Measuring superheat is not all that difficult. All you need are a manifold gauge, a clamp-on thermometer, and a bit of math. We will show you exactly how to measure the superheat for any air conditioner (central air conditioner, split system ACs unit, etc.).
We are going to include a step-by-step way of how to measure superheat, but let’s first start with the definition:
Superheat is the increase in refrigerant vapor temperature:
- From the saturated state (liquid and vapor mixture at constant temperature) at the beginning of the evaporator coil (indoor unit). For example, this temperature can be 40°F.
- To where the vapor exits the evaporator coils. For example, let’s say this temperature can be 48°F.
We are basically measuring how many degrees over the saturation temperature the vapor in the suction line (vapor line) goes; you can read a detailed explanation of what superheat and subcooling are here. In this case, we have an 8°F superheat since 48°F – 40°F equals 8°F. This is the ‘a bit of math’.
We will use the manifold gauge (low side suction line; blue gauge) to measure the saturated state temperature (40°F in the example), and we will use a clamp-on thermometer to measure the superheated vapor temperature (48°F in the example).
Needless to say, we:
- Don’t want a very low superheat (0°F, 1°, or 2°F) since this indicates liquid refrigerant might be entering the compressor. The compressor can only handle vapor, not liquid.
- Don’t want a very high superheat (15°F or more) since we are sending very hot vapor into the compressor; this can overheat the compressor.
The ideal superheat should be anywhere between 7°F to about 15°F.
When measuring the superheat, our goal is to measure only 2 temperatures. However, do note that the way we measure them is quite important as well. From purging air from the HVAC gauge, turning the AC off and on (and waiting 10-15 minutes) to wearing gloves for protection (seriously, whenever dealing with a refrigerant, do wear gloves).
Alright, let’s go through the whole procedure exactly how an HVAC technician goes through it on a step-by-step basis:
Step-By-Step Of How To Measure Superheat
Despite measuring the temperature increase in the indoor evaporator coil, we position ourselves in front of the outdoor unit. We are going to measure everything we need to calculate the superheat there. Let’s start:
- Shut off the air conditioner. We need an equalized system before we start measuring superheat. The AC should be turned off for at least 30 minutes before we start.
- Take your manifold gauge. This gauge will measure the pressure and convert (automatically on the meter) the pressure into temperature. That’s why it’s best to use an HVAC gauge that has a pressure-temperature meter for your specific refrigerant (R-22, R-410A, R-134A, R-404A, etc.).
- Prior to using the gauge, open the valves to let the air out, then screw them back tightly. Potential leftover air is sucked into the compressor if we don’t adequately let it out, and that’s not recommended.
- Attach the low side gauge to the suction line near the compressor. For superheat measurement, we are only using a low side gauge (blue gauge). The suction line is the bigger vapor line; locate the suction line service port, and screw the blue line from the gauge on there. Here we will measure the lower saturated temperature (40°F in the example above).
- Attach the clamp-on thermometer to the suction line. It’s best to clamp it on near the service port; near the compressor. Here we will measure that 48°F temperature from the example above. Note: You could use an IR thermometer but a clamp-on digital thermometer gives us a more stable and accurate thermometer reading.
- Turn on the air conditioner and let it run for about 15 minutes. You should see the pressure and temperature increase on your blue gauge. Additionally, you should see the temperature measured by the clamp-on thermometer stabilize. Once it stabilizes (moving up and down 0.2°F of a degree or less), you can start reading the temperatures.
- Read the temperature measured by the blue low end gauge and note it down. Note: For this, you will have to know how to read a gauge. It measures the pressure exerted by the saturated refrigerant; the circles in the gauge convert that pressure directly into the temperature for 3 to 4 refrigerants, depending on which HVAC gauge you have.
- Read the other temperature measured by the clamp-on thermostat and note it down.
- To calculate the superheat from the measured temperature, subtract the lower temperature measured by the compound manifold gauge from the higher temperature measured by the thermometer.
- The resulting difference in the superheat you were hoping to measure.
Let’s look at an example of how to measure superheat to illustrate how this goes down in practice:
Example With A R-410A Refrigerant
We have a 5-ton mini split AC unit and want to measure the superheat. We know that the refrigerant used is R-410A, and we have a manifold gauge for R-410A.
We shut off the unit, connected the blue gauge and thermometer, and are now running it for 15 minutes.
On the manifold gauge, we can read that the pressure in the suction line is 125 psi. On the gauge, we can see that this 125 psi pressure for a R-410A refrigerant translates to about 43°F. Great, we have the 1st temperature read that we need.
To get the 2nd temperature read, check the temperature on the clamp-on thermometer. Let’s say this temperature is 52°F.
Now we can easily calculate superheat using this simple formula:
Superheat = Tclamp-on thermometer – Tgauge
We input our measured 43°F and 34°F and calculate the superheat like this:
Superheat = 52°F – 43°F = 9°F
Now we know that superheat is 9°F.
Simple as that. Hopefully, you can use these instructions to measure superheat by knowing where to measure superheat, which gauge to use, and how to use the simple superheat formula to calculate superheat for your AC unit. Thank you.