Superheat measurement & use questions & answers.

What is superheat and how is that measurement used in charging or adjusting air conditioning or refrigeration or heat pump systems?

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.

Superheat FAQs

How Do I Use 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. 7/22/2014 Tom

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

see REFRIGERANT 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

Continue reading at DEFINITION of SUPERHEAT or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• BLEVE EXPLOSIONS - dramatic example of the effects of superheat when a water heater explodes
• DEFINITION of HEATING, COOLING & INSULATION TERMS - home
• DEFINITION of HEAT FLOW DIRECTION
• DEFINTION of JOULE
• DEFINITION of LATENT HEAT
• DEFINITION of SEER RATINGS
• DEFINITION of SENSIBLE HEAT
• DEFINITION of SUBCOOLING
• DEFINITION of SPECIFIC HEAT
• DEFINITION of SUPERHEAT
• REFRIGERANT PRESSURE READINGS & CHARTS - home

Suggested citation for this web page

SUPERHEAT FAQs at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to DEFINITIONS

Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia

Ask a Question or Search InspectApedia

Questions & answers or comments about the definitions of heating & cooling terms.

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Note: appearance of your Comment below may be delayed: if your comment contains an image, photograph, web link, or text that looks to the software as if it might be a web link, your posting will appear after it has been approved by a moderator. Apologies for the delay.

Only one image can be added per comment but you can post as many comments, and therefore images, as you like.
You will not receive a notification when a response to your question has been posted.
Please bookmark this page to make it easy for you to check back for our response.

---

 

IF above you see "Comment Form is loading comments..." then COMMENT BOX - countable.ca / bawkbox.com IS NOT WORKING.

In any case you are welcome to send an email directly to us at InspectApedia.com at editor@inspectApedia.com

We'll reply to you directly. Please help us help you by noting, in your email, the URL of the InspectApedia page where you wanted to comment.

Citations & References

In addition to any citations in the article above, a full list is available on request.

---

ADVERTISEMENT

---