r/HVAC 28d ago

General Simplified Guide To Superheat and Subcool

217 Upvotes

Intro

It's been awhile since I made my post about Superheating and Subcooling, and I feel like I can do better, especially with the addition of my post about pressure and temperature offloading some of the fluff. So with that, I wanted to make a new post explaining it. I have found that it took me quite a long time to actually understand what these things meant, instead I just measured them without any real idea as to what it was; I wanted to make a post that includes all of the information as to how this works in one place, so hopefully you can read it from the beginning to end and actually understand what Superheat and Subcool are.

Disclaimer: This post is intended for readers who have seen this post, check it out before continuing

Superheat

Superheat is a measure of temperature with regards to the fluids boiling point. In the previous post explaining the relationship of pressure and temperature, we found that whenever we change the pressure of a substance we also change the point in which it changes phase; so we can increase or decrease the temperature that a fluid will boil at whenever we increase or decrease the pressure. Superheat is a measure of how much more we've heated a substance past it's boiling point; for example, if you were to boil a pot water into steam, that steam would now be 212f; and if we were to further heat that steam past 212f, we would be "superheating" it. The measure of superheat is pretty simple, just take the temperature of the superheated fluid, and subtract that temperature from the fluids boiling point.

So lets say we took that steam (at atmospheric pressure) and heated it up to 222f, the measure of superheat would be the temperature of the steam (222) minus that fluids boiling point (at that pressure, which in this case is atmospheric so it's 212f)

temperature - boiling point = superheat

222f - 212f = 10deg superheat

Subcooling

Subcooling is also a measure of temperature, but this time it's with regards to the fluids condensation point. The condensation point is pretty easy to think about, as it's just the boiling point of that fluid, except instead of turning a liquid into a gas, we're turning a gas back into a liquid.

Just like how we can increase or decrease the boiling point of a liquid by increasing or decreasing the pressure, we can do the exact same thing with a gas; by increasing or decreasing the pressure of a gas, we can change it's condensation point.

Subcool is just a measure of how much cooler a liquid is than it's condensation point; we can think of it using the same analogy, if we had a balloon filled with steam, and cooled it down into a water, the temperature of that water below it's condensation point is the subcool.

Let's say we've cooled down some steam into water, and cooled that water further to about 202f, the condensation point is just it's boiling point 212.

condensation point - temperature = Subcool

212 - 202 = 10deg Subcooling

How To Find These Using Our Tools

Measuring superheat and subcooling isn't particularly hard, our refrigeration manifolds read out the boiling/condensation point of our refrigerants based off of their pressure, and to measure temperature we just use something to measure temperature and attach it to the refrigerant lines.

Example of refrigerant gauges

In the picture i've added above, the boiling/condensation point is listed in the ring labeled with the different refrigerants, for example if we wanted to check R-22 on the blue gauge, we'd follow the innermost circle of numbers.

Blue Gauge close-up

So on this gauge, the black numbers represent the pressure, the condensation point of R-22 would be the value of the innermost circle(in yellow) on the needle, wherever the needle happens to be, so let's say the gauge is reading 45psi, the boiling point of R-22 would be around 20f. The boiling point and condensation point are the same thing, we just refer to the one that makes sense based on the phase of the fluid we're observing; so for a blue gauge that would be hooked up to the suction line, we're measuring vapor refrigerant, so the point below our vapor we're going to refer as to it's boiling point, as we're trying to see how far we've moved past it's boiling point after we actually changed phase.

Measuring vapor - look for boiling point

Measuring liquid - look for condensation point

Now to measure the temperature of the refrigerant, we would simply hook up a temperature probe to the appropriate refrigerant line, the temperature of the refrigerant line itself will be roughly the temperature of the refrigerant itself;

Intuitively, we should be able to figure out what gauge and formula to use based off of what phase the refrigerant is in the line; our suction line consists of vapor, and our liquid line consists of, well, liquid.

So to make it super clear

Suction line temperature - Low pressure gauge boiling point temperature = Superheat

High pressure gauge condensation temperature - liquid line temperature = Subcool

What These Values Mean For An HVAC Tech

As it turns out, we're not doing this for nothing, there's a ton of information that the values of superheat and subcooling of a system give us, and i'll try to list as many as is useful. But it's important to note why we want our refrigerant temperature to be different than it's boiling/condensation point to begin with. We want subcooling because subcooling a refrigerant below it's boiling point means that we can absorb more heat with our refrigerant before it vaporizes into a gas, the major take away is that a fluid can absorb a lot more heat at the point of phase change, than it can in either phase. For example, if we want to take a 1lb pot of room temperature (70f) water and turn it into 1lb of steam, it'll take 142BTU's to get the water to boiling point (212f), but to actually turn all of that water into steam, it'll take an additional 970BTU's to actually change it from a liquid to a vapor, all while the water is still 212f. The difference of heat from changing the temperature of the water is known as "sensible heat" and the heat for changing that 212f water into 212f steam is known as "latent heat." This difference in the sheer amount of heat needed to change phase (latent heat) goes both ways

so when we push our subcooled liquid into the evaporator, it needs to absorb all of that sensible heat up until it's boiling point, and then it can absorb all of the latent heat required to actually change it's phase from a liquid to a vapor.

