Superheat and Subcooling: How to Measure Them

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Last updated: 
February 7, 2018

This brief guide explains how to measure superheat and subcooling. As a review, superheat is heat added to the refrigerant above its boiling point/saturation. Subcooling is removing heat from refrigerant below saturation. Measuring the superheating and subcooling of the refrigerant tells you if it is under the proper amount of pressure and can lead to a diagnosis of the refrigerant level and mechanical issues.

Step-by-step instructions on how to calculate superheating and subcooling values plus possible diagnosis using the measurements are included. Furthermore, the tools, tips, and techniques below assist you in streamlining the process for faster maintenance and diagnosis of the problem.

Illustration with 4 thermometers in different colors

Why You Might Need to Know How to Measure Superheat and Subcooling

Knowing how to calculate superheat and subcooling (aka sub-cooling and subcooling) values is essential to ensuring proper refrigerant charge in air conditioning systems and refrigeration units. The measurements also assist in troubleshooting problems with the equipment. This is important because knowing superheat, subcooling and temperature difference (Delta) across a coil:

  • Allows an HVACR technician to ensure refrigerant levels are properly maintained for maximum efficiency and performance
  • Safeguards against improper diagnosis and repair such as diagnosing and replacing the compressor when the problem is something else. That’s a costly but very common mistake.

Tools and Specs to Keep in Mind

The primary tool for the job is a superheat/subcool meter or gauge. They’re produced by many brands in a price range from about $70 to $250. We suggest looking for one with presets for the refrigerants you work with most commonly – R22, R410a (Puron), R134a and R404a, for example. That eliminates the need for inputting the specific refrigerant’s specs.

A handy assistant when converting low-side and high-side pressure to temperatures is a phone app like Emerson’s Check & Charge. The free app is one of many great HVAC troubleshooting apps every tech should have. Its features include superheat (non-TXV) and subcooling (TXV) calculators for R22 and R410A. Check & Charge is available here for iOS and here for Android. A companion to it is the Danfoss Refrigerant Slider with pressure-to-temperature conversions for 80 refrigerants. Those are specs you must know to successfully evaluate the performance of the equipment you’re evaluating.

How to Measure Superheat and Subcooling

Here’s how to determine these values in a few easy steps for each.

Calculating superheat:

  1. Measure the low-side pressure with your gauge.
  2. Convert pressure to temperature with an app, slide or chart.
  3. Next, take the temperature of the suction line where it leaves the condensing unit, though stay 4-6 inches from the compressor.
  4. Use your measurements to determine the amount of superheat. For example, if suction line temperature is 65 degrees and your conversion of the suction pressure to temperatures yields 50 degrees, then the difference is 15 degrees of superheat.

Calculating subcooling:

  1. Measure the high-side pressure with your gauge.
  2. Convert pressure to liquid temperature with an app, slide or chart.
  3. Measure the temperature of the liquid line where it meets the evaporator and before the metering device.
  4. Finally, determine the difference in the temperatures determined in Steps 1 and 2 by subtracting the liquid temperature from the saturation temperature of the refrigerant you’re working with. A difference of 12-15 degrees is standard. For example, if your conversion of pressure to temperature yields 115 degrees for saturation and the liquid line temperature is 100 degrees, that’s 15 degrees of subcooling.

Note: Superheat should be 12-5 degrees in ambient air temperature below 85 and 8-12 degrees in warmer air. Subcooling should be 5-18 degrees, at the high end of that spectrum when the equipment has a TXV. Always go by the manufacturer’s specifications.

Using superheat and subcooling data in troubleshooting:

  • Low superheat: Too much refrigerant in the evaporator. It’s likely overcharged.
  • High superheat: Too little refrigerant in the evaporator. This might indicate that the system is low on refrigerant, but it’s just as commonly caused by insufficient heat getting to the evaporator (dirty filter or blower, undersized or blocked ductwork, plugged evaporator coil) or a dirty/defective metering device.

Caution Notes

Don’t try this at home! Diagnosing and repairing air conditioning and refrigeration is one of those jobs experienced DIY homeowners are wise enough to steer clear of. Moreover, working with refrigerant is not a DIY job unless you’re a licensed HVACR technician.

Only licensed techs can legally handle refrigerants. Additionally, overcharging and undercharging a system lead to serious issues with air conditioning and refrigeration equipment. There’s no reason to turn what might be a small problem into a major mechanical failure.

And here’s an important caution: Refrigerant might not be the issue. If you misdiagnose the cause of the problem, your solution won’t work. Consequently, the right approach is to leave the work to an experienced HVAC technician.

Hiring a Pro for your AC or Refrigerant Job

Finally, if this post interested you, it’s possible you ARE an HVACR pro. If so, we hope it was a good refresher in superheat and subcooling basics. We’d enjoy getting your feedback in the Reply box below. If you’re a homeowner diagnosing an AC or refrigerant issue, we’d recommend discussing the issue with a professional.As such, look for one certified by N.A.T.E., the North American Technician Excellence. The organization provides the most broadly respected training, testing, and certification for professionals in the HVACR industry. If you’re a hands-on homeowner that likes to know what the issue is, so you can fix it yourself or knowledgeably discuss it with a pro, you probably know other homeowners who feel the same. In the end, perhaps they’d benefit from this information on superheating and subcooling, so feel free to share!

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