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

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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 https://www.hvacrschool.com/.

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ben vashchenko says:

Thank you for making this material available

Mark Honea says:

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.

Mark Honea says:

We haven't used Freon since 1990. I am surprised this guy uses that word.
Freon is the DuPont brand name for r-12. Then Forane for 134a.
No brand name for the non patented r600a. How about that. DuPont is hating it.

edubracer15 says:

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

Sem Meh says:

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

Big Dog says:

Uhhhh where's the information for a HEAT PUMP system? This ALSO must be mentioned. Simply because this video entered a place like Idaho!!! Plus IDAHO is a different atmospheric pressure… than Florida

Jon Cavender says:

This helped me out a lot


It’s called refrigerant not Freon

Efrain pelaez says:

Excelent and magistral. My respect for you !

Duke daDog says:

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

Conrad March says:

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.

Magnus Jansson says:

Very good tip 👍

newstart49 says:

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.

F1reguy says:

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.

What the phuck says:

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


Hi sir my name is vimal i am from dubai i have 2.5 ton ductable split ac in this ac supply temp not down below 60.8 ° F this is r-22 refrigerant suction pressure 80 psi
Suction gas pipe temperature – 46.4°F
Metering device – orifice type

Brian Mcdermott says:

Great info. Bryan. Thank you.

Donnell Cairns says:

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

Brian D. says:

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.

Abel Garcia says:

Anybody here because they want to know more about Thermo Kings and Carries?

Matt Baker Design-Re-Build says:

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.

Gone Here says:

Your awesome

mrrock nobodylostinspace says:

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?

jjengelbert says:

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

Tyrell Rutledge says:

He's going to put FREON out of business!

Larry Iaquinta says:

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…

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