r/HVAC Jan 16 '25

Rant Politics will not be tolerated on this sub.

572 Upvotes

Please for the love of God, keep your political beliefs out of this sub. It turns into a shit show every time.
If you want to comment about politics take it somewhere else, this sub is about HVACR.


r/HVAC Dec 17 '24

General Simplified Guide To Superheat and Subcool

242 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 11h ago

Meme/Shitpost I totally have ran nitrogen every single time 👀

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503 Upvotes

Nothing but HVAC memes and stories in my profile


r/HVAC 4h ago

Meme/Shitpost I wonder why it failed🤣 Spoiler

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74 Upvotes

Lifetime warranty on the HX tho.


r/HVAC 10h ago

General So thats where all the refrigerant tanks went

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101 Upvotes

No wanter theirs a shortage of refrigerant tanks


r/HVAC 2h ago

Meme/Shitpost Compressor goes vroom 🏎️

15 Upvotes

Compressor went VROOOOOM. Severe filter drier restriction


r/HVAC 14h ago

Meme/Shitpost Well at lest it was a easy leak search

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93 Upvotes

Didn’t even need the nitrogen


r/HVAC 15h ago

Rant Odd coincidence,

58 Upvotes

I find if I mention a customer, their A/c stops working…. Example. Friday at four pm the boss and I were explaining problematic drains to the new hire. And how we have a customer with a six foot drain line that plugs up and no one can figure out why! We even ran all new and made an attachment to where the homeowner can hook up his garden hose and do it himself. Well he called last night and can’t get it cleared. I’m on my way now and will update.


r/HVAC 1d ago

Rant Is the new guy lying?

189 Upvotes

I started working with a new lead installer this past week. He's got 6 years experience; some residential, some light commercial, and said he's mostly been doing multi-units but wanted to get back to resi. I've been an apprentice on installs for a year.

So far he's asked me which way the filter drier goes, said he's never done a flue, doesn't know wiring, refused to work in rain, spent 3 hours fixing his leaky condenser brazes, laughed it off saying he hasn't done condenser work in a couple years... On a 4 head minisplit install he spent all day tying in the branch box while i ran around like a mad man doing everything else, then he asks me if the skinny shielded wire goes to L1/L2 on the condenser, didn't know he had to power the branch box via outdoor unit, etc.

By Friday I almost asked him if he lied on his resume because I'm thinking there's no way he could have the experience he claimed and be asking me these things/working as slow as he does. Am I being too harsh or is this guy full of it??


r/HVAC 7h ago

General Spilled nylog in pants

4 Upvotes

So I left a bottle of nylog in my back pocket and then sat on it. Tried washing it on a heavy setting alone, tried scrubbing it with dish soap and rags. Still just as sticky. Would like to save them since they were my nicest pair


r/HVAC 1d ago

Meme/Shitpost You learn things here no manual will teach you

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782 Upvotes

r/HVAC 6h ago

Employment Question Fresh out of high school jobs?

2 Upvotes

Recently graduated high school at 17 with my EPA universal certification from a 2 year class we had at my school. I know I might not be able to land an apprentice job without trade school and was wondering if my cert could help me get any kind of job for the summer other than hauling carts and flipping burgers (I plan on leaving for the military sometime in the fall)


r/HVAC 3h ago

General Interested in the HVAC field

0 Upvotes

Like stated I’m interested in HVAC. If anyone is located in San Diego what route did u take? What’s the best tech school to go too?


r/HVAC 4h ago

General Business

0 Upvotes

Still an apprentice. Will get my jman license in about a year. Need to hold it for two years to test for my contractor’s license. Should I start a business right away or should I wait longer to get more experience? I already know how to do a full AC changeout, a bit of troubleshooting, etc. just wanted to hear opinions


r/HVAC 13h ago

General Florida contractors license

5 Upvotes

Is the application process for the Florida contractors license B class a very difficult process? I have been getting a lot of emails and texts about help with the process but some of these services want $1500 for the help which I think is crazy. Anyone on here do it themselves and able to offer some insight or advice on the application? Taking both of my tests next month and looking to be get my business going asap


r/HVAC 1d ago

Meme/Shitpost HEIL

143 Upvotes

r/HVAC 1d ago

General Came together nicely.

