Superheat measurement & use in HVACR systems:

Here is a tutorial on superheat and how that measurement is used.

We discuss: What is the definition of superheat? How and where is superheat measured in HVAC systems? What are typical superheat numbers?

This article series defines terms used in air conditioning, heating systems, refrigeration systems and other physics applications.

Page top photo: taking advantage of a trip to Italy we use the Venice lagoon temperatures as an example while we explain saturation temperature and subcooling temperature and similar heating, cooling, temperature and pressure terms in this article series.

Definition of Superheat or the latent heat of vaporization, condensation, solidification

Definition of Superheat

Superheat is the amount (degrees of temperature) by which a liquid or gas under pressure has been heated above its boiling point.

Superheating of a liquid occurs when the liquid heated in a closed system or container (such as a water heater tank) to a temperature above its boiling point.

Se can also superheat a gas such as a refrigerant gas or steam (water vapor).

Superheating of a gas (such as refrigerant or steam) occurs when the gas has been heated to a temperature above its temperature of saturation.

Definition of saturation temperature

The saturation temperature is simply the boiling point of the liquid form of the substance - the temperature at which the liquid will boil into a vapor. HVACR experts and physicists use the term saturation temperature to describe a condition at which a liquid (or gas) has been saturated with thermal energy - that is, it has absorbed as much energy as it can without changing state (boiling to a vapor or gas).

The "trick" of superheating, or getting a substance to a temperature above its boiling point is the use of pressure - that is, heating the substance inside of a closed container.

At higher pressures we can heat a substance to increased temperatures.

You'll see this expressed in charts showing the change in refrigerant saturation temperature as a function of change in refrigerant pressure.

As long as you keep the scale consistent across various measurements, tables, standards, the temperature scale can be in Kelvin, Centigrade, or Fahrenheit - it doesn't matter.

[Click to enlarge any image]

Definition of superheated

Any liquid (such as liquid refrigerant or ordinary water) that has been heated above its boiling point without having converted to a gas or vapor has been super-heated.

Relationship between Degrees of Superheat and Degrees of Temperature Above Boiling

1 degree of superheat = 1 degree of temperature above the substance's boiling point (in °F or in °C).

Where is Superheat Measured?

A superheat measurement as used in air conditioning and heat pump systems, is the number of temperature degrees to which a refrigerant (in liquid OR in gas form) is above its boiling point.

Danfoss (2005) TEV fitters notes provides details about where and how to measure superheat on HVACR equipment, from which we quote this excerpt:

Superheat is measured at the point where the bulb is located on the suction line and is the difference between the temperature at the bulb and the evaporating pressure/evaporating temperature at the same point. Superheat is measured in Kelvin (K) or °C and is used as a signal to regulate liquid injection through the expansion valve. - Danfoss (2005) 

Also see THERMOSTATIC EXPANSION VALVES

Steam superheat example

The boiling point of water is 212 °F (100 °C) - the point at which it will form steam. At that point both the water and the steam are both at a temperature of 212 °F (100 °C).

If we keep boiling the water (at sea level) in an open pot, we will not raise the temperature over the boiling point. Water has changed state from liquid to gas (water to steam) at 212 °F (100 °C) .

If, however, we boil water in a closed container (perhaps a steam heating system) we can continue to add heat to the container, raising the temperature of its contents (both water and steam) to above the boiling point.

Every degree of temperature that we raise our steam above 212 °F (100 °C) is a degree of superheat.

So if we heat our steam up to 242 °F (117 °C) we have superheated our steam by 30 °F.

Refrigerant Superheat Example

If our liquid refrigerant X boils at 40 °F (4.4 °C), as X passes through the evaporator coil across which warm building air is moving, the liquid refrigerant boils, that is converts to a refrigerant gas

In a properly adjusted system, by the time the last bit of liquid refrigerant gets to about the end of the cooling coil, it has all boiled into a refrigerant gas.

If we measure the temperature of the refrigerant at a foot past the end of the cooling coil (aka the evaporating coil) and we find that the refrigerant is at 45 °F (7.2 °C) then we have added 5 degrees of superheat to the refrigerant (45 - 40).

In a refrigeration system the refrigerant is in a closed network of tubing and perhaps a receiver and an area in the compressor motor. In a closed refrigeration system, pressure increases raise the boiling point while pressure decreases lower the boiling point of the refrigerant. For this reason a service technician cannot simply measure temperature, she also has to know the pressures in the system to make sense of the numbers obtained.

That's also why if you look at any table of pressures for a refrigerant you will see that the table specifies both the temperature and the pressure of the refrigerant together.

