Calculating superheat in HVAC is super easy. We only need 2 temperature measurements and a minimal amount of math. We are going to show you exactly how to calculate superheat.
To help you out, we have prepared these 3 valuable resources:
- Superheat Formula. This is a very simple equation anybody can use to calculate superheat directly.
- Superheat Calculator. When you have 2 temperatures (measured by manifold gauge and clamp-on thermometer) you can just insert them in the calculator, and you will get the superheat immediately.
- 3 Superheat Calculation Examples For R-22, R-410A, And R-134A. We are going to showcase how to calculate superheat for R-22, R-410A, and R-134A since these are the 3 most commonly used refrigerants today.
Alright, let’s first look at what superheat actually is (what we are calculating):
Namely, superheat is the temperature increase of vapor refrigerant above its saturation point. It is defined as the temperature difference between:
- Temperature of refrigerant vapor when it exits in the evaporator coil (when we have 100% vapor, the temperature of vapor will increase).
- Temperature of refrigerant in the saturated state (mixture of liquid and vapor that has constant temperature).
We are usually looking for 7°F to 15°F superheat. Too low superheat (below 2°F) = Risk of flooding the compressor, too high superheat (above 15°F) = Risk of overheating the compressor. More about that in our general superheat and subcooling article here.
Alright, to calculate superheat, we only need to measure 2 temperatures, and then use the superheat formula to calculate the superheat. Let’s start with the formula and then proceed with the superheat calculator and R-22, R-410A, and R-134A superheat examples:
Superheat Formula
This is the superheat formula by definition:
Superheat = Tvapor after evaporator coil exit – Tsaturated refrigerant
In practice, we have to measure both of these temperatures. As you can read in our article about how to measure superheat here, you can see that we use a manifold gauge to measure the temperature of the saturated refrigerant, and we use a clamp-on thermometer to measure the temperature of the vapor after the evaporator coil exit.
That’s why in HVAC we can use this more practical superheat formula:
Superheat = TClamp-On Thermometer – TGauge
As we can see, we have to subtract the temperature measured by the gauge (saturated refrigerant temp.) from the temperature measured by our thermometer (superheated vapor temp.).
Quick Example: Let’s say that we have a 3-ton 16 SEER AC are using R-410A refrigerant and want to calculate R-410A superheat. With low side blue gauge on the suction line service port, we measured 130 psi pressure that correlates (according to R-410A PT chart) to about 45°F. The clamp-on thermometer shows that the saturated refrigerant temperature is 56°F. Here’s how to calculate superheat for this system:
Superheat (R-410A) = 56°F – 45°F = 11°F
We see that we have calculated 11°F superheat for this system.
To make this calculation even easier, you can freely use this superheat calculator (we’re going to use it for R-22, R-410A, and R-134A superheat calculations in our examples further on):
Superheat Calculator
Here we are using the R-410A example from above. Move the 1st slider to ’45’ since this is the superheated vapor temperature as measured by HVAC gauge, and move the 2nd slider to ’34’ since this is the saturated temperature as measured by the clamp-on thermometer.
Here is the result: We have 11°F superheat. This matches our manual calculation from above.
Alright, let’s look at R-22, R-410A, and R-134A superheat calculation examples:
R-22 Superheat Calculation (#1 Example)
Let’s say we have a 5-ton 14 SEER AC with a R-22 refrigerant and we want to calculate R-22 superheat in this system.
With the manifold gauge, we measured the superheated vapor pressure of 91 psi. According to the R-22 PT chart, this is equal to about 54°F.
With the thermometer, we measured the saturated R-22 temperature of 45°F.
Here is how we can calculate the R-22 superheat in this system:
Superheat (R-22) = 54°F – 45°F = 9°F
The superheat calculator also gives us 9°F superheat for R-22 refrigerant.
R-410A Superheat Calculation (#2 Example)
Alright, we have already demonstrated one R-410A superheat calculation. However, in different systems (Lennox, Carrier, etc.), and at different refrigerant charges, we usually get different vapor and saturated temperatures, and hence different superheat.
Suppose we have a 2.5-ton 18 SEER unit that runs on R-410A superheat. We use a gauge with an R-410A temperature meter, which shows us the temperature of 51°F. The thermometer shows us that the superheated R-410A vapor is at 54°F temperature. Now we can calculate R-410A superheat:
Superheat (R-410A) = 54°F – 51°F = 3°F
In this case, we have a dangerously low 3°F superheat. This presents a risk that the suction line will deliver liquid R-410A to a vapor-only compressor and flood it.
In most cases with very low superheat, the problem is overcharged line (we have an excess of R-410A refrigerant). The solution here is to remove refrigerant from the line.
R-134A Superheat Calculation (#3 Example)
R-134A is another commonly used refrigerant we have to calculate superheat for.
Let’s say we have a 4-ton 16 SEER AC unit, and we measure 60 psi pressure (equal to 62°F temperature, according to this R-134A PT chart) with the gauge, and the thermometer shows us 80°F. Here is the R-134A superheat calculation for this system:
Superheat (R-134A) = 80°F – 62°F = 18°F
In this case, we have a too high superheat. Namely, very hot R-134A vapor (with 18°F superheat) is being channeled into the compressor. This can cause the compressor to overheat and break down.
In many cases when we have a high superheat, the problem is that the system is undercharged, and we have to charge it in order to ensure adequate superheat and thus protect the compressor (as well as increase the performance and efficiency of the AC unit).
Hopefully, you can now use the superheat formula, calculator, and draw knowledge from these 3 calculation examples to easily calculate the superheat yourself. If you need any help, you can use the comment section below, give us some numbers, and we can help you out. Thank you.