Understanding Superheat And Subcooling In HVAC: 101 Explanation (2024)

Superheat is one of the most commonly used terms in HVAC. We use quite a lot of superheat and subcooling calculations as the No. 1 air conditioner diagnostic tool. It is only appropriate that we adequately define what superheat and what subcooling are.

Example: You will very often see an HVAC technician with a manifold gauge. That HVAC gauge is used primarily to determine the superheat and subcooling. A doctor uses a stethoscope to listen to how your heart is beating; an HVAC technicians use a manifold gauge in much the same way. Measuring superheat and subcooling is alike to listening to how well the heart of an air conditioner beats (and we can diagnose problems before a heart attack).

Superheat and subcooling are quite a broad topic. That’s why we will take a structured approach you can use to fully understand superheat and subcooling. Here are the topics we are going to cover:

1. What is superheat? Superheat definitions, total superheat vs. superheat vs. target superheat. This is the refrigerant temperature increase in the indoor unit (evaporator coil). Easy way to remember what superheat is: an increase in vapor temperature (vapor line).
2. What is subcooling? Subcooling is quite the reverse to superheat. This is the refrigerant temperature decrease in the outdoor unit (condenser coil). Easy way to remember what superheat is: a decrease in liquid temperature (suction line).
3. How to measure and calculate superheat and subcooling. Measuring is a bit hard (pressures and P/T charts), but the superheat calculation is quite easy.
4. What are superheat and subcooling even used for? It’s all about ensuring adequate refrigeration charge (R-22, R410A, R134A, and so on). The main thing you want to avoid by measuring superheat is to protect the precious compressor (if superheat is 0° or close to 0°, you have to act immediately).
5. What to do if you have high superheat low subcooling, low superheat low subcooling, high superheat normal subcooling, low superheat normal subcooling, and so on (all scenarios explained).

Overall, measuring and calculating superheat and subcooling is very useful in HVAC. It’s something every HVAC technician must know exactly; correctly determining superheat and/or subcooling will let us know if we have adequate refrigerant charge, or if we should add or remove freon.

Alright, let’s start with the definitions so that we know what we are talking about:

What Is Superheat?

Here is the setup in a split AC system that will serve as a good example:

We are looking at refrigerant states (liquid vs. vapor) and temperatures in the indoor unit (evaporator). The refrigerant entering the indoor evaporator coil is in a saturated state (about 80% liquid and 20% vapor). While traveling through the coils, the refrigerants absorb heat from warm indoor air. Eventually, we go from 80% liquid and 20% vapor refrigerant to 100% vapor refrigerant (this happens within the evaporator coil).

But this is not the end of it:

When we have 100% vapor refrigerant, it will still absorb heat. Thus far, we had a constant temperature (since we have a saturated state prior to hitting 100% vapor; let’s say the temperature here is about 40°F). Now, the temperature of 100% vapor refrigerant increases. This temperature is increasing all through the end of the evaporator coil; let’s say that the temperature of the vapor that comes out of the evaporator coil is 55°F.

Alright, where is the superheat in all this?

Superheat is the temperature difference between:

• Temperature of the refrigerant vapor when it comes out of the evaporator coil. In our example, this is 55°F. We measure this with a digital thermometer.
• Temperature of refrigerant when it comes out of the saturated state (within the evaporator coils; when it hits 100% vapor). In our example, this is 40°F. We measure this temperature with a compound manifold gauge.

To calculate superheat, we have to subtract the temperature of freon when it comes out of the evaporator coil (55°F) from the initial saturated state refrigerant temperature (40°F). We can quickly see that the calculated superheat is 15°F.

Quite useful to remember about superheat is that we are:

1. Measuring vapor refrigerant temperatures (vapor line).
2. Everything is happening in the evaporator coil (indoor unit).

Now, in HVAC manuals, you will see “superheat”, but you also might see “total superheat” or “target superheat”. It is important to note that superheat is not equal to total superheat (target superheat is quite different).

Here is the difference between “superheat” and “total superheat”:

• Superheat is the temperature difference between the temperature of refrigerant exiting the evaporator coil and the saturated state (standard temperature within the coil).
• Total superheat is the temperature difference between the temperature of refrigerant entering the compressor (at the measuring service valve) the evaporator coil and the saturated state (standard temperature within the coil).

