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    subcool (32 Posts)

  • Paul S Paul S @ 10:17 PM
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    subcool

    Hey everyone....what would cause a high subcool on a under charged system....its a r410a system...very low load that day it was 50f outside...return air was about 65f....txv system...I don't remember the exact pressures....95 psi lowside....200 highside....17f subcool....just would like to know....was it the low load conditions?....temp split was 12f....thanks Paul S......ohh coils were clean....brand new install
    This post was edited by an admin on April 12, 2013 10:19 PM.
  • Techman Techman @ 9:28 AM
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    subcool

    I think that it can't be subcooling because subcooling is a drop in temp of liquid freon.A system very low on freon would have lots of activity in the sight glass/liquid line.
  • Techman Techman @ 2:50 PM
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    The Shrinkage Factor.

    What application is this for? Does the AC mfrg require some sort of minimum head pressure control? Most AC systems are properly charged to a minimum ambient of 70* and that properly charged unit will show an undercharge of freon  at 50* ambient due to the shrinkage factor.
  • unclejohn unclejohn @ 10:54 PM
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    Sounds like

    A bad TEV not letting refrig through and causing refig to back up into condenser.
  • Spence Spence @ 3:37 PM
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    High SC

    Sounds as if you are slugging liquid with your high sub cooling and low TD at the evaporator.  Your unit is not designed to operate below 55 degrees, so go back on a warmer day.  You may have to create a load by running the furnace for a half hour or so if the ambient is still around 60 degrees.  Hint: weigh in your charge per factory specifications in cold ambients, then go back in May to fine-tune.  Never use a sight glass with R-410A.

    When you go back, just to make sure all of the moisture was taken out at installation, do a static test. Hook up your head pressure gauge (the system must be off for at least 20 minutes) and check the ambient temperature. It, and your head pressure converted to temperature, should match. If your condensing temperature is higher than ambient, you may have non-condensables in the system, such as moisture or remaining nitrogen. If you do, the 1st thing that will occur is high sub cooling. Remember, R-410A POE oil is a sponge!
    This post was edited by an admin on April 16, 2013 3:55 PM.
  • Techman Techman @ 9:11 AM
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    No moisture indicator/sight glass ?

    I'm curious about that! I have installed MI/SG on every regular  split system that I worked on . That includes new installs and service on other's . Generally I do not put MI/SG on ductless splits ,but ,I have on some. I am not aware of any problems that I might have.
  • Spence Spence @ 10:37 AM
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    Sight Glass

    The problem is not you. It is others who use them as charging devices. I cringe when I hear someone who charges a system to "a clear sight glass". Yikes! Plus, since it is so hard to get all of the moisture out of an R-410A system, I rely on the LLD temperature drop as a moisture indicator. As pipe joining appears to be a lost art, one less component means two less potential leak sources.
  • Techman Techman @ 10:53 AM
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    Temp drop of LLFD

    Hi Spence! It is my understanding that moisture alone does not cause a press/temp drop across a F/D as compared to "debris" causing a P/T drop. I have replaced lots of F/D due to a "caution or wet " indication of the Moisture Indicator although I didn't notice a temp drop of the LLFD ,but,honestly I wasn't looking for said temp drop.I would also think that having any ammount of moisture floating around the system will create acid and kill the comp.
    This post was edited by an admin on April 21, 2013 10:56 AM.
  • Spence Spence @ 11:48 AM
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    LLD Drop

    Don't forget that temperature drop also means pressure drop. Since we can't see inside the drier, we can assume the drop equals either moisture or some other nasties. Either way, it's bad news for the system.
    This post was edited by an admin on April 28, 2013 12:01 PM.
  • drhvac drhvac @ 8:50 AM
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    good idea

    Spence that is a good idea to weigh in the charge on a cold start up. It is so difficult almost impossible to get it right when it is cold out and there is no load on the system. I tried charging a new American Standard system with a TAM7 air handler the other day and a couple of times while the system was running and I went inside to do something, when I went back out the unit was off. I think the freeze sensor on the air handler was kicking it off. However you can not get an accurate subcooling reading when there is no load on the system. Weighing in is a good idea.
  • Spence Spence @ 12:10 PM
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    Weighing In

