Low AC Refrigerant Charge - How to be SURE (Does it really need Freon?)

Does the system really need Freon? In this video, Bryan shows how you can answer that question by covering the signs of low AC refrigerant charge. That way, you can be SURE about the answer. He describes the role of superheat and subcooling to verify that a system is low on charge so that you don’t add refrigerant unnecessarily.
Subcooling tells us how much liquid is stacking in the condenser, with higher numbers representing more liquid and lower numbers indicating less liquid. We need to feed the metering device with a full column of liquid, but we don’t want excessive subcooling; too much liquid in the condenser will raise the head pressure and the compression ratio. In refrigeration, there will be a sight glass to indicate that you have a full line of liquid going to the metering device, so connecting gauges to check the subcooling is less important.
Superheat indicates how much liquid we’re feeding the evaporator coil. High superheat indicates a starved evaporator, whereas low superheat may bring liquid back to the compressor. The superheat should be high enough to ensure that we won’t deal with compressor flooding, but it shouldn’t be high enough to indicate a starved evaporator coil.
A visual inspection is also critical; filter restrictions, dirty coils, and improper thermostat settings may cause your readings to be off. Once you’ve verified that there are no obvious airflow issues, you can check the superheat and subcooling to begin verifying the charge level. Subcooling will generally be the main indicator of the charge level for modern residential systems.
After subcooling, suction pressure warrants the most attention. You can figure out the suction pressure by figuring out the evaporator TD. You can then use that temperature to refer to a P-T chart to get the corresponding suction pressure. If the TD is lower than the design temperature difference (DTD), the system will have a lower suction pressure than normal. If the suction pressure is low, double-check to make sure that the coil isn’t frozen and that there are no airflow restrictions.
We can also look at the condensing temperature over ambient (CTOA), which is usually more variable than the suction pressure. The CTOA refers to the difference between the saturation temperature at the condenser and the ambient temperature. In cases with a 15-degree design CTOA, the condensing temperature would be 15 degrees higher than the ambient temperature (105 degrees on a 90-degree day). We can then use the condensing temperature with a P-T chart or app to figure out the head pressure.
A low delta T may also be an indicator of low charge. The telltale signs are a low condensing temperature, low evaporator temperature, low subcooling, and high superheat. To get the most accurate readings, you should ensure that there are NO airflow restrictions and that the Schrader cores have been depressed if you take your readings with gauges.
Read all the tech tips, take the quizzes, and find our handy calculators at www.hvacrschool.com/.

[@benvashchenko5696]

Thank you for making this material available

[@aneskazic1933]

You’re a beast. We need to get you a raise man. You’re an amazing teacher, and you’re helping thousands and thousands of techs around the world be better. Thanks!

[@kevinkrug466]

He's the owner of the company

[@mattsprinkle2145]

Lol^^

[@jibletjibletstein8040]

He’s fucking loaded dude

[@jibletjibletstein8040]

B dawg has all the money

[@EdgarLopez-rf6tu]

as a brand new tech , %100 true

[@brunonikodemski2420]

Very good info. We had a AC scam-shop come in to check our R22 AC, due to what seemed like short-cycling. Tech went straight to compressor, checked pressures, no temperatures, but it was obvious that compressor was working OK, since heat outflux was obvious by feel, and no motor noise. Never checked anything around Evap system, then told us we had "low suction pressure", and recommended a new compressor. 5K$. Had a competent AC company come in, and they found one bad/intermittent wire connection to the Evap blower-motor. Fixed for 89$. Too many scammers around.

[@neverendingstudent]

Someone wanted 5 grand for a compressor!? Wow, yeah... Sadly, you have to look out for bad actors who serve their bottom line first and the customer a distant last.

[@americanpie8059]

Mr Bryan, your information is invaluable! Thank you for all of the videos. I really appreciate you putting in the time and the effort doing these.

[@conradmarch804]

I am a beginner. A/C Automotive only. I've been a mechanic for decades. All this is alot to absorb but with this and many of your other videos I'm understanding the theory more than ever. I'm the kinda guy who want's to know enough before I pull the trigger. Thanks you are an awesome teacher. Good Cadence and clear definitions.

[@jjengelbert]

Really great info. Thank you so much for existing and sharing. We are all better for the help and teaching you provide. Peace.