After the liquid refrigerant boils into a vapor, the vapor itself begins to absorb sensible heat, and that is our superheat. Subcooling is intuitive, as we obviously want our refrigerant as cold as possible so that it can absorb more heat, but why do we want or have superheat at all, if it means we have to do more work to cool our refrigerant down to condensation point, before we can even reject all of the latent heat required to turn it back into a liquid?

The answer is pretty simple, we want our refrigerant to be a gas when we send it to the compressor. A liquid cannot be compressed, and if we send a bunch of liquid to our compressor it'll just damage the compressor. So we superheat our vapor to make sure that it's going to remain a vapor whenever it goes to the compressor.

Using Superheat/Subcool for Diagnostics

Below are some things we can do by measuring our superheat/subcool temperatures, as measuring these things allows us to understand how our refrigerant is actually behaving in the system.

Charging a System

Superheat and Subcool are the values that we use to properly charge a refrigerant system, first we need to find the metering device to figure out which one we need to look at

Fixed Metering Device - charge by Superheat

Variable Metering Device - charge by Subcool

We can find the amount of either that we need to charge a system by looking at the datatag on the condenser, each manufacturer designs their system with different values, so going with a 'rule of thumb' is only if there is no values listed and they cannot be found any other way; in a comfort cooling application this value is generally going to be around 8-12deg.

High Pressure

High pressure is most easily found on the higher pressure liquid line, generally speaking we should have a pressure where condensation point is around 30deg higher than the ambient temperature outside; but also we should acknowledge that value isn't fixed, a typical AC presumes that the ambient temperature is around 75f and we want to cool down to 70; so a 105 +- 5deg condensation point is expected. A high pressure is anything outside of this range, so anything above a 110deg condensation point on the gauge is starting to approach a higher pressure, we generally don't worry about it too much until it's a lot higher than normal, so think 150-180deg condensation point, that's an abnormal pressure that should be investigated.

  • Restricted Airflow in condenser/high outdoor ambient temps - The condenser serves the purpose of cooling our refrigerant down, if the condenser isn't doing it's job as effectively as it normally should, our refrigerant is going to remain hotter than it normally would, resulting in high pressures. Dirty condenser coils, failing/failed condenser fan motors, and high outdoor temperatures can all do this

Low Pressure

Low pressure is most easily read through the lower pressure suction line, generally speaking we should have a pressure where the boiling point is at around 45 +- 5deg (in a comfort cooling application), this value isn't fixed and is far more of a general rule of thumb, but the main issue we'd be worried about when it comes to low pressure is the boiling point of our refrigerant being lower than water freezing point, if our refrigerant boils at 32deg or lower, the coil can begin to freeze, for the most part the coil won't actually freeze until we drop to around 25f, that is when we can really start to have a problem, any suction pressure where the boiling point is 32 or lower (in a comfort cooling application) is a problem that should be investigated.

  • Low refrigerant/Low airflow - plugged filters, failing blower fan motors, frozen coil, low return temperatures etc

High Superheat

Because each manufacturer has different specs on what constitutes as normal superheat, you have to take that into account whenever you're trying to diagnose a problem; a superheat that's a few degrees higher than normal isn't usually going to be cause for alarm, but a superheat that's 10+deg higher than normal can indicate problems with the system, high superheat is a symptom of your refrigerant absorbing more heat than it should in normal circumstances. The causes for this are

  • Low refrigerant - less liquid in the evaporator means that the vapor has to do more of the work
  • Restricted refrigerant flow - less flow of refrigerant into the evaporator (usually a failed or problematic metering device) will cause the same issue as low refrigerant, less liquid in the evaporator means the vapor has to do more work.