At REFRIGERANT PRESSURE READINGS & CHARTS you'll see some typical refrigerant pressures at specific temperatures.

You'll see that R410 at 80 °F produces a pressure of 236 psi while at 101.1 °F the system pressure would be expected to be 322 psi.

This is why I disappoint readers who often write to ask "what's the normal pressure of R410?".

It's sort of like saying what's the size of a cardboard box. The answer is, as Mark Cramer says, "it depends."

Rules of Thumb for A/C Superheat Target & Charging

Indoor Evaporator Coil Superheat Target

A common rule of thumb for superheat in an air conditioning system: set the thermostatic expansion valve to 8 - 12°F of superheat. - ( ACHR News 2001)

[Click to enlarge any image] Chart adapted from Trane & ACHR News 2000 cited below.

Outdoor Compressor/Condenser Superheat Target

When taking outdoor temperature account (Dry Bulb Temp. or DBT) by making use of Trane's Chart of Superheat vs. Outdoor Temperature,

If the superheat at the compressor (measured at the suction line close to its point of entry into the compressor) is more than 5°F above the chart reading you need to add refrigerant gas by low-side charging.

Watch out: Abnormallhy-low temperature can indicate low refrigerant and probably a leak that should be found and fixed.

If the superheat at the compressor is more than 5°F below the chart reading, the system is overcharged and you need to remove refrigerant by recovering it in an approved manner. - ( ACHR News 2000)

Watch out: be sure the system is allowed to run long enough to reach stable temperatures before and after adjusting a refrigerant charge and thus when making follow-up superheat measurements.

How to Measure Superheat at an Air Conditioner or Heat Pump

Superheated high pressure refrigerant gas exits the compressor where it is condensed to a liquid in the compressor/condenser unit outdoor condensing coil.

Superheat is determined by taking the low side pressure gauge reading, converting that pressure to temperature using a PT [pressure - temperature] chart [for the specific refrigerant involved], and then subtracting that temperature from the actual temperature measured (using an accurate thermometer or thermocouple) at the same point the pressure was taken.

Superheat gives an indication if the amount of refrigerant flowing into the evaporator is appropriate for the load. If the superheat is too high, then not enough refrigerant is being fed resulting in poor refrigeration and excess energy use.

If the superheat is too low, then too much refrigerant is being fed possibly resulting in liquid getting back to the compressor and causing compressor damage. Emerson Climate Technologies (2005)

Reader Question: 7/22/2014 Tom said: using subcooling or superheat to charge an AC system

Very interesting and informative article. I've recently had "added" to my responsibilities a lot of A/C kit. Although an engineer I haven't any qualifications in A/C but would like to be aware of what's happening (or isn't) when these guys visit the site. So I bought books on the subject and read up on it.

Question: I've just been reading a book (printed 2009) where the author lists and describes different methods of charging an A/C system.

One method he describes uses the compressor motor current to indicate when the unit is full of gas and none of his other methods mention subcooling anywhere. Comments please.

Reply: What are typical A/C superheat numbers?

Tom,

Indeed we've read & discussed measuring current draw which is an indirect measure of motor loading which in turn can describe what the motor is "seeing" in discussing evaluation of water wells. So in concept I agree that one could have an idea of the refrigerant charge by noting whether or not the compressor were running under load - as would happen when there is enough refrigerant present to lead to high pressures on the high side.

But frankly I'm dubious that one could have an accurate idea of the actual charge level with this approach.

It may be possible that with a specific compressor motor, TEV, a specific refrigerant gas, and other variables held constant and with some experience with having put in a measured charge and then measured motor current draw one could infer something about that specific set-up.

But as motor properties vary and refrigerant gas properties vary, and as normal operating pressures vary among HVAC systems by refrigerant gas itself, I'm skeptical about the claim that merely measuring current could be at all accurate as an indicator of the precise charge.

You could, in sum, determine if there were a significant undercharge (low current) or a stuck TEV (high current) ... maybe.

And many texts including my own pay too little attention to superheating or subcooling though those are important measurements, perhaps because deeper understanding of refrigeration principles is required. As you've probably read, subcooling and superheating are important conditions that can be measured in a properly-functioning HVACR system.

Watch out: improper adjustment of a TEV or thermostatic expansion valve or any other refrigerant device can result in sending liquid refrigerant into the compressor, causing catastrophic damage.

Liquid slugging at the compressor: The damage referred to is liquid slugging. An air conditioner or heat pump compressor's valves are designed to pass gas, not liquid. Slamming a piston into liquid destroys the compressor.