[@scottallen5182]

Great job as always Bryan. I find its pretty easy to assume a low charge when it is actually a failed exp valve if you don't pay close attention to the subcooling.

[@Cynic58]

This is an awesome source of knowledge and much appreciated! Your teaching style is top rate and I hope you're well rewarded.

[@wilfredovega577]

Once again great detailed info on CTOA and evap design temp. In other word what to expect on most modern systems.

[@FiercefoxProductions]

I’m currently a student for a 2 year commercial HVAC program and your videos really help to recap and explain everything in a way I can understand a lot easier 🤙

[@arthurhartwick7974]

That is such a great video. I wish I had seen that when I was just starting out. I learned a few lessons the hard way.

[@johnfrye3368]

One of the best videos I've ever seen on low charge. Every tech needs to sub your channel.

[@larryiaquinta428]

Very nice segment on adding Freon to a system that’s right you have cover all your bases can’t jump to conclusions…Great Job sir…

[@sethmeistergee]

Really appreciate your teaching style. Yes this is a lot of info but I think you've booked it down pretty well.

[@abdelalielmellouki6406]

Man you're a good instructor. I learned a lot from watching your videos. Great work!!!!

[@brianfueston7402]

this was exactly what i needed to hear and you did a great job explaining it. Thank you

[@huntercovington9421]

This was a great refresher for me thanks Bryan.

[@karljg19]

Thanks for the in-depth content. That Refrigerant Slider app is awesome.

[@donnellcairns3137]

Thank you for your time that lays out alot !!! I injoy some one teach this !!! I gained so much about your teaching, I had a customer that keep say I replaced the filter and I would look at return and the filter looked clean , but because of your teaching I refused to put freon in to system, even tho the customer said it just needs freon , I explained with freon being so high let me run a few test , he agreed!!! Turned out that the filter was causing a big pressure drop across the evap coil so I showed him and this is where service tech do explaine every thing !!! But I also find that it help if you show them the problem, I removed the filter and every thing whet normal again, the cool thing about it was the customer did A/ C work like 15 years ago , And he thank me for my time showing him !!! Thank you

[@edubracer15]

First time i put gauges on a system my hoses were on my manifold backwards lol I knew what to expect but the pressures were all wrong.... great video

[@donaldclark1872]

Thank you for sharing your knowledge. It was very informative

[@Stuart68505]

Very helpful review. Now it is 15 deg over ambient for the expected effect on the condenser discharge temperature. Thirty years ago It was 20 to 25 over ambient as I understood the concept.

[@danielcarrillo9122]

Mr Bryan for all the information you are a good teacher

[@Nba19980]

Good stuff, the residential exposure to these obvious things is about 98% less than if u do commercial refrigeration or industrial. Far superior honestly

[@garypoplin4599]

3:40 - It’s now easy for me to remember the effects of SH/SC: SH is directly related to the volume of vapor I the evaporator coil and SC is directly related to the volume of liquid in the condenser coil (higher SH = more vapor and higher SC = more liquid). Thanks, Brian!

[@efrainpelaez1246]

Excelent and magistral. My respect for you !

[@pabigurung5135]

Very useful video!! Amazing

[@dylanvillano5317]

You are an amazing human! Thanks for all you do! See you in January 🤙😎

[@gold4honey]

Awesome video Brian!

[@brianmcdermott1718]

Great info. Bryan. Thank you.

[@flaplaya]

Even pressures sometimes lie it's a tricky business. Thanks for the information.

[@bigdog2084]

Only way i can think of a pressure lieing is. 1 not checking all the systems other issues. For example air flow. You also need to be sure every part of system is working properly. The teacher mentioned that. He mentioned Even a shrader valve. Teacher also used a specific way to check your pressures according to atmospheric pressure. Which is different in Colorado vs Florida.

[@mohammadmahdawi4821]

good job thank you for slowing down

[@hvactechwanttobe1499]

Thank you I watch over Few times to get it

[@influenz1490]

Great video! Thanks

[@trunkmonkey355]

A lot of information, but I love being able to rewatch until it sinks in.

[@ronaldclark8187]

Thanks! Fixed metering device, super heat.