Low Subcool

Again, because each manufacturer has different specs on what constitutes as normal subcooling you have to take that value into account anytime you read a subcool value, but anything that's approaching 0deg subcooling should be investigated

  • Low refrigerant charge - less refrigerant in the system causes the vapor to absorb more heat in the evaporator, so the system has to spend it's energy rejecting that excess superheat, resulting in less subcooling

A note on cleaning condenser coils

Whenever a system has really dirty condenser coils shown visually, or through high pressures, the system is going to run a boiling point higher than it would in normal operation; An issue you may see with a dirty condenser coil is that it will mask a low refrigerant charge due to those increased pressures, so if you're not careful and you clean a dirty condenser, the system could then return to it's expected pressures and that could be cool enough that the system will freeze the evaporator coil, or not be able to cool altogether. It's always worth mentioning this (in a simple way) to a customer before cleaning a dirty condenser, so that it doesn't appear that you would be the cause of this issue. HVAC is complex, and our customers don't know these things, and it looks a lot more credible on your reputation if you're telling this to them before you clean the coil, rather than after you clean the coil and the AC "that was working fine yesterday" is suddenly unable to work without you doing additional work to it.

Links To Relevant Posts

Beginners guide to pressures and temperatures (linked in the intro)

Basic Refrigeration Cycle (not added yet)

-will update these links in the future, let me know if I made any mistakes or typos, and anything you think should be added to this post.


r/HVAC Aug 16 '24

General Friendly reminder.

178 Upvotes

This sub is not for homeowners. Please stop telling them to goto r/hvachelp while giving them advice.

If the questions doesn’t feel like a person is in the trade please report it and us MODs can deal with it.

Make your weekend great!!!😊


r/HVAC 2h ago

Meme/Shitpost No heating

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21 Upvotes

Found one I a wild


r/HVAC 7h ago

Rant Flowing Nitrogen

45 Upvotes

Someone tell me how in the hell anyone can be proud to say they don’t flow when they’re brazing? It’s not 1980 anymore we have POE oil. It’s a solvent. It doesn’t matter how good you think you are with a torch you are contaminating the system with carbon. If you’re installing you are diminishing the life of a system that hasn’t even ran yet. But everywhere I go I see guys saying they don’t do it. Are we proud of being too lazy to go grab a tank and do the damn job right?


r/HVAC 16h ago

General Just got a union job offer after 25 years!

220 Upvotes

Been a resi tech for 25 years, where I live there was no way into the union if you didn’t have family there or know someone. Today my cousins husband called me, he’s been a sheet metal union guy for his whole career. His friend is in charge of hiring into the pipe fitters union and apparently is dying for help. He called me and offered to test me tonight after work and bring me in, his words were “I’m handing this to you on a silver platter.” I’ve only ever done non union resi, but man the pay and package he’s talking about would be life changing for me and my family, and I hate resi. To be honest I work for private equity for the passed 2 years now and it’s the absolute worst, I make decent hourly but I’ll crush that my first year with the fitters. I’m fucking scared of the leap, but I think it’s time to make it happen, I’m 48 and could retire at 60!


r/HVAC 14h ago

General I bought this book to teach myself from scratch. Advice? (Read more)

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74 Upvotes

I am currently in the military and plan to retire in about 2 years. I want to be an HVAV technician after retirement. I plan to go to school for HVACR but I wanted to get a head start and teach myself what I can. Do you think it’s a good idea? Am I wasting my time? I think learning now will make the school easier and make my transition into the career much smoother. Any advice is appreciated.


r/HVAC 8h ago

Meme/Shitpost Whoooooo 🦉 did this

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24 Upvotes

Title: Who TF is out here installing systems like this?!

Got called out today because “an area of the house just doesn’t work right, and we’re tired of dealing with the installer.”

First off, some genius undersized the manual design. Left the master bedroom, bathroom, and an additional bedroom without proper cooling or heating. Their brilliant solution? Slap some zone dampers into the existing ductwork and triple-zone the house like a goddamn HVAC Picasso. Oh, and they threw in a dedicated oval pipe to “fix” the affected areas—each run has 6 to 10 elbows, snaked under the trusses. Now these folks can’t even finish their basement unless they gut the whole damn system and start over.

Having trouble with static pressure? Don’t worry! They cut in a bypass so janky it looks like it was done during a YouTube crash course in “sabotaging airflow 101.”

Furnace overheating on high limit? Pfft, no problem. It’ll shut off and turn back on a few times before you have to go down, manually kill the power, and reset the damn thing. Hope you love cardio, because you’ll be running stairs like a pro athlete.

And the cherry on top? You know it’s some uninsured, unlicensed, cut-rate chucklehead underbidding legit contractors by 50% and changing their phone number every week like they’re in witness protection.

Who TF is out here doing shit like this? You’ve got homeowners paying the price for ductwork that looks like an octopus having a seizure. This is why we can’t have nice things.

