Negative Superheat? Time to Check Your Tools

I was fresh out of school working as an apprentice at my first real HVAC job and I was listening in on a shop conversation between a few techs.

They were talking about finding so many overcharged systems and one of the techs turns to me and says “I had a unit yesterday that was so overcharged it was running minus five degrees of superheat”. I don’t remember EXACTLY what I said in response to that but it started a miniature argument and set me on a crusade against misinformation that led me here all these years later.

When in doubt check your tools

Before we move on I want to mention something that Jeremy Smith pointed out to me. When working with a zeotropic refrigerant blend that has “glide” the change from liquid to vapor and vapor to liquid occurs over a range of temperatures and not at a single temperature. When calculating superheat we use the “Dew point” and when calculating subcool we use “Bubble Point” the saturation temperature is the range of temperatures between those two points meaning that it could be “interpreted” as negative superheat or subcool when it is actually just in the saturated range. In air conditioning the traditional R22 and R410a refrigerants do not have any significant glide but newer blends do so it is something to watch out for.

Here is a list of things that if you observe them, it will be worth checking your tools to make sure they are set up correctly, connected correctly and properly calibrated BEFORE you start making an exotic diagnosis.

Negative Superheat   

Superheat is the temperature gained in the refrigerant once it is completely boiled into a vapor. When it is still in the process of boiling it will be in a mixed state and will be at saturation temperature for that given pressure. Zero superheat is something you will see often when a system has a flooded coil and liquid still boiling in the suction line. While this generally isn’t a good thing it is something that you will observe from time to time and will usually result in you as the tech taking corrective action.

Negative superheat goes by another name SUBCOOLING and the only way a substance can be in the subcooled range is if it is 100% liquid and has given off additional heat below the saturated (mixed) state. It is impossible in a running air conditioning system for the suction line to be 100% liquid subcooled below saturation, therefore it is impossible to have negative superheat both by definition or in practice.

So what happens when you measure negative superheat you may ask? Good question.

It is one of a few possibilities

  1. You are looking at the the wrong refrigerant PT scale
  2. The refrigerant is mixed (somebody put something in on top of the original refrigerant)
  3. You are dealing with a blended refrigerant with “glide” like many of the new 4 series blends such as R407c
  4. Your suction gauge is reading too high
  5. Your line clamp thermometer is reading too low
  6. You do not have a good connection on the line / schrader core isn’t depressing / king valve isn’t open
  7. A combination of the items listed above

Negative Subcooling 

Just like we mentioned above, negative subcooling is superheat. There is no such thing as negative subcooling.

Is it possible for the liquid line to contain superheated vapor? It is THEORETICALLY possible but not practical. For example if someone short circuit nearly the entire condensing coil and connected to the liquid line you could see superheated vapor…. but let’s be realistic.

When techs measure a negative subcooling (superheat) at the liquid line it could be

  1. You are looking at the the wrong refrigerant PT scale
  2. The refrigerant is mixed (somebody put something in on top of the original refrigerant)
  3. You are dealing with a blended refrigerant with “glide” like many of the new 4 series blends such as R407c
  4. Your high side gauge is reading too low
  5. Your line clamp thermometer is reading too high
  6. You do not have a good connection on the line / schrader core isn’t depressing / king valve isn’t open
  7. A combination of the items listed above

Liquid Line Cooler than the Outdoor Air 

There are two cases where the liquid line can be cooler than the outdoor air when measured at the condenser outlet

  1. A Wet Coil
  2. A restriction inside the condenser cabinet in the liquid line, usually in a factory installed filter drier

Because the liquid line temperature will often be VERY close to the outdoor temperature on new, high efficiency system this is often a point where you will measure a liquid line as colder than the outdoor air when that may not really be the case.

Often you may SEE a liquid line colder than outdoor ambient and it may be simply be

  1. Miscalibration of the line clamp or the ambient air thermometer
  2. Measurement of the ambient air in sunlight where the probe can be effected by sunlight
  3. The coil is still damp after cleaning or a rain (evaporative cooling)

It is always a good practice to have a back up set of thermometers and gauges so you can double check the calibration of your tools against one another. Whenever possible, test them under the conditions that you are using them.

If you have two clamps, place them on the same line right next to one another, when testing two air probes, stick them both in the same return air stream side by side. For temperature measurement you may also test in an ice bath just make sure that the water is pure and that the water and ice are fully mixed and circulating when you test for 32°F(0°C) degrees.

Also keep in mind that every measurement device has “uncertainty” in the measurement of +/- a certain amount depending on the tool. Don’t expect your tools to provide a greater accuracy than what is published in their specifications.

— Bryan

P.S. – If you don’t have a “backup” set of temperature and pressure measurement instruments I will suggest the Testo refrigeration smart probes kit for line temperatures and pressures and the Testo 605i for air temperature and humidity. They are a FANTASTIC price and you will get an additional 8% off by using the offer code getschooled at Trutechtools.com

2 comments

  1. DAN GARSKA says:

    Two of everything is good! Back up plans….

  2. Karl Fischer says:

    Great post Bryan.

    I have overheard other techs seriously discussing “negative superheat” before, I agree with you about battling mis-information in our trade.
    I’d like to share one rule of thumb that I learned long ago.

    ~ If you have liquid at vapour saturation there can be NO SUPERHEAT.
    ~ If you gas / flash gas at liquid saturation there can be NO SUBCOOLING.

    I would, however, like to contribute to our trade by coining some new terminology that will hopefully help to clear up the matter at hand.

    Instead of using the terms “negative superheat” or “negative subcooling”, I propose that we use the terms “subheat” for negative superheat and “supercooling” for negative subcooling, that should help to clarify the subject for everyone…

    Have a good day.

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