[@joncavender9117]

This helped me out a lot

[@rain8228]

Great clear understanding indepth knowledge and teaching 👏 👌

[@JamaicaBayRat]

Great class, thank you

[@MattBakerDesignBuild]

Really Thorough As Expected, From and for Technicians who are troubleshooting and diagnosing systems that use some of the most hazardous gases related to ozone depletion and global warming.

[@bigdog2084]

What does global warming have to do with this presentation? Oh wait you thought that you were watching a COW farm video? Yes believe it or not a commercial cow business has a TREMENDOUS effect on global warming most likely the MOST as well...and they use seriously enormous amount of water. Hence DROUGHT issues in California..Oh BTW it also puts CRAP in the aquifers...So please choose your EXTRA points of interest with care. Nobody really understands what your trying to do when you mention things that have nothing to do with the video. Now go eat your hamburgers? While you enjoy them in your air conditioned establishment/home. With a cup of chilled water with a filtration system? 🤦‍♂️🤷‍♂️🥸💩

[@MattBakerDesignBuild]

@@bigdog2084 My EPA refrigerants license restricts me by law from releasing Refrigerants into the atmosphere because of the global warming potential of these chemicals. I'm only trying to show my respect for teachers who produce these videos that help us mechanics and technicians stay tip top on our skills.

[@timstanford3443]

Great instruction video!

[@semmeh4918]

Hi, thank you for shareing your knowlage, and really it helps, so please if you can do more videos for mini split units

[@brunotaretto]

Thank You. You are the Best!

[@fuzzyvision1219]

You seem like your having a rough day brother... we appreciate you

[@magnusjansson4831]

Very good tip 👍

[@jerryhuff86]

I'm gonna have to watch this more than once

[@ericreinoehl9402]

Well said, I agree 100%.

[@grashoprsmith]

Thanks Bryan!

[@austinperez826]

Great comprehensive troubleshooting video Bryan. Just one question, where does amp reading play out? I know we cant rely on it solely to tell an undercharged system but is it safe to say that we can add it as one of the parameters to check to reinforce whatever is our conclusion in the troubleshooting process?

[@Shameful97]

This is a guy I would love to work for.

[@discoverneweyes]

Luv it, just bumped in..!

[@j303ti]

If the house is above design temperature (80°) and you add refrigerant using subcooling method, you must be careful. Add a little bit and wait for the house to get down into the 70's before setting the final charge. If you charge to 12° subcool when the house is 95° when the house gets down to 75° it will be overcharged.

[@cheddymachete]

Sounds like a cuck behavior

[@jasoncole3833]

Generally speaking if you follow these 6 checks and the result is Low SST, Low SCT, Low delta T across indoor coil, Low Compressor Amp draw, High superheat, Low subcooling the system is short of refrigerant. Good video 👍

[@realSamAndrew]

What is SST and SCT?

[@jasoncole3833]

@@realSamAndrew Saturated Suction Temp and Saturated Condensing Temp

[@realSamAndrew]

@@jasoncole3833 thanks for the reply. I feel like they use a lot of contradictory terms in this industry. If one is SCT, the other should be SET. On the SMAN display they use LSAT and VSAT, which is consistent, but then they say SLT and LLT, which is technically correct but doesn't match convention. Then you have other companies that use even other terms that nobody else uses. It's no wonder many techs get confused.

[@joez.2794]

@@realSamAndrew I calibrated the probes on my SMAN just the other day, and it was recommended to label the plugs so you're always using the same probe for high and low. So I drew an 'S' on SLT and an 'L' on- wait, now that just doesn't make any goddam... 🙂

[@realSamAndrew]

@@joez.2794 that's funny 🤣. Say, aren't SMAN and probes two completely different options? Did you just call it SMAN because they are both Fieldpiece? Or, are you using the SMAN as a wireless screen instead of a smartphone?

[@tonym6920]

Excellent! 👍👍

[@yvonneagundez1994]

Thank you

[@hliz8818]

BRYAN! BRYAN! I hope your team shows you this comment. A Weak compressing compressor can also give you low charge readings right. My co worker had a call where he got low charge readings (106 L/ 275 H), high super heat and low subcool, as he added refrigerant system turned OFF. Overload switch broke open. He recovered refrigerant, hosed down compressor, system turned on but gave same readings. Now I asked him to take Amp draw on common wire of compressor, he got 4.3 amps out of 18 RLA. This is signs of a weak compressor that doesnt have the strength to put in the work to move that refrigerant, hence the low charge readings he interpreted. Condemned compressor, installed new one, amp draw was in the 65% range of RLA, happy customer. Hope this comment finds you or if someone can share some insight. Best of luck brothas. Miami, FL

[@The1realro]

Appreciate that insight thank you so much. Checking the amperage of the compressor seems logical. What does RLA represent ? And could your compressor been saved by a capacitor or hard start kit?