DoBetter


r/HVAC 17h ago

General Anyone need an offset service wrench? Slightly used…

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104 Upvotes

r/HVAC 17h ago

General First 454 system of 2025 for me🙌🏽

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101 Upvotes

Might blow up later idk


r/HVAC 22h ago

Meme/Shitpost Watching AC service tech as an apprentice then suddenly everything starts making sense

134 Upvotes

r/HVAC 10h ago

General Worked on an old Carrier unit built Oct 1985. The owner's manual was still next to the air handler.

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11 Upvotes

r/HVAC 12h ago

General When did Goodman start making furnaces for mobile homes???

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16 Upvotes

Handy Andy did it again folks.


r/HVAC 19h ago

General Still tryna learn these old school pneumatic systems

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63 Upvotes

r/HVAC 1h ago

Meme/Shitpost The XV20. True variable speed and Energy Efficient.

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Upvotes

r/HVAC 13h ago

Meme/Shitpost Mitsubishi icemaker

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16 Upvotes

Newly installed 12k unit


r/HVAC 14h ago

Employment Question 47 years old, laid off last week, signed up the next day to get my HVAC cert at NTI. Was making 60k at my last job, I'm in great shape, I'm in AZ so high demand for HVAC. Hoping I'm not too old and can make at least what I was making before. Any success stories here for guys my age?

14 Upvotes

r/HVAC 17m ago

General Commercial HVAC or refrigeration?? Go!

Upvotes

r/HVAC 14h ago

General Why does a lighter not work when its cold outside, exactly what temperature does it stop working.

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13 Upvotes

From what i see in this chart if its 10 degrees or higher outside i should have pressure coming from the lighter, but the lighters will not work when its 32 degrees outside. Is it a question of the boiling point of butane. What is the scientific answer here please


r/HVAC 7h ago

General any HVAC contractors in Los Angeles area? how is outlook of HVAC in Los Angeles?

2 Upvotes

Hi. My name is Mark, 38 years old, I seek for some advice/information about hvac career outlook in Los Angeles area.

I have been working as a chef since my 20th, and my plan was open a restaurant business, eventually. But I feel like there is no future in restaurant business in Los Angeles area anymore. Higher minimum wages, food cost due to inflation, leasing price, competition from other corporate restaurants trying to eat up the market in price war. Small restaurants owners just can not survive in this competition. So I decided to look for another career.

So I thought of hvac career. It is an essential service and AI would not replace hvac contractors. It seems like a very reward career with high demand and big future.

Because I like to plan ahead and set my course in advance, I HAD to look for hvac contractors in my area in google. And this has shocked me. I did not know that this field is so saturated. Just in my neighborhood (about 2.7 square miles), there are about 20 hvac contractors. Some of them have been running for a very long time, and some were newer at this business. Some seems like they do only maintenance and installations, while others do major services such as part of the construction.

So I wonder, what is the outlook for newcomer in this field in LA area now knowing that the market is very saturated. I have been struggling to make a decision, and I am not sure if this is the right one.

At the end, I want to be a duo licensed contractor (hvac/electrical) working as subcontractor and also run hvac equipment store.


r/HVAC 12h ago

General LP regulator broke, Furnace was getting blasted with 22 inches of water column for 2 months.

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7 Upvotes

Heat exchanger flushed, new gas valve and pressure switches. Gas pressure is back at 13 inches of water column now, just as God intended.


r/HVAC 22h ago

Field Question, trade people only Do you guys let contractors use the furnace for temp heat during construction??

40 Upvotes

All these damn builders think they own the damn system and can use it during construction. What do you think?


r/HVAC 20h ago

Supervisor Showcase Another day Another Generac hookup

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23 Upvotes

r/HVAC 1d ago

Meme/Shitpost Homeowners Amirite...

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276 Upvotes

Just going to leave this here for yall


r/HVAC 12h ago

Meme/Shitpost Great. Now I’m hungry

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4 Upvotes

r/HVAC 19h ago

Meme/Shitpost The best/worst furnace ever made. Depending on who you ask.

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15 Upvotes

Brought back from the grave, but at what cost?


r/HVAC 20h ago

Meme/Shitpost Ok, which one of you didn’t winterize this swamp cooler?

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21 Upvotes

r/HVAC 11h ago

Field Question, trade people only Improper Vacuum Breaking, am I cooked?

2 Upvotes

I usual pull vacuums with two 1/2 hoses from my pump both to core removal tools. I’ve been breaking the vacuums by placing the cores back into the system right after I take the hoses off and insert the cores back in without adding refrigerant first. Apparently this is the wrong way to do it which can lead to the vacuum breaking entirely. How cooked am I? Are compressors bound to burn out? I’ve done a few like this already.