AC Pressures, Subcooling and Superheat

Bryan with HVAC School goes over AC pressures, subcooling, and superheat in his troubleshooting mindset presentation from the BTrained HVAC training event in Birmingham, AL. Bryan also talks a bit about his educational photo-sharing app, MechPic.
The suction pressure and suction saturation give us an idea of the refrigerant’s boiling temperature. Evaporation happens at the surface and not necessarily at the boiling temperature; on the other hand, boiling happens throughout the entire substance and occurs at the boiling temperature. So, the “evaporator” is really a heat absorber where boiling occurs.
A rule of thumb for suction saturation is that it should be around 35 degrees below the indoor ambient temperature at 400 CFM per ton. That differential or temperature split, also known as the design temperature difference (DTD), is typically 30-35 degrees in high-humidity markets that use 350 CFM per ton like Florida and Alabama, where the conference took place. Lower suction pressures generally result in a higher DTD, and higher suction pressures usually result in a lower DTD.
When we look at head pressure, we’re actually learning about the condensing temperature over ambient (CTOA). The CTOA is the temperature split between the outdoor ambient temperature and the condensing temperature. In general, CTOA tends to fluctuate a bit more than evaporator DTD, but a rule of thumb is that it should be about 15-20 degrees above the outdoor ambient temperature. SEER ratings may also affect CTOA.
A ratio of head pressure to suction pressure is your compression ratio. Higher compression ratios tend to lead to reduced compressor performance (moving fewer BTUs).
Even though we tend to see high head pressure as the main problem to avoid, we should be aware that low head pressure is possible. There needs to be an energy or pressure differential for matter to move from one place to another, and we’ll have trouble moving refrigerant if the head pressure is too low. The most important thing is to make sure that the system is working in accordance with its design.
Superheat is the degrees a vapor is above its saturation temperature, and it really tells us the amount of liquid that’s stacked in the evaporator. However, the evaporator and compressor superheat could be quite different due to heat gains in the suction line. Generally, we want to see 10 degrees of evaporator superheat (+/-5 degrees) on many AC systems. The TXV is supposed to keep superheat constant; however, some possible system problems can affect superheat beyond the TXV’s control (such as significant liquid line restrictions, though there are exceptions to that example).
On the other side, subcooling refers to the temperature of a liquid below its saturation temperature. In an HVAC system, subcooling indicates how much refrigerant we’ve “stacked” in the condenser; lower subcooling indicates that we have little liquid refrigerant stacked in the condenser, and higher subcooling indicates that we have a lot of liquid refrigerant stacked in the condenser. (There can, but not always, be temperature drops across liquid line restrictions.) Refrigeration systems with receivers don’t necessarily have to rely on subcooling values to indicate refrigerant storage (a rule of thumb for residential A/C systems is typically 10 degrees +/-3). When we measure subcooling, we really want to know if we’re feeding the metering device a full column of liquid. If we want to increase subcooling, we can do that by reducing the liquid temperature via mechanical subcooling or increasing the condensing temperature. (However, we don’t want too much subcooling.)
When we use the manufacturer’s specs to set our superheat and subcooling, we might actually be reducing the equipment performance, so we have to know exactly what our goals are.
Read all the tech tips, take the quizzes, and find our handy calculators at www.hvacrschool.com/.

[@davidswitzer8994]

A very good follow up! I like it!🙂

[@mikaelg5840]

Been in the trade 24 years, I love listening to your videos before work during my work out or morning walk, knowledge is power. Your videos reaffirm my work. Please keep spreading the love - one day I hope to be at one of your seminars. God bless, your a legend to the hvac family.

[@christophercurcio4535]

I hear his voice in the back of my head when I'm working lol.

[@mike-yp1uk]

I like these discussions because I can see my weakness in theory of operation and principals

[@ebfsystem]

Same here 😏

[@jollyscaria1922]

Using ice ...$ mechanial Refrigeratio ..absorption Lihuim rimide Absorption systtem steom leror vacuum systom booster cycle extremeey lowtemperture systom flooded coil system floodded coilsystem hermetic packaged systom ..refrigration basics . Systom

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[@nicolelarrance1417]

That is a very good way to put it. And I like your comment about you seeing your weakness in theory of operation and principle, I can relate.