[@hliz8818]

@The1realro Compressor wasnt locking up. And capacitor was good. Was just compressing really weak.

[@michaelproust7891]

Excellent

[@anthonybarragan1687]

Great video, just curious to where you got that 35° from?

[@discoverneweyes]

Old man I gettin' - forgot a lot, need to figure out the new shit, this brought back my dementia.. thanks Dude! 🤪

[@holdurhorse9149]

Add a little catch-up, when the measured Enthalpy is less than designed values at the suction line.

[@billbaber6653]

I’m a ole man not in the business but enjoying the ride.

[@jonas3598]

@HVAC School, is there anyway you could update your Playlist on KZbin? Specifically putting the videos in order for the HVAC fundamentals? I noticed a lot of your newer content isn’t in there.

[@bcracing204]

Make sure your gauges are on the right refrigerate scale

[@jjjohn5914]

Excellent content Bryan. In 35 years of owning central a/c units, I never had a tech perform the tests you mentioned...usually just feel pipes and say need freon or check hi/lo pressures...thats it. Where can I find a tech that will do the right job?

[@denrayr]

This might be controversial, but an experienced tech can have an intuition without specifically measuring everything. This checklist is geared towards inexperienced technicians that won't have that intuition. Listening to the compressor and feeling how much heat is being blown out, along with feeling the suction and liquid lines can tell an experienced person a lot about how the system is performing.

[@bigdog2084]

Idaho has a school. With a teacher/professor that taught me. It's been a long time ago. He was freaking awesome. I understood this presentation/video pretty well. He's got some great troubleshooting tests. We were taught to troubleshoot..always. BSU was the college. I miss using those skills. I do other type of job now.

[@bigdog2084]

@@denrayr uhhh no. I respectfully disagree. He mentioned MODERN system. You go around doing beer can cold checks. Then that's why Copland MADE every state in the USA send their technicians to college/school For AC. 👉or absolutely NO warranty.. Can you guess why? Yup one simple thing a compressor that was still working. I won't get into details. Experience is based off many skills. Troubleshooting with your hands is a dime a dozen these days. The technology is there now. If your going around changing parts. Your doing something wrong. The professor that taught me. Had a system that worked..still worked then for 25 years! Maintenance and System testing kept it going. Air conditioner has a closed loop. No atmospheric contamination. That's all you gotta remember when you do these mentioned tests. If something fails. Use your tools not your hands. We get old our blood flows differently our hands change...the beer and tools don't.

[@denrayr]

@@bigdog2084 it sounds like you didn't work in the trade long enough to develop that intuition. Modern or not. An ac system is going to behave a certain way. If I'm checking a system I'm familiar with, I'm not going to automatically hook up the gauges. If performance has changed, or is a new install or component replacement, then I absolutely will. You're guessing if you're not testing. Intuition can be part of the testing process if you have experience.

[@bigdog2084]

@@denrayr ok cool you know best. Then you should be a teachers. Or man up and make a video of such experiences. I can already imagine. For example in Florida..Colorado. or even Hawaii. How that would go. Beer can cold is over boss. You know it. My assumption is you have NEVER done service in those areas. To your assumption you are correct im not experienced enough. Yet I know what I gotta do. I was taught well enough to do Industrial refrigeration. Yes i was only an operator. We weren't allowed to overhaul the 200 ton compressors. I guarantee also we didn't use our hands to explain what's going on. 3 years of freezing temperatures year round. You learn a thing or 2. No hands needed.

[@solehhermanto6688]

Euleh euleh is ok i think cukup understand mebi harus banyak praktek...

[@eniniyathamizha2049]

LCT/LET/LSC/HSH = time to charge. Thank You.

