Class - What Superheat Signifies

Bryan teaches a class about what superheat signifies. Superheat is the difference between a vapor’s actual temperature and its suction saturation temperature; it lets us know how much an HVAC system feeds its evaporator coil with boiling refrigerant.

Liquid refrigerant goes into the metering device, and there needs to be enough liquid going into the metering device to achieve the desired effects of air conditioning but not so much that it floods the evaporator coil.

We want to know the evaporation temperature (the temperature at which the refrigerant boils), which we can determine with P-T charts or apps like the Danfoss Ref Tools app. A cold evaporator coil is desirable for moisture removal, but an evaporator coil that gets too cold may freeze.

We also don’t want the coil to get too cold because it could negatively affect the compression ratio by dropping the suction pressure. A cooler, lower-pressure vapor is less dense than a warmer, higher-pressure vapor, and the compressor has to do more work to raise that vapor’s temperature and pressure with each stroke or oscillation. So, you’re moving less refrigerant.

As long as a substance is still boiling as a liquid-vapor mixture, it will maintain a constant temperature as heat continues to be added to it; the temperature won’t rise or fall until boiling or condensation has been completed. Refrigerant with a 45-degree evaporation temperature will be 45 degrees as it boils, but it will go higher than 45 degrees once it has completely vaporized. That additional heat is called the superheat.

According to those rules regarding latent heat, it would stand to reason that lower superheat makes for a more efficient evaporator coil; there would be more boiling refrigerant in there. However, low superheat would put a compressor at risk of flooding if the refrigerant were to condense in the suction line. TXVs also have a minimum stable superheat that must be met. So, the efficiency of a lower superheat comes at the expense of increased flooding risk (which can lead to costly failures).

TXVs can set the superheat, and they must be charged by subcooling. However, older piston systems would require the superheat to be set, and you would need to do that with the indoor wet-bulb temperature, outdoor dry-bulb, and a superheat calculator as you charge a system.

If the superheat is too low on a TXV system, that indicates that the TXV is overfeeding the evaporator coil. On the other hand, if the superheat is too high, the TXV is likely underfeeding the evaporator coil. To prevent a failed TXV misdiagnosis, you must check several other things than the superheat; look for temperature drops across the liquid line filter-drier, airflow problems, and improper subcooling. Even when charging a system by subcooling, it helps to be aware of those conditions, the evaporation temperature, and the superheat.

On residential TXV systems, a typical rule of thumb is that the superheat should be 10 +/-5 degrees. The readings can deviate from the rule of thumb depending on things like long line sets and the location of your data point. In some cases, up to 20 degrees of superheat is acceptable in those exceptional cases where we can’t do anything about the system design, even though that may not necessarily be good for the system over the long term.

[cocusa]

New in the channel, really easy to understand. 👍🏻

[aday]

Superheat is the heat picked up after a liquid completely evaporates and becomes a gas. Nothing more, nothing less. Knowing you have some superheat before the compressor is important so you don't have liquid or partial liquid going into the compressor. Liquid cannot be compressed without damaging the compressor. So having some measurable superheat is a good thing.

[garypoplin]

0:10 - Superheat is a number that signifies the volume of vapor refrigerant at the top of the evaporator; subcool is a number that signifies the amount of liquid refrigerant at the bottom of the condenser. The higher the superheat the higher the volume of vapor. The higher the Subcool the higher the volume of liquid.

[luke]

Been in the trade for almost 20 years, but these videos give me the itch to learn to this day more than ever. Bravo Bryan!!

[tomh]

As an ASE certified auto technician who jumped ship to do residential HVAC I'm glad to see I'm not losing my mind, I keep saying that basically were moving energy/heat, laws of thermodynamics state basically hot wants to go somewhere cold or technically it wants to calm down lol, every HVAC tech I've worked for and with since my switch in careers says I'm wrong or don't get it, but he just totally redeemed me lol, your just moving heat one way or another especially with a heat pump, furnace just makes its own heat but still moves it lol. I noticed that alot of HVAC techs are stuck up and cocky, meanwhile I just want to talk shop and openly yet fairly compare equipment and setups but always get slack for being "a greasy car mechanic" . Love the videos!