After exiting the evaporator coil (55°F), the refrigerant can be further heated up (let’s say to 57°F). We usually have a measuring valve before the refrigerant enters the compressor; that’s where the 57° measurement is taken.

In this example, the superheat would be 15°F and the total superheat would be 17°F. We’ll look at why we even need two different superheats in the 3rd charter.

Let’s now look at the subcooling:

What Is Subcooling?

Subcooling is basically the same as superheat but in reverse. Here’s what this means:

For subcooling, we are looking at the condenser coils (outdoor unit). The compressor turns low temperature low pressure vapor in high temperature and high pressure vapor. This 100% vapor is channeled into the condenser coil, where it ejects heat, and starts to condense into liquid (hence “condenser” coil).

When it reaches 100% liquid state (within the evaporator coil), the previously constant saturated state temperature (let’s say 90°F) starts to decrease. We are basically cooling liquid refrigerant below its saturated temperature; this is the subcooling phenomenon. Let’s say that when the refrigerant liquid exists in the condenser coils and reaches the service valve, we measure the 78°F temperature.

Subcooling is the temperature difference between:

• Temperature of refrigerant when it comes out of the saturated state (within the condenser coils; when it hits 100% liquid). In our example, this is 90°F.
• Temperature of the refrigerant liquid when it comes out of the condenser coil. In our example, this is 78°F.

We can calculate the subcooling by subtracting refrigerant temperature at saturated stated (90°F) from refrigerant liquid temperature when it comes out of the condenser coil (78°F). In this case, the subcooling is 12°F.

A quick way to remember subcooling is that we are:

1. Measuring liquid refrigerant temperatures (liquid line).
2. Everything is happening in the condenser coil (outdoor unit).

In contrast to superheat and total superheat, with the subcooling, we don’t have total subcooling. That’s because there we don’t have two places to measure the temperature of the refrigerant exiting the coils (superheat at the TXV valve and total superheat at the service valve). We only have one place to measure the exiting liquid temperature, and that’s at the liquid line service valve.

Let’s look at how we measure the temperatures needed to calculate superheat and the calculation itself:

How To Measure Temperature And Calculate Superheat, Subcooling?

Calculating superheat and subcooling is pretty straightforward. We are basically calculating the difference between two temperatures (we just need to know exactly which refrigerant temperatures we have to choose).

Here is the formula for calculating superheat and total superheat:

Superheat = T_{vapor after evaporator coil exit}– T_{saturated refrigerant when it turns to 100% vapor}

Total Superheat = T_{vapor at service valve; prior to entering the compressor} – T_{saturated refrigerant when it turns to 100% vapor}

Quick superheat and total superheat calculation example: Let’s say that we measure the saturated refrigerant temperature within the evaporator coil (when it hits 100% vapor). That temperature is 40°F. The vapor temperature at the TXV valve (vapor after evaporator coil exist) is 50°F. The temperature at the vapor line service port (located before the compressor) is 52°F. Here are the superheat calculations:

Superheat = 50°F – 40°F = 10°F

Total Superheat = 52°F – 40°F = 12°F

Alright, calculating subcooling is simple as well. Here is the subcooling formula:

Subcooling = T_{saturated refrigerant when it hits 100% liquid}– T_{liquid after condenser coil}

Quick subcooling calculation example: Let’s say that we measure the saturated refrigerant temperature inside the condenser coil when it hits 100% liquid state. This temperature is 100°F. At the liquid line service line, located after the condenser coil, we measure the temperature of 92°F. How much is subcooling? Here’s the calculation

Subcooling = 100°F – 92°F = 8°F

The calculation itself is the easy part. The harder part is how to measure the needed refrigerant temperatures for superheat and subcooling calculation.

We use an HVAC manifold gauge and a clamp-on thermometer for all superheat and subcooling measurements. With the gauge, we are measuring refrigerant pressures, and we use the pressure-temperature chart (PT chart) to convert pressure into temperature.