[@briand.1694]

Other than cost considerations, is there a reason why HVAC manufacturers do not have temperature sensors placed at key locations throughout an HVAC system to monitor system performance? Wouldn't this take a lot of the guesswork out of troubleshooting systems? I would imagine that a relatively simple processor/software and interface would be a huge advancement in energy savings. It could also be remotely monitored by the servicing company who could contact the customer when the system is showing signs of falling out of 'tune'. This seems like a win-win for both the service company and the customer. The customer could have his system repaired before it totally fails, thus not sweating while waiting for a service call. And, the service company could ensure that the customer will be reliant upon them to maintain their system/systems. On a much more basic level, I monitor my system's performance with with a remote thermometer placed just above the evaporator coil to monitor the air temp there vs the ambient air temp at the thermostat. Making sure that the system is cooling as it should.

[@pointblankmatt6580]

youre describing most lennox units

[@jordanhenshaw]

Too expensive.

[@basspig]

When you refer to 10 to 20° of superheat are you referring to the difference in temperature between the high pressure line and the low pressure line?

[@efrainpelaez1246]

very good video. I have a question. Is the condenser designed with 20% more than the sum of the energy absorbed by the evaporator plus the energy consumed by the compressor? Would it be a good idea for sub cooling to be 20% higher than superheat? What do you thing about?

[@johnthompson9918]

No. Even though condensers are bigger than evaporators for obvious reasons. If you look on a P-T chart there is more heat rejection than refrigeration effect. Also pipe runs/bends/joints and units that differ in height (some condensers are on a roof or down in basements etc). If your condenser is much lower than evaporator liquid will start to flash off and pressure will reduce as the liquid ascends up towards the metering device and finally evap coil. All of this must be taken into account when sizing equipment.

[@Jobychayan4085]

Low charge also air mixed refrigerant, txv fully open one system but running low efficiency

[@marekdorobczynski1727]

Hi, what unit are you using while talking about superheat/subcooling ? are these Kelvins or Fahrenheits ?

[@n.gineer8102]

No subcool, high suoerheat, 104° hot Texas afternoon, dew point 75 so heavy air. Looked like low charge so added but SC didn’t really change. Actually the condenser was dirty. Looked clean but a quick wash and wait and suddenly the SC is -15. Now have remove refrigerant to get back to 8. Did I mention it was 104°F? Two hours to sort it out, very exhausting. Watch out for condenser that looks clean because on a hot day it makes a huge difference.

[@ozziewelcome5612]

Good 👍 👍 👍 👍 👍

[@damnitdang]

Make a vid about non condensible. In my line of work. So many new tech fail to to a proper charge and vacuum and always introduced air into the system. Units always getting worked on and technicians are half assing stuff alot

[@gonehere4330]

Your awesome

[@BartlettTFD]

Thoughts on the need for a sight glass in the liquid line❓

[@chrismyers5758]

I had a weird situation can you please explain this? -7.6 subcool with 13 superheat 120 psi low side 229 psi high side. The charge looked low from the way below normal subcool but the superheat was about perfect so I thought it must be something else and not a low charge. I added Freon and it did end up being low. However from a diagnostic standpoint I do not understand this. And it makes me not trust that superheat reading at all when dealing with a txv system. Which it was.

[@MrFla954305]

What about a r410a Rheem split system with a 200psi suction and 300psi head 80 degrees inside the house could it the compressor not compressing or low on Freon or bad txv? Which would be likely?

[@Garth2011]

And of course, look for the leak before adding charge. All of that data on the cart is also on your typical analog gauges...just memorize the pressures and temps and you're good.

[@Echo024]

Anyone who says the trades are unskilled professions needs to watch this video.

[@faafetaileota1340]

What is the 35 degrees you are subtracting from your suction line?

[@markhonea2461]

Try doing a visual inspection of the indoor coil on the dirty side. It basically impossible without cutting a hole. All you can access is the 'clean' side. We stopped using the electronic air filters because they allow so much to get by and into the coil. It has been my experience that buildup on the supply side of the indoor coil is the most overlooked problem with systems that aren't working well. Very low superheat during cooling, and increased sweating on the suction line. Long run times in any mode. It is difficult to clean, but necessary, and any tech worth his/her salt will find a way to do it. Stuff gets beyond the filters. Always. And gets trapped in the coil. Of course it does. But nobody is talking about it. It throws the entire system off and leaves many people scratching their heads. Looking at the clean side of the coil doesn't help.