    Good for you! Weighing in with the correct amount of refrigerant for your lineset means you could be mere ounces off instead of not even being in the ballpark. Your outdoor unit comes with a charge for it, "x" feet of tubing, and the smallest AHRI matched coil. If you have used a larger evaporator to up the EER, then you know right away your charge is a little off, yet not enough to cause poor performance until you can return in the right weather. Remember we said earlier to create some indoor load if you have to. Turning on the furnace is all sensible load, so while you check your sub cooling, look at your superheated as well. Both within acceptable range means job well done, and the unit will be happy under any condition.
  • Techman Techman @ 9:18 AM
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    Charging to a full sight glass

    I just have to ask, under "normal conditions" why can't I charge to a full glass?
  • Spence Spence @ 12:00 PM
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    Sight Glass Charging

    R-410A operates at pressures 70% higher than we are used to, so a clear glass could easily mean overcharge. The best method is RTM (read the manual) and introduce refrigerant the way your manufacturer wants it done. Then use his charts in the unit to make your system happy. A sight glass won't tell you that you have 10 degrees of sub cooling if that is what your unit needs. What is sub cooling? It is to ensure a 100% column of liquid at the evaporator, yet too much liquid means you know what.
  • Techman Techman @ 6:27 PM
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    Sight Glass

    I believe every mfg of TXV's requirers a "full stream of liquid" entering the TXV to prevent errorion of the TXV vavle seat. I agree that a full glass does not mean a SubCooling of 10-15*, nor does it mean that 35' away at the TXV that the liquid line is "full". Full at the TXV is required.
    Sure R410a operates at a higher press than R22 or R502 or R12 or R500 or R409 or R134a or most "common" refrigerants that we commonly use, but, the saving grace is that they ALL (KindaSorta) operate at the SAME TEMPERATURE as far as WE are concerned!!!!! Right or Wrong?
    This post was edited by an admin on April 29, 2013 6:33 AM.
  • Spence Spence @ 7:20 PM
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    Sight Glass

    The 100% column of liquid your TXV is looking for starts at the exit of the condensing coil. If you don't have it there, it won't magically appear on its way to the TXV. That is what sub cooling is all about; ensuring there is no flash gas in the liquid line and keeping the flash gas that naturally occurs during the metering process within a tolerable percentage. With respect to refrigerants operating at the same temperature, your liquid line would be the same temperature if the pressure is 115 PSIG for R-22 and 254 with R-410A, but I am confused as to how this apples to the use or non-use of sight glasses. Again, if your manufacturer wants you to use one as a charging device, then you are an RTM technician and bless you for it. Nationally, 72% of all installations do not have the correct charge, so we owe our customers and our industry a healthy dose of "get it right."
  • Techman Techman @ 5:39 AM
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    Sight glass

    I don't recall very many AC mfrg's telling us to charge a system to a "full glass" and your done,as compared to checking the SubCooling (TXV).I also don't recall too many AC mfrg's telling us to use a SightGlass/MoistureIndicator at all.Having a system running with an almost perfect S.C. does not mean the L.L is full up there in the attic of a three story house.All mfrg's of SG/MI show picturers of the SG/MI on the outlet of the filter drier, and the preferred location for the F/D is right before the TXV.Being a lazy mechanic I put my SGMIFD at the cond unit. But I do check for a "full"liquid line at the TXV.The temperature being the same for all of the freons only means this, operating at 95* condensing temp on a 72* day,the hiside press is, r22 = 180psig & r410a = 295psig & r134a=114psig. Sure you don't have to put in  a SG or a FD for that matter, lots of guys don't,but I put in LOTS of SGFD's.For me, the added benefit is that MI part of the SG.And on a PM inspection if there is even the slightest activity in any of my SG's I know I'm going to be leak checking, long before i get the "no cooling" call.So I charge a system to a full glass and then some ,before any SC/SH readings,but, that's just me, and my fellow wokers.
    This post was edited by an admin on April 29, 2013 6:21 AM.
  • unclejohn unclejohn @ 10:53 AM
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    If you charg to a full

    Sight glass on start up, on a hot day with a big load on the evap you are over charged. I don't normally use a sight glass on A/C mostly refrig. systems. I chagre to sub cool after a pull down and the evap tep at or less then 40* F. So there you have it 9 different ways and they all work.
  • Spence Spence @ 11:24 AM
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    If You Charge......