Here’s a quick procedure for how to measure superheat and subcooling (starting with the shut-off air conditioner; for practical step-by-step guides, you can check how to measure superheat here and how to measure subcooling here):

1. First, we need to attach the manifold gauge. For superheat, we use a low side gauge (blue one) and attach it to the vapor line service port (this is the larger vapor line). This is for later measuring the temperature of the saturated temperature within the evaporator coil (temperature measurement is via gauge PT chart for the specific refrigerant; R-22, R-410A, R-404A). This is the 40°F measurement in the example above.
2. For subcooling, we use the high side gauge (red one) and attach it to the liquid line service port (this is the smaller liquid line). We are going to use this gauge to measure the liquid refrigerant after it exits the condenser unit; this is the 92°F temperature in the subcooling example above.
3. We wait until the pressures are equalized (both low side and high side gauge should show the same pressure, and thus the same temperature). Remember that the AC is off now; that means that the all lines and coils are in saturated state (mixture of refrigerant liquid and gas). This means that the pressure/temperature throughout the lines is the same.
4. Additionally, we attach the clamp-on digital thermometer to the suction side (low pressure side; bigger line).
5. Then we turn the AC on and wait for 10-15 minutes. You will see the low side gauge pressure (and thus temperature) decrease, and the high side gauge pressure (and this temperature) increase.
6. Now we have all 3 temperatures we need to calculate superheat and subcooling. For superheat, we subtract the temperature on the low side gauge (red gauge) from the temperature on the clamp-on thermometer (suction line temp). For subcooling, we subtract the temperature from the high side gauge (blue gauge) from the temperature of the clamp-on thermometer.

This is the most common way how to measure superheat and subcooling.

Now, what do we even measure superheat and subcool? Let’s look into the reasons why we measure these temperature differences:

Why Do We Measure Superheat And Subcooling?

Whenever you see an HVAC technician with a manifold gauge (very often), he or she is most likely measuring superheat and subcooling. The manifold gauge is like a doctor’s stethoscope to an HVAC technician. By measuring refrigerant pressures and temperatures, we are closely monitoring the beating heart of an air conditioner.

By measuring superheat and subcooling, we are checking these key air conditioner parameters:

• Performance. A 3-ton AC unit will produce 3 tons of cooling output only if the superheat and subcooling are correct. If not, we will get less than 3 tons of cooling output. If we know how much off the superheat and subcooling are, we can add or remove refrigerant to get up to 3 tons of cooling again.
• Efficiency. Similarly, a 16 SEER unit (well, from 2023 on, we have to say 15.3 SEER2 unit) will be less efficient if the superheat and subcooling are off. Basically, if you paid for a 16 SEER unit, you would have for the energy efficiency to drop to 14 SEER, 12 SEER, and so on. If and when that happens, we can use the superheat measurement to adjust the freon charge in order to ensure that 16 SEER efficiency.
• General air conditioner troubleshooting.
• Protect your AC unit from the oh-so-feared 0°F, 1°F, or 2°F superheat. Namely, zero or very low degree superheat means that liquid refrigerant might be exiting the evaporator coil and, God forbid, liquid refrigerant is entering the compressor. If that happens, the compressor will quickly break down (it can only handle vapor, no liquid whatsoever), and you will have to replace a few $1,000 worth air conditioner.

Here are a few scenarios of how we can decide if we need to add or remove refrigerant, based on the measured superheat and subcooling:

High/Normal/Low Superheat High/Normal/Low Subcooling Scenarios

In the ideal situation – when the AC is working as it should – we will measure normal superheat and normal subcooling temperatures. In a lot of cases, however, the superheat might be high or low, or subcooling might be high or low, and so on.

There are a couple of these superheat subcooling scenarios (based on the target superheat and the target subcooling chart for specific refrigerant). Let’s quickly go over them and list if we need to add or remove refrigerant for each of them:

• High superheat low subcooling. This might be a case of a low refrigerant charge. Action: Add refrigerant.
• Normal superheat low subcooling. This might be a case of a high refrigerant charge. Action: Remove refrigerant.
• Low superheat low subcooling. Orifice-related problems.
• High superheat high subcooling. Usually caused by a blockage in the line, coils, or in the orifice.
• High subcooling normal superheat. Might be caused by a flooded condenser.
• High subcooling low superheat.This might be a case of a high refrigerant charge. Action: Remove refrigerant.

This has been a 101 guide on understanding what superheat and subcooling means in refrigeration. We hope you now have a bit clearer picture of these two key HVAC diagnostic parameters, how they are measured, calculated, and how we can use the superheat and subcooling measurement to diagnose a problem your air conditioner might be having.