[@braaap6292]

Depends on the system I guess, I've seen plenty that flow the other way so they're easy to check or have access upstream to see the dirty side.

[@markhonea2461]

@@braaap6292 of course. The problem lies with the significant percentage of the inaccessible supply side coils. My boss had a cow when he first learned what I had done to a system he had installed 8 years earlier until he saw the picture I took of the restrictive build up, that could only be viewed with some invasive surgery to the ducting. Actually it solved a number of units issues that had both the owners unhappy and techs scratching their heads and throwing parts at it. I had long suspected this condition. Doesn't mean I'm smart, but that one thing was the only factor nobody has investigated. And that was because no one has the guts to cut a hatch into the right spot and look.

[@captainnutty4168]

What if you have a txv and the superheat is like 40 but the subcool is within range?

[@f1reguy587]

Anyone running an inficon d tec 3, a fieldpiece 82, or a bacarach informant 2? Love to hear the good,the bad and the “i wish”. Cheers team.

[@jordanhenshaw]

Every apartment maintenance guy: "Nah, man, just charge it up!"

[@stephwaylonwells3218]

👍

[@tyrellrutledge6251]

He's going to put FREON out of business!

[@newstart49]

Subcooling and superheat MUST be checked along with psi for a TXV or piston or you will get snagged some day. Mostly subcooling for TXV and mostly superheat for a piston or cap tube system.

[@ablejohnson]

SOLID

[@dpolendo]

So it's always going to be 35 deg minus my return air temp for evap TD?

[@CHOPPA78]

Why top up if its low , you might have a leak therfore leak more refrigerant into the atmosphere , shouldn't you decant and pressure test to know for sure ?

[@topfell8277]

120 on the low 300 on the high clean the coils in and out only add what's Ness. Little at a time

[@reidarnold1992]

Does this also apply to fixed orfice/piston systems? I was also told to charge those off of superheat and that you really shouldn't have and subcool.

[@d3adp94]

Doesn’t hurt to know both. Orfice, you have to adjust amount of refrigerant. Txv does this for you.

[@briangc1972]

@@d3adp94 You still have sub cooling, just not as much. Just as a TXV system still has superheat, just not as much.

[@manuelmejia76]

I have a question about an anyone know an app that help you with subcooling when you put in certain data it will tell you suggested subcooling thanks

[@shifter8207]

If you have a packaged unit (it was a Trane rtu) and it had a liquid line access and a discharge line access I had a system that had no subcooling should I set the subcooling off of discharge pressure or liquid line pressure ?

[@briangc1972]

They are the same pressure up to the TXV. Same with some Carrier models

[@shifter8207]

@@briangc1972 this is thing on carrier you can read discharge pressure and due to the increase of pressure in the discharge line you will artificially higher subcooling numbers than using liquid pressure so on a Trane package unit I’m still unsure if proper readings are taken on the liquid or discharge line press tap

[@briangc1972]

@@shifter8207 The pressure is the same everywhere between the compressor and TXV. There should be no pressure differential according to the engineers.

[@Ojeramup12]

I will be glad when we can add freon just like we do on cars. Someone could make a fortune.

[@jordanhenshaw]

I keep hearing people I work with diagnosing low charge exclusively based on outside temperature and pressure. I ask what the superheat/subcool is and they don’t know. I ask what the evaporator temperature is. They don’t know. But they charge it up because “the pressures are lower than what I expect at this ambient temperature.” But because these apartments are so leaky, they get away with it. And then I come in, I don’t have my EPA yet, but I learn all the observational skills. So I find all this other stuff and I’m book smart from studying 3 hours a night. So I come up with an idea for what’s happening and I explain my thought process. Other dude has no clue what the fuck I’m talking about. No clue at all. But somehow he’s right and I at least appear to be wrong, but not conclusively. So frustrating.

[@ApexRefrigerationBoilerC-rz8wz]

What if you got good subcooling but very high superheat?

[@THEONLYETP]

Why would a tech say the unit is a liitle low on freon in the morning (77 degrees) and way too much freon at 93 degrees the next day when no freon was added?

[@moneybags7482]

Instructions unclear… I’m just going to clean toilets 🤦🏾‍♂️😂

[@h5v1y0]

You’ll get it bro, just stick with it