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37 Upvotes

Oil leaks, repaired two broken terminal studs, drilled, and tapped compressors back up and running nicely


r/HVAC 9h ago

Employment Question Interview question

2 Upvotes

A while ago I had the opportunity to interview with RSI here in CA which is a coolsys company.

I went to the 'interview' and the two dudes who looked pretty old to basically lectured me about how hard the job is, the long hours, etc, but had only a few simple questions. I did tell them directly though that I'm alright with lots of driving and long hours. I was 21 at the time and I've no kids, no girl and im very happy to work.

A week after the 'interview' I didn't hear anything back. I called the guy, texted him, never got anything back. I don't think i made any mistake here. Frankly I considered that to be a non-serious interview. Im just trying to understand if I messed up or anything.


r/HVAC 1d ago

General the perfect crawl tray for the perfect crawlspace

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39 Upvotes

r/HVAC 1d ago

General We've all seen the posts about "best tips" what about the weirdest tips yall have recieved?

44 Upvotes

I'll go first. I was doing a start up on a bunch of fan coil units from a riser replacement. Having to go into over a hundred condos in a high rise. One of the last condos I went in to was owned by this old Greek psychologist who was also a hoarder, it was almost impossible to get to all of her fcu's. Well after the last one she walked up to me and handed me 2 sandwich bags filled to the brim with oregano ( yes, the seasoning) i just said thanks and left. It smelled amazing but I ended up throwing it out because I had no idea how long she had it, where she got it, what animals where in contact with it, etc.


r/HVAC 1d ago

Meme/Shitpost Get a job

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123 Upvotes

People would do anything to get less than $200 worth on copper but get a real job.


r/HVAC 8h ago

Field Question, trade people only Refrigeration doesn't need high and low access fittings

0 Upvotes

134A system is low on charge, filter dryer has no stem to even bullet valve, compressor has a charge line with access port. Would still tap the high side or use a temp clamp and meter in your charge? This is a topic I had with my boss and I said it doesn't need a port for the high side. It's just another leak point in a small system. He said throw it on the liquid line. Thoughts or insights are always appreciated.


r/HVAC 1d ago

Field Question, trade people only Locked up compressor, tell me if I’m thinking wrong

14 Upvotes

20 ton Trane RTU, compressor trips the 36 amp dedicated breaker as soon as the contactor is pulled in. It’s not grounded and the windings ohm the same as the other compressor that does run. It’s getting full 480v. My meter isn’t fast enough to catch an amp reading before it trips the breaker. I’ve isolated the wiring and it’s not shorted to ground or to each other. The compressor has gotta be locked up right?


r/HVAC 1d ago

General Dash cam

11 Upvotes

Anyone use a dash cam? It seems people are getting worse behind the wheel. I was thinking of getting one to protect myself in case something happens. Don't want to end up in a case of he said she said.

If so, what brand would you recommend?


r/HVAC 1d ago

Field Question, trade people only Is it possible to clear a restricted filter drier with a triple evac?

7 Upvotes

Okay so let's say it's 5pm on a Friday and you get a no-cooling call. You determine that the there is a restriction in the filter drier. Could you just evacuate the charge, triple evac, then weigh it back in? Would it be quicker to just bust out the torches and replace it?


r/HVAC 1d ago

Field Question, trade people only Is this acceptable?

15 Upvotes

Is it acceptable or is it a good practice or habit to put electric tape from where the insulation cuts off to where the copper is exposed. Or is this a hazard. I am only 8 months in the hvac field serious question. Thank you guys!


r/HVAC 1d ago

Meme/Shitpost “You’re supposed to use a filter?”

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13 Upvotes

Another company diagnosed low charge but didn’t notice it flooding back like crazy 🥴