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.
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 seenthispost, 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.
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.
I know better now but when I first started as an install helper I didn’t know about recovery I just knew ductwork. The company I worked for also didn’t care to spend money or take the time on recovery. First unit I ever changed out had a bad compressor. He told me “since we can’t pump it down we will have to recover it so close the valves and I’ll be back” he comes back with one of those big empty monster energy drink cans and said “I call this a monster recovery” fills it up with water and starts venting the Freon into the can. I asked him if that was the right way and he said “do as I say not as I do. Our boss is too cheap to buy another recovery system for the install crew to have one and not just service so this is what we have to do, watch some YouTube videos of the proper way when you get home”.
Now I work for a company that actually gives a shit about following laws and regulations I haven’t had a moment to say this other than like changing the adjustment valve on a gas grill without turning the gas off. Have you guys?
Do any business owners or techs have a cut off time for any service calls or just field work in general? I’ve been really giving this some thought and just wondering how good is it really to keep working nights and being away from family. Anyone have intake or experience?
This is a super positive post for my people out there struggling. It’s possible to stop getting bent by your current employer. I was being paid 22.60 6 months ago with around 3 years of experience doing light commercial work. I officially start Monday and my hourly will be 35.47 an hour with a union. Quit letting people take advantage of your hard work and dedication and go chase the money you deserve.
6th year in and this is the first time I've seen this type of corrosion. The compressor is the same age as the unit, 2010. So original compressor. It's a 2 stage ASXC Amana unit. It's flaking off in layers at the suction line on the compressor.
Been looking at the Malco turbo shear and want to know if this can really mostly replace aviation snips for cutting duct work and what not. I hate aviation snips. I hate cutting panning. If not these is there any other alternative that is easier?
I preface this by saying I’m only a 2nd, almost 3rd year apprentice, but I’m coming up on the time where I really gotta decide if this is what I truly want to do and if I’d realistically be able to make it as a service tech. When I started I was as green as green can be and I truly feel I’ve come a long way, but it always feels like it’s never enough for any employer I work for. I had a difficult start with employers getting slow and getting laid off. It definitely unmotivated me then my father passed away from cancer which took its toll for a long time. Basically I feel like I’m not good enough when I compare myself to others who are also 2nd/ 3rd years and I’m under a lot of pressure from my employer to get ahead of the game so I can be sent out on my own. I understand that as I’m starting to get too expensive to simply keep pairing me up with other guys to learn.
I broke the gauge off the top of my manifold trying to take it off to replace it. The brass threaded part is stuck in the manifold body and i need to get it out. I tried to use a screw extractor to take it out an still couldn't twist it out.
Just wondering if anybody else had this issue and how to remedy it.
The only thing that could possibly work would be to try to drill out the remaining brass and maybe re thread the aluminum manifold where the gauge goes in. Its an old refco 4 way manifold used daily and was pretty pricey back in the day when i got it. hoping its not a lost cause
Cleaning out bag after snow storm here in the north. I've slowly moved to service after installing for a few years so it's a bit of a mixed bag. Probably carrying too much but at least it's lighter then my install bag! Hopefully after another week I'll find another tool I haven't touched and can move it out to make it more efficient.
I am going to be a new tech coming straight out of trade school in 2 weeks, I have a job offer for Commercial Refrigeration.
I did an over the phone interview yesterday & a in person interview will be scheduled later next week.
Benefits are $19-21/hour (starting no experience)
$1,000 sign on bonus
Company vehicle supplied as soon as I’m done with training
1 week paid vacation year 1, 2 weeks in year 2
Medical, Dental & Vision insurance
401k 5% match day one of employment
No less than 40 hours a week aswell
21 & in Illinois, I feel like this is good starting out. What do you guys think?
When I'm browsing jobs and it says Refrigeration Technician, is that in reference to refrigerated appliances like a refrigerator itself and.. water coolers, these types of appliances ? How often do you all find yourselves working on these types of systems in a residential setting ? I think I could carve out a nice niche for myself repairing refrigerators in homes and perhaps even in small convenience stores, maintaining those systems but as I think.. if a refrigerator's problem would mostly come from its compressor, I'd imagine a homeowner would just purchase a new refrigerator.
So our company has decided to switch to pro press exclusively from here on out, and though we buy all our own tools (gauges, torches, vac pump etc) this is one that I feel they should be buying.
These things cost around $3,300 and they said each crew of 2 will be having $100+ a check taken out until they are paid off and then will be ours.
So in the event of someone leaving, one party will either have to forward the $1,650 to the one that wants to keep it or agree to take a loss since we can’t both own it, same if it’s broken, I’m supposed to front another $1,650 cause my partner broke it?
Nor do I want one, I am perfectly fine brazing like we have for years, and if the company wants to make a decision that we are strictly pressing our fittings from here on out, I feel they should foot the bill.
This is a nexstar company that made $50mil last year so forgive me for feeling wronged they expect me to casually cover the cost for a $3,300 tool.
I just started at a new company this week doing resi/light commercial install and service. Yesterday I watched the lead I'm shadowing carry a 70lb Tech Pac up 4 stories to change a bad thermostat. I can't imagine bringing 14" pipe wrenches and sheet metal tools to diagnose something. That seems to be the standard here, but it got me thinking about how others breakout their tools.
Do you have a small diagnostic pouch and a larger service bag for additional stuff? Does your install bag contain everything or do you break it down by gas, sheet metal, etc.?
I was out for a job at a place called Del air. When I originally apply I applied for a residential apprentice. I went to the interview and they were talking more along the line that they needed a runner to pick up stuff in the mornings and say after that, I could get some training in Now they have extended the job offer to me, and I am worried that if I take it, I will be stuck only doing that stuff and they won’t train me. Do you think that they’re just selling me a dream because they need a body?