[@roymedlin946]

It's very unlikely that you're going to ever read this Mr. Orr, but I have learned more from your & Craig's videos than on sites in Commercial jobs for a company & some of the residential ones as well ,even after 4 years, If I could ,I'd love to come to Kalos & really learn

[@coldfinger459sub0]

I can never watch your videos enough even if it’s a repeat on the same topic that you have done before I always learn something new on a method of teaching other people through your videos. As you grow and learn and improve in your industry I grow and learn and improve in my industry by watching your videos. I want to become an HVAC instructor as I approach retirement in a local HVAC college and adopting your methods and style of teaching along with Ty and Craig and Chris combining all these styles and methods together will definitely excel my students and myself in a teaching and learning environment. 👍. Thank you for all your efforts and time involved in producing these videos so we can all share and pass the knowledge along

[@performancehvacr6629]

I was at this seminar. I thought about not going because I am familiar with the material. I have watched videos and listened to podcasts multiple times. I am glad I went. I learn more and more and think about things in different ways. It also helps to be able to communicate it better. And now, I'm watching it on KZbin. lol.

[@coldfinger459sub0]

@@performancehvacr6629 I hope it’s in my plans to attend this event sometime you’re in the future and I think there’s also another train the trainers event I think it’s called something like ( ESCO ) I think I’m pretty sure I got the abbreviations wrong but if you know what I’m talking about you’ll be able to correct me also want to attend that event since I want to be a HVAC college instructor

[@The-LongRoad-Home]

Excelsior My Friend ! Smashing through all your Videos... Cooling season if fast approaching and I'm all about Complete solutions found efficiently and professionally.

[@okayjacobs8569]

Hi, Bryan. I appreciate you for the great works you guys are doing. I would like to know if you have an accelerated HVACR training program? Alternatively do you have a packaged and elaborated videos for a new technician? Trust you will give these questions a response. Thanks.

[@honestman276]

I have been looking for such types of videos before. Thanks from Bangladesh.

[@alexanderreyes6546]

Very good seminar for the trade thank you for sharing 👍👍👍

[@brianmcdermott1718]

Great info. Bryan. Thank you.

[@ebfsystem]

Great video! I liked the idea of having a community chat,I need that app

[@EverythingHVACR]

Great video as usual!

[@jollyscaria1922]

Super heat a liquid under pressure above its boiing point wirh out vaporization non the excessof tempertcureof saturtuion..

[@KevinMartin-dp8qf]

I call that the mirror effect on subcooling it is the opposite of increase condesor temp for max flash and flow bubble point and flash and sat temp

[@johnnya4236]

HVAC school at min 17’21” you mentioned 20 degrees superheat is too high in a 10ft line set but Copeland minimum is 20 degrees to ensure no liquid comes back. I believe your compressor SH overrides Evap Sh in this example. If you want 10 degrees in the evap you could risk flooding back. So you asked if 20 degrees SH is good back at the compressor and that would be the least you’d want at the compressor it should be that or higher all depends on length of lineset

[@vincentpribish9276]

YES compressor superheat should always trump evaporator superheat. However in the real world sometimes that's not always doable. As an example, a walk-in freezer with the condensing unit sittings on top the box. Suction line could be less than 3' and typically we set freezer superheat between 4-6°f. So what's the solution ? We call it an accumulator.

[@basspig]

What about mini splits that use EXVs? For instance, the techs at Mr Cool say you don't measure the superheat or sub-cooling because in their systems those numbers change constantly. They want to rely upon the suction line pressure.

[@jasoncicaroni9939]

Hey brian, maybe you can help me out with a small disagreement, I was telling another tech about what another one does when he cleans a condensing coil with a water hose while the unit is running, one its counter productive to try and hose off the coil while its sucking in won't get clean , but that it's not good for the system because your now stacking the condenser coil with liquid by doubling the amount of subcooling going on from air and water, he says there not a problem, so I'm asking it is detrimental to the system to do it that way, would I be correct?

[@onlyulee866]

If you oversized piston would that lower subcooling, superheat and delta?

[@luisvillatoro6730]

Where would I start with you’re vids ? I’m an apprentice

[@nguyenson3112]

Hello bryan and everyone, I'm having a problem that I don't understand and hope you can help. I tested a bottom mount refrigerator with 3 compartments, the fresh food compartment on top, the chiller compartment in the middle and the freezer compartment below. The target temperature is 4/-3/-18 respectively, testing at ambient temperatures 16, 25 and 32 degrees all gave OK results. When tested at 38 degrees, the chiller compartment decreased to only - 0.5 degrees. What is the cause?

