so important.
It is complicated to calculate superheat, as you need to take multiple measurements, which you may not always have the time for. That being said, once you get the hang of it, it becomes second nature and only requires a few extra minutes of your time while providing valuable information in exchange.
What is superheat in HVAC?
Superheat looks at the refrigerant within an evaporator coil and is the difference in temperatures between its boiling temperature and the temperature when it exits the evaporator.
The refrigerant boils, and after that, it keeps heating up, with that additional change in temperature referred to as superheating.
The following images offer two ways to visualize superheat and subcooling. These two concepts are typically connected.
Subcooling and condensation occur in the condenser but superheat, and evaporation occurs in the HVAC’s evaporator.
Types of superheat
While most people only discuss a single type of superheat, there are technically three. All three types can provide valuable insights into an HVAC system.
- Duct system superheat: This superheat is not directly related to the refrigerant, but it still plays a pivotal role in HVAC due to the duct system’s importance for HVAC performance.
- Evaporator superheat: This is the superheat of the indoor coil or evaporator and what most people measure when determining the superheat.
- Total superheat: This superheat includes the heat that the refrigerant likely gained while moving in the suction line. This measurement can help determine how long the compressor will last.
Why is superheat important?
One of the most important reasons to measure superheat is its ability to improve the air conditioner’s efficiency and prevent damage.
Air conditioning manufacturers always have charts published informing you of the ideal superheat, based on the outdoor temperature and the wet-bulb measurement inside. Accessible charts help you avoid issues with the liquid getting into the compressor, which could happen if the refrigerant boils closer to the evaporator coil’s end.
In simpler terms, having the right superheat measurement ensures that all of the liquid refrigerant boils off, so it does not enter the compressor as a liquid.
You do not want the superheat to be too high or too low, as this leads to inefficiency and the risk of compressor flooding, respectively.
Essentially, superheat lets you calculate the proper charge and, therefore, maximize an HVAC unit’s efficiency and reliability.
Overall, measuring superheat is crucial because it:
- Helps prevent flooding of the compressor
- Helps you keep the compressor cool
- Lets you know where the saturated refrigerant feeds
- Helps you understand the evaporator’s load
- Helps you diagnose how your active metering device is working
Consider an example situation where you notice the suction pressure of a condensing unit is low. While some may just choose to add refrigerant, you first need to make sure that it is not another issue. In this case, the superheat can let you know if you measured low suction because not enough heat gets to the evaporator.
Superheat and subcooling frequently go hand in hand. In the same example situation, calculating subcooling can let you know if the problem is with the metering device, specifically if it is undersized, clogged, or otherwise defective.
What is normal superheat?
If your superheat measurement is higher or lower than it should be, you just add or remove refrigerant, assuming everything else is working properly.
The following chart shows the ideal superheat for air conditioning units with fixed orifices.
You will notice that the “normal” superheat depends on the wet-bulb temperature. As a general rule, with TEV/EEV systems, superheat should be 6 to 14 degrees in a high-temp AC system, 5 to 10 degrees in a medium-temp system, and 4 to 10 degrees in a low-temp system.
The absolute best way to see the “normal” superheat for a given air conditioner is to check the manufacturer’s specs, as there is a great deal of variation between models.
With all that information in mind, it is time to look at how you measure superheat.
1. Measure the refrigerant.
You will start by measuring the refrigerant. To do this, you need to let the refrigeration system run for a minimum of 10 minutes, as you want to steady the temperature.
Once it has been running long enough, find the suction service valve that separates the compressor and evaporator. Connect this with your refrigerant manifold gauges. Now, connect the suction line located close to the suction service valve with your pipe clamp thermocouple. Finally, you can use a digital thermometer.
2. Measure the pressure in the suction line.
Now, look at the refrigerant pressure gauge to check the pressure in the suction line. Write down this figure and the temperature.
3. Look at the chart.
You can now use a chart for the temperature and pressure of the refrigerant to see what the figure from the pressure gauge means for the temperature of evaporation saturation.
Next, take the thermocouple temperature and subtract the temperature of evaporator saturation from it. That figure will let you know the system’s superheat.
5. Compare the superheat to the recommended value.
Now, you can see how your measured superheat compares to the one suggested by the manufacturer. These reference measurements should be in your air conditioning system’s operating instructions.
6. Use the information from the superheat.
Once you measure superheat and compare it to the ideal value, you can see what information it provides. Remember that the following are some of the potential problems that you may want to check if the superheat is too high or too low:
- The liquid in the compressor, which could damage the compressor
- A wrong refrigerant charge
- Issues with the thermal expansion device
- A condenser coil that requires cleaning
- A filter-drier that is clogged
Remember that too high superheat values can indicate that there is not enough refrigerant in the evaporator, which would lead to inefficient operation. If the values are too low, this can mean there is liquid getting into the compressor, where it could potentially cause damage.
7. Or use a tool to streamline the process.
Because measuring superheat is important but not done as frequently as it should, some companies have developed devices that complete some of the steps for you. One type of product will measure the suction line temperature and the suction line pressure. It will then use that information to calculate the superheat for you automatically.
8. Know when to check superheat.
In addition to knowing how to measure superheat, you should also know when to do so. Ideally, you should take the time to calculate it if:
- You installed a new compressor
- You changed the TX valve
- You added refrigerant
- The system’s refrigeration is not acting as it should
- You are doing preventative maintenance
- Any other time you are troubleshooting problems and need to rule out possibilities
9. Bonus: measure the subcooling as well.
Ideally, you should consider measuring the subcooling in addition to the superheat. This involves a similar process to measuring superheat, but with a few small variations.
- Have your refrigeration system run a minimum of 10 minutes.
- Connect your pipe clamp thermocouple to a part on the refrigerant line that separates the thermal expansion valve and the condenser’s discharge.
- Connect your refrigerant manifold gauge with the discharge service valve.
- Read the refrigerant manifold gauge to see the condenser pressure.
- Use a digital thermometer to check the thermocouple temperature.
- Check your temperature and pressure chart for the refrigerant to turn the condenser pressure reading into a condenser saturation temperature.
- Start with the pressure from the thermocouple and subtract the temperature of the condenser saturation from it.
Keep in mind that you may notice insufficient subcooling because of:
- Issues with your thermal expansion valve
- Not enough refrigerant charge
- Not enough airflow on the condenser
Conclusion: Why superheat is necessary for HVAC.
Measuring the superheat of your HVAC system helps you ensure its overall functioning. Remember that the superheat can let you know if there is a risk of liquid reaching the compressor and if the system is not operating efficiently. It can also provide other insights.
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