    I agree that many methods may work. However, I still maintain that best method is the one your manufacturer wants.
  • drhvac drhvac @ 6:01 PM
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    weigh in

    If you weigh it in, you will always be right, It doesn't matter if its too hot or too cold.
  • Techman Techman @ 8:03 PM
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    OK Mister Nice People,

    I would like to thank you for lableing this particular AC unit as having 45.32451 oz of freon R410a , so I can recover whats left and put back in the exact(or close) ammount of new R410a and be on my way! Otherwise I would have a little bit of figuring out to do, to know what to do, on this FRIDAY EVENING at 8:30PM with my wife sitting in the service van, THANK YOU!!!!!!!!!!!!!!!!!!!!! Really?
  • drhvac drhvac @ 7:17 AM
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    no

    techman I'm talking on a new install which is what this post is about. The original ask about charging by subcool on a cold day. Spence mentioned about weighing it in. I agree with him, whether its 60 out or a 100, if you weigh it in you will be right there. Charging with a sight glass went out with the twist brother, plus they are another thing to leak down the road, especially with R410A. They are more for refrigeration jobs, not the high efficiency AC equipment that is out there today.
  • Spence Spence @ 9:22 AM
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    No

    Perfectly said; end of story. Now, if you change your login name to Chubby Checker,........
  • Techman Techman @ 2:06 PM
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    subcool

    Just for giggles, lets say Paul S and I work for the same co. and I got the service call for not enough cooling on that Friday evening. According to some of you I would HAVE TO WEIGH  IN THE FREON CHARGE. So thats the ONLY option I have? How many of you guys weigh in the freon and mark the unit with the ammount of freon that is required? How many of you guys do refrigeration work where we have to know how to properly charge a system in - 10*f  and +10*f and 100*f. AC work is so much different? What about AC for computer rooms and/or Fire Dept dispatch rooms that require AC in ALL different ambients? How many check for a full L.L. at the TXV after the "correct" charge was weighed & put into the unit?
    This post was edited by an admin on May 5, 2013 10:42 PM.
  • Spence Spence @ 11:47 AM
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    Sub Cooling

    I believe there has been some misunderstanding here, and I will certainly blame myself for not communicating well. It was my impression that Paul's question involved a new air conditioner or heat pump installation (not a refrigeration unit) and you can follow that train of thought through these posts. For a "no cooling" call in which you have eliminated other possibilities (refrigerant should always be last on the list) and believe the malfunction is charge related, my part of this session was my firm belief in charging a unit via the manufacturer's recommended method, period, whatever that method may be. Hence the charts inside the unit. In response to your question as to how many of us check for 100% liquid at the TXV, the answer is everyone of us who checks their sub cooling. Yet that is not a stopping point; even with a TXV one should still look at the superheat to ensure health and happiness for the compressor. If the charts are gone on that Friday night call and you're at 10 degrees of sub cooling and 15-18 degrees of super heat, at least you know you're in a safe enough range until you can obtain the maker's data. If you have a fixed metering device, the super heat calculators are OK provided you know the indoor wet bulb temperature.
  • Spence Spence @ 11:48 AM
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    Sub Cooling

    I believe there has been some misunderstanding here, and I will certainly blame myself for not communicating well. It was my impression that Paul's question involved a new air conditioner or heat pump installation (not a refrigeration unit) and you can follow that train of thought through these posts. For a "no cooling" call in which you have eliminated other possibilities (refrigerant should always be last on the list) and believe the malfunction is charge related, my part of this session was my firm belief in charging a unit via the manufacturer's recommended method, period, whatever that method may be. Hence the charts inside the unit. In response to your question as to how many of us check for 100% liquid at the TXV, the answer is everyone of us who checks their sub cooling. Yet that is not a stopping point; even with a TXV one should still look at the superheat to ensure health and happiness for the compressor. If the charts are gone on that Friday night call and you're at 10 degrees of sub cooling and 15-18 degrees of super heat, at least you know you're in a safe enough range until you can obtain the maker's data. If you have a fixed metering device, the super heat calculators are OK provided you know the indoor wet bulb temperature.
  • Techman Techman @ 5:02 AM
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    subcool

    Spence, are you saying that w/ a good subcooling reading at the begining of the LL that under ALL conditions the end of the LL is ALWAYS full?
  • Spence Spence @ 12:03 PM
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    Sub Cooling