[Harrys]

I've been doing HVAC for 32 years and I also teach it. This is one of the best explanations of superheat I've seen. Great job! I've found through teaching about units from 2 tons to 1000 tons, too many techs do not really understand the refrigeration system, how a metering device works, and what superheat and subcooling is and how it can effect system ops. Most techs think a/c units add cooling to a space but that is not true. They actually remove heat and move it to then release it via air, water. or earth's temps.

[keithmcgee]

Damn ur voice is able to keep up with ur brain, just a smart individual, good job

[Kevin-mmxm]

This is for anyone who thinks you talk too fast, Brian. Love you, your knowledge, and your videos and hope this helps you become a better presenter.
Yes, he does talk too fast, but what he does worse is put 3 different subjects into the same sentence without pausing, giving a student no time to think about the first item, and while thinking about the first item, completely missing the next two. Brian is great and has a tremendous amount of knowledge, but like many on YouTube who have years of experience and knowledge, they have poor communication skills. There is an art to communication, and it is a skill that needs to be learned and developed, just like a skilled technician. That's why there's a 4 year degree in communications. Talk about one subject at a time when speaking, speak 100 to 150 words per minute with pauses, and pause between sentences to allow the listener to absorb what they just heard. There are good YouTube videos by professors on communication that will help immensely. There you go Brian. A 4 year degree in just 10 seconds!!

[frenx]

Helpful video. Tech for 13 years but sometimes you get foggy about the principles learned in AC night School. Great video.

[superdfad]

Excellent explanation!! I learned in 17 minutes what I couldn't learn in my 2 years at college, where I got my associates degree.

[robertwalker]

Thank you for this video. Been out of school for a few years and haven't had to do much HVAC at the maintenance job I've had before. New job is asking a lot more of me so I decided to brush up on some of the basics and you really took me back to when I was at Ranken Technical College..

[samyz]

I'm a new subscriber seeking knowledge that I didn't find in HVAC school neither as apprentice. I have been thrown to the Wolfs in the second week. I'm studying your videos, taking notes, and trying to make it make sense to me. This stuff, superheat and subcooling hard to be explained even by technicians that have been doing it for a decade . Please do more for us the self thought people.
It's funny, the video is just 17 min. It took me 2 hours between taking notes and analysis.
Thank you
Peace ✌️

[coldfingersub]

That was really really good explanation and demonstration‘s. 👍. These videos really must become standard required watching for students in HVAC colleges

[harleyhawk]

was BT in the navy, when ships had boilers and steam turbines. super heated steam was a point where there was no water vapor left. it was super heated to point where it was a hot steam gas. used to turn the steam turbines so the turbine fans wouldn't be contaminated. the pressure in the boilers were 650 psi and some were 1250 psi. that was in a 8" steam pipe.

[brianlittle]

Good video Bryan. I’ve been saying this for years that it tells you how “full” the evap is. Not really an accurate way of saying it, but it’s easy to understand. Something else I will do is inspect the evap. If the upper third looks brand new while the rest of it is rusted, then I will add more charge so that I have more “cold” surface area. I might even physically inspect how much of it is cold, especially for those systems where all the measurements look good, and it should cool the building, but for some reason it isn’t. Thanks for sharing.

[melvinjohnson]

New tech here, graduate from my trade school in June and I've worked in the field for 6 months. I loved seeing the video and I learned some more. THANK YOU!!!!

[Cynic]

I have a NM Refrigeration Journeyman license and mostly work with industrial refrigeration, up to 165 ton units. However, I have gained immensely from these videos, which has given me a galactic advantage while keeping my own HVAC alive and well here at home. I greatly appreciate these awesome videos and instructors! God bless you all!

[heatpump]

The best tutor I have ever had the pleasure to listen to.

[bill]

This is a great series! Thanks for putting it on YouTube!

[LionelLiftsVegas]

You have gotten better at your videos!! I haven’t been around for a long time because I basically work on all chiller system now. However chillers use superheat and without proper superheat my approach will be way out for wack on a chiller. I like this video!

Thanks for helping the HVAC industry and giving a damn.