We discuss refrigerant flooding and six common causes of liquid refrigerant slugging the compressor in more detail at REFRIGERANT FLOODBACK, LIQUID SLUGGING

For discussion of the refrigerant pressure / temperature chart shown above seeREFRIGERANT PRESSURE READINGS

Question: can I measure compressor current draw to determine when refrigerant charge is correct?

Tim said:

I've just been reading a book (printed 2009) where the author lists and describes different methods of charging an A/C system. One method he dscribes uses the compressor motor current to indicate when the unit is full of gas and none of his other methods mention subcooling anywhere. Comments please.

Reply:

Tim,

Indeed we've read & discussed measuring current draw which is an indirect measure of motor loading which in turn can describe what the motor is "seeing" in discussing evaluation of water wells. So in concept I agree that one could have an idea of the refrigerant charge by noting whether or not the compressor were running under load - as would happen when there is enough refrigerant present to lead to high pressures on the high side.

But frankly I'm dubious that one could have an accurate idea of the actual charge level with this approach. It may be possible that with a specific compressor motor, TEV, a specific refrigerant gas, and other variables held constant and with some experience with having put in a measured charge and then measured motor current draw one could infer something about that specific set-up.

But as motor properties vary and refrigerant gas properties vary, and as normal operating pressures vary among HVAC systems by refrigerant gas itself, I'm skeptical about the claim that mere measuring current could be at all accurate as an indicator of the precise charge.

You could, in sum, determine if there were a significant undercharge (low current) or a stuck TEV (high current) ... maybe.

And many texts including my own pay little attention to superheating or subcooling though those are valid measurements, perhaps because deeper understanding of refrigeration principles is required. As you've probably read, subcooling and superheating are important conditions that can be measured in a properly-functioning HVACR system.

Put simply for other readers, "superheat" is the number of degrees to which a refrigerant gas is above its boiling point.

Superheated high pressure refrigerant gas exits the compressor where it is condensed to a liquid in the compressor/condenser unit outdoor condensing coil.

Thanks for the question, Tim, it was helpful. To assist other readers I've included our notes along with some expansion & citations in DEFINITION of SUPERHEAT

and at THERMOSTATIC EXPANSION VALVES

and at SUBCOOLING DEFINITION

Superheat Calculation Examples, Charts, Rules of Thumb, Tables

While I'm uncertain about the use of amps or current measurements to estimate superheat settings in an HVACR system discussed above with Tim, there are some useful examples and some rules of thumb that help understand the relationship between superheat and other air conditioner, heat pump, or refrigeration data.

[Click to enlarge any image]

• Sporlan PRESSURE-TEMPERATURE CHART [PDF] (2009) normalized for an altitude above sea level of 5000 ft. , for R12, R22, R134A, R401A, R402A, R404A, R407C, R408A, R410A, R422D, Sporlan Division, Parker Hannifin Corporation, 206 Lange Dr., Washington MO 63090 USA, Tel: 636-239-1111, Website: www.sporlan.com retrieved 2018/06/24, original source: https://www.parker.com/literature/Sporlan/Sporlan%20pdf%20files/Sporlan%20pdf%20Miscellanous/001B_012009.pdf
  
  Example: What is the superheat value for R-22, suction pressure measured at 71 psig, suction line temperature read on the suction line at the sensor bulb at 52 degF.
  
  In the Sporlan chart above, in the PSIG column see the two entries for 70 & 72 psig in the left column. [As there is no entry for 71]
  
  then read across to the second colunn (R22) to see the saturation or superheat temperature.
  
  Since at 70 psig the saturation temperature for R22 is 39°F and at 72 psig the saturation temperature is 41°F, we interpolate between tose two readings to use a saturation temperature for R22 of 40°F at 71 psig suction pressure.
  
  To obtain the number of degrees of superheat, subtract the table temperature (saturation temperature - 40F) from the measured temperature (52F).
  
  52°F - 40°F = 12°F = the amount of superheat in the system. That's within the recommended superheat range of 8°F to 12°F.

Watch out: there are at least three types of refrigerant pressure/temperature chart, each of which may be used slightly differently. The Sporlan chart offers tables that use at least two of these methods. See Lavelle 2006 cited below.

On 2015-05-30 by Mod: where to find information on measuring superheat

Jim: see complete details at DEFINITION of SUPERHEAT

On 2015-05-30 by Jim

I have a climatrol package unit model 750-1-201-07. I can only find one service port that is located near the top of the compressor. Is this port high side or low side? If it is high side how do I check superheat for charging purposes? The metering device is fixed.