[@Chubbadubz]

Brian, can you share your powerpoint? I'd like to have this class with my techs.

[@baskorobas7550]

Saya slalu menyimak vidiomu......thanks

[@stepanvinnikov6024]

What’s the best online school for hvac can you guys give some insight

[@abazism]

In PT chart, what would be the evaporator temp ? Bubble point ? Or dew point ?

[@manuelcabral8974]

Very interesting

[@DanielMalkokian]

how do you take superheat on a digital compressor. The superheat will go up and down with little to no stable point. ( this is for refrigeration measure by the evap case ).

[@jkbrown5496]

Bit of trivia from hi-fi audio. Generally, they pitch is the amplifier has 30db and that means is amplifies the signal 30 times. It just so happens 30db is about 30 times. It's really 29.xx db, but it just happens at that point, 20 db is not 20 times, etc.

[@leftover7766]

Sorry, but 30 dB amplification is 1000 times. 3 dB is a doubling, while 10 dB is 10 times and 20 dB is 100 times.

[@hugogarcia7574]

I appreciate this video . help a lot

[@realestateservicessaleshea99]

You cut it when it made sense!🤣🤣🤣 Thanks again for the videos. 🥃🥃🍺🍺🍺🎳⛳ Stay safe. Retired (werk'n)keyboard super tech. Wear your safety glasses!

[@russm1757]

Hey Brian how do I find Mech pci ? Russ

[@iancunicolae2626]

what is considered low liquid line restriction on a txv 410a; 18seer system? in terms of Low suction pressure? if the system is looking for 130 psi and you have 120 or 125 ? is that considered low? therefore a liquid line restriction?

[@jollyscaria1922]

R410 psi sucshio 120.. discargeing psi 420.to 450. Stand220to230.

[@performancehvacr6629]

When everyone raised their hand saying they were in a humid market, I thought to myself, Edward Janowiak would disagree.

[@sahiljuned3762]

Dear sir please explain vrv and vrf system working and testing system

[@hectorgarcia-zv1wi]

I’m new ,came from trucking just wanted a new trade something fun and challenging

[@emailfilters]

5:31 - The cat's outa the bag. Is this app available?

[@leonardlewis5586]

OH SNAPS! I got light bulbs going off!

[@bmabellada]

You have to tie it with psychrometric charts to be accurate of what you are saying. CFM, moisture content of air on both evaporator and condenser affect the heat exchange,. Rule of thumb is not accurate way to define heat exchange and cooling/heating results. Heat exchanger surface areas are also to be included.

[@leftover7766]

Early on you argus that the evaporator should be called a boiler because you think the phase transition is happening inside the liquid, but this is incorrect. The refrigerant is evaporating off of the surface of the droplets of the refrigerant that were created by the metering device. The surface area is immense at this point allowing it to quickly absorb the latent heat of vaporization.

[@muhammadusamakhan5295]

very difficult sir

[@josephrostkowski8631]

Sub(cooling) bub(ble) Su(preheat)dew(point) sub bub su dew!!!! Your welcome 😊

[@KumiTeePoke-bv9bs]

The mp are looking good, they might expect grants from pm, but don't. Contain the number and fix the problem please, it's our country, it's our future.

[@hooverlondon]

I personally think alot of these guys should start talking more about window air conditioners, central air and portable air sucks, window units are the best, window units are always needed more than the others...

[@fuzzypickle5307]

Cost of repairing window shakers exceed cost of a new window shaker.

[@henrysapp4631]

People ask me all the time if I work on window units. I tell them NO. Because by the time you pay me to fix it, you could have just replaced the unit and save money. Not in all situations, but most the time.

[@jimflakker2981]

Can a D😅😊😊

[@jimflakker2981]

Can a DTV scroll compressor be used in a walk in freezer set up ??

[@spartan5106]

I don't know what the hell he talked about for half an hour

[@zesty2023]

Hey HVAC School is it possible for me to watch your trainings like this one and get credit for NATE hours? My job has absolute terrible training, we're talking like 1 zoom training for an hour a month maybe. They made this big haruff about getting me NATE certified and then don't give me enough training that I think I'll be able to keep it. They want me to use Praxis to get my hours but honestly I think their training videos are terrible compared to you and AC Service Tech's channel. I love watching your stuff but it pisses me off that I'm not getting any credit for this time learning on my own time... so any way to make this count towards NATE?