    You are exactly right, provided the following conditions apply: someone has not installed a longer lineset than the factory allows without making approved adjustments; you do not have more than 5 uninsulated feet of LL in an attic, your sub cooling temperature is within the factory's happy range, and your superheat is within the designed range. It is a piece of cake to check just to reassure yourself. If it is 90 degrees outside, then the LL is going to be roughly ambient temperature at the entrance to the evaporator. If the two temperatures (1 at the service valve and 1 at the ID coil) have a substantial difference, you may have a crimped or undersized LL, or any of the above conditions. That is what sub cooling is all about; making sure the TXV gets a mouthful of liquid ONLY. However, I will again stress that the refrigerant charge should be the last thing on your list. Clean ID and OD coils, clean filter, duct pressures within the correct range should ALWAYS be before grabbing the gauges.
    This post was edited by an admin on May 4, 2013 12:26 PM.
  • Techman Techman @ 10:53 PM
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    subcool

    I know i'm a little different than some in somethings, so here goes. Getting a set of gauge readings is usually the first thing I do. I see from the get go how this system IS running as compared to how this system IS SUPPOSED TO BE running.If my gauge readings are "normal" I have nothing to do with tape measurers or Ductalators. So what I'm an oddball , I straighten out lots of screwed up systems and I have fun doing it!!! Even if an extra 20' of suction/liquid  line was added ,that changes the SC reading so the unit can't be charged? That 20' gets added into the weighed in charge so it shouldn't make any difference. Don't forget to mark the unit w/ the charge weight for me ,my friend! So out of the 8-14* of SC at the cond unit ,how much is  gone by the time the freon reaches the TXV? I like that "insulated LL in the attic" thing. So by you doing that , and most don't, does that make you "odd"  or one of the few "normal"  techs out there?
    This post was edited by an admin on May 5, 2013 11:15 PM.
  • Spence Spence @ 8:50 AM
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    That Sub Cooling Thing

    Provided your lineset is sized within factory tolerances, if you have the correct sub cooling, you're a winner. It doesn't go any farther than that, yet again, check your superheat too. Weigh-in, as we have discussed earlier, is for the smallest AHRI coil. It is easy to add refrigerant for longer linesets, yet you don't know how many ounces you may be off if there is an up sized evaporator. Your sub cooling and superheat checks will tell you that. I don't believe you're oddball at all. I take a firm stand against field-invented charging methods such as sight glasses, beer can cold, or charging to a 40 degree evaporator or a certain TD. Our customers and our brother and sister technicians deserve better than that. I mean no disrespects either, but this wall should be an educational tool to keep us all at the top of our game. By the way, regarding LL insulation in an attic: if you have more than 5 uninsulated feet, you're going to create flash gas, and tons of it. So dump away with refrigerant trying to get your sub cooling in line, and you have changed your condensing coil into a liquid receiver. Yikes! I understand your thoughts by putting gauges on first, but allow me to present this: if your airflow is not right, your charge will NEVER be right, and there is no duct calculator involved because that won't tell you anything because you can't read a duct calculator directly. It's all about pressure!
  • Techman Techman @ 5:21 AM
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    subcool

    This post was edited by an admin on May 8, 2013 6:12 AM.
  • Techman Techman @ 5:22 AM
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    subcool

    Using a piece of cardboard to control the MINIMUM HEAD PRESS on a cool/cold day is right out of my R.S.E.S. manuals.Nothing field invented about that.Your LL has to be full,so does mine . Your system wants 12* SC ,so does mine. I get to watch my LL filling up, you can't. I get to watch my SG be full ,you can't. I get to check for excess moisture in my systems, you can't.
    My longest running comp(cap tube) was around 30 years old . Luck? Skill? Another customer had gone thru 6-25hp Copelametic Comp in 12 years,then I got the my foot in the door, and my comp lasted 18 years.After 18 years we turned off the disconnect and put in 5 small systems. Luck?Skill?
    The bottom 2-3-4 passes of the cond coil IS A RECIEVER.It sure acts like a receiver! As the TXV opens/closes the extra Freon has to come from/go to someplace.
    Low air flow shows up as a low, lowside press on my gauges.So inside the bldg I go . Filters, ducts, dirty coil. I'll get it. If the air flow is wrong and my charge was "weighed in" my pressure readings will be "NOT NORMAL", so there is something wrong somewhere.
     I don't loose compressors. Luck?Skill?
    This post was edited by an admin on May 8, 2013 6:20 AM.
  • Techman Techman @ 7:49 AM
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    longevity

    Just got back from a quick service call on an ice machine. While there I looked at a compressor that I put in WAY back in 1987 and it is still running.SH and SC was applied back then too. Luck?Skill?
    This post was edited by an admin on May 8, 2013 7:50 AM.
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