Superheat References

These notes and speculations on superheat are also indebted to the following:

• Alan Carson, Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. The firm provides professional HOME INSPECTION SERVICES,        HOME INSPECTION TRAINING,        Horizon INSPECTION SOFTWARE    &    PUBLICATIONS.
  
  Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides:
• ACHR News, "Superheat Charging Curves for Technicians", [Web article], 3 August 2000, ACHR News, 2401 W. Big Beaver Rd., Suite 700, Troy, MI 48084 USA, Website: www.achrnews,
  Phone: 248-362-3700retrieved 2018/06/24, original source: https://www.achrnews.com/articles/83025-superheat-charging-curves-for-technicians
  
  Jill L. DeVries, Reprint Manager 2401 W. Big Beaver Rd., Suite 700 Troy, MI 48084 Phone: 248-244-1726 Fax: 248-244-3934 Reprints to order reprints online now.
• Christopherson, Norm, "Superheat and Subcooling Made Simple", ACHR News, 11 January 2004, ACHR News, 2401 W. Big Beaver Rd., Suite 700, Troy, MI 48084 USA, Tel: 248-362-3700, original source: http://www.achrnews.com/articles/94178-superheat-and-subcooling-made-simple
• Christopherson, Norm, "Rules of Thumb for Truobleshooting", [Web article] 4 February 2001, Op. Cit., retrieved 2018/06/23, original source: https://www.achrnews.com/articles/86221-rules-of-thumb-for-troubleshooting
  Christopherson San Jose City College, San Jose, CA.
• Tomczyk, "Understanding Superheat", [Web article] 3 June 2004, Op. Cit. retrieved 2018/06/25, original source: https://www.achrnews.com/articles/96890-understanding-superheat
  Tomczyk, Ferris State University, Big Rapids, Mich. e-mail tomczykj@tucker-usa.com.
• Ferguson HVAC, SUPERHEAT CHARGING METHOD [PDF] presentation, Ferguson Heating & Cooling, Website: https://heatingcooling.fergusonhvac.com/ retrieved 2018/06/24, original source: https://heatingcooling.fergusonhvac.com/wp-content/uploads/2017/03/11-Superheat-Charging-Method.pdf
  Website Excerpt:
  
  Ferguson plc, headquartered in Zug, Switzerland, is the parent company of almost two dozen subsidiary enterprises, including Ferguson Enterprises. Formerly known as Wolseley plc prior to August 1, 2017, Ferguson plc is the largest trade distributor of plumbing and heating products in the world and a leading supplier of building materials ...
• Lavelle, Jim, Refrigerant Blends: P-T Charts and System Troubleshooting [PDF], RSES Journal (March 2006), 1911 Rohlwing Road, Suite A, Rolling Meadows, IL 60008-1397 USA, Tel: 800-297-5660, Website: retrieved 2018/06/24, original source: http://www.refrigerants.com/pdf/RSES%20Article%20Part%203%20Mar06.pdf
• Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
• Trane Corporation, CHARGING CHARTS and SUPERHEAT, p. 23 in Split System Air Conditioners Odyssey™ R-22 Dry Charge Heat Pump Condenser — 7.5, 10, 15 and 20 Tons INSTALLATION & OPERATION MANUAL [PDF], Trane Corporation, Website: www.trane.com, retrieved 2018/06/24, original source: https://www.trane.com/ content/dam/Trane/Commercial/ global/products-systems/equipment/unitary/ split-systems/odyssey-6-to-25-tons/SSP-SVX15F-EN_09132017.pdf
• Trane Corporation, SPLIT SYSTEM XR15 HEAT PUMP SERVICE FACTS[PDF] ], Trane Corporation, retrieved 2018/06/24, original source: https://www.bayareaservice.com/ wp-content/uploads/2011/12/XR15-Service-Facts.pdf
• Trane Corporation, HEAT PUMPS INSTALLERS GUIDE 4TR4 [PDF] ], Trane Corporation, retrieved 2018/06/24, original source: https://www.trane.com/content/ dam/Trane/ Commercial/ global/products-systems/equipment/ unitary/split-systems/ Small%20Splits/Heat%20Pumps/ 18-BC95D1-4-EN_08012016.pdf

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Continue reading at DEFINITION of HEATING, COOLING & INSULATION TERMS - home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

BLEVE EXPLOSIONS for a dramatic example of the effects of superheat when a water heater explodes.

Or see REFRIGERANT PRESSURE READINGS & CHARTS - home

THERMOSTATIC EXPANSION VALVES

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• DEFINITION of SUPERHEAT

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