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  1. #1
    Join Date
    Aug 2012
    Location
    west burbs,il
    Posts
    145

    subcool/ superheat troubleshoot

    I've been in the trade for several years now and I go thru the forums reading comments of guys screaming at others to post their superheat and subcool readings because its the ONLY way to solve the problem, or saying that these readings will tell you exactly what is wrong with an a/c system. Honestly the only thing I use subcool and superheat for is to charge a system. Whats subcool and superheat gonna tell you that your pressure readings won't? Of course now I can see refrigeration but not a/c work. I'd like see to your guys opinions because to me taking the additional time to take these temp readings and still hunting down your problems seems like a waste of time. I personally only take average of 15 minutes to figure out the problems, worst cases with multi issues maybe 30 min. I'm interested in your guys feed back and seeing how subcool and superheat really help you in your efforts of finding the solutions with a/c issues.

    " Tis easier to get hands dirty than to keep clean"

  2. #2
    Join Date
    Jul 2005
    Posts
    5,571
    I have to wonder how many systems you've left running with a partial restriction, and how many you've left running with an undercharge or overcharge. Both are common problems, especially the latter. How do you know the TXV is operating correctly without a SH reading?

  3. #3
    Join Date
    Aug 2012
    Location
    willowick ,oh
    Posts
    261
    Don't worry. One day you will realize that these are part of a complete system checkout and troubleshoot. Air conditioning is no different than refrigeration. Refrigeration just has more controls and use various refrigerants due to the temp ranges. Lower temp applications typically use a lower sh range than a/c. You should realize that any real refrigeration issue can be found with the help of these readings. It doesn't matter what temp application it is for. Get used to doing this on a regular basis. I know it takes time but it makes you a better tech than the guy before you or the guy who trained you.

  4. #4
    Join Date
    Apr 2006
    Location
    Toronto
    Posts
    275
    SC and SH tell you a whole lot of what is going on with a system. How do you diagnose refrigerant related issues without them? Get a book called System Diagnostic and Troubleshooting Procedures and start reading.

  5. #5
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    18,571
    These readings tell you the what and where and how much of the refrigerant in the system.

    How much refrigerant is in the condenser? How much is making it into the evaporator? Is either value "enough" or is it "too much?"

    You will NEVER know what a system is doing using pressures alone.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist

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  6. #6
    Join Date
    Apr 2012
    Location
    chicago burbs
    Posts
    75
    Touchy feely days are gone. I think since txv's came along for better efficient systems its much more important to be exact on measurements all around.

    Sent from my SPH-D710 using Tapatalk 2

  7. #7
    Superheat and subcooling tells you where the liquid is and where the vapor is. You can tell if you're stacking liquid in the condenser or if you're starving the evap. Pressures alone can't do that. Pressures will vary with the load. It's like Dan Holohan says, "You have BE the refrigerant." Well, he was talking about hydronics so he said, "you can't see the water in the pipes so in order to know what it's doing you have to "be" the water,... it's a zen thing."

  8. #8
    Join Date
    Jul 2012
    Location
    Western KY
    Posts
    1,221
    while I can agree that alot of AC problems can be diagnosed and repaired without this data it should not be dissmissed so arrogantly. The only way to know what is actually going on in the system is to gather and analize as much data as possible. I believe you can find and fix a problem with out them but are you analizing the system for problems? I would have to, respectfully, say no you are not. another thing to consider is that on this forum we cannot see or touch the equipment that posters are asking about. Soo in order to paint a picture of the system in our heads we need as much data as possible. SH and SC will tell you alot about a system if you understand what they are and what they mean. You say you rarely use them I reccomend you start. Check them on every system you touch for the next week, after diagnosing without them the way you usually do. Perhaps you will not find additional problems but maybe you will but regardless you will learn alot more about each of those systems I promise..

  9. #9
    Join Date
    May 2011
    Posts
    54
    Yeah and try explaining to the compressor warranty department how these readings do not have any place in air conditioning. Yes as tommy knocker said that they are needed in the field as a means of setting txv valves but also they are needed to figure out flood back/starve issues leading to premature compressor failure. Most importantly the idea that SH/SC is overkill to you might be true as a lot of diagnosing can be accomplished through touch and feel, and simple pressure readings but walking away knowing your SH is doing the customer, your boss, the equipment manufacturer and yourself a favor. Yeah I used to get furious when the boss started requesting it when you had 20 calls lined up that day and you are about to have heat stroke but I tell him my readings even before he calls or before I even get to the service call. Hahaha JK

  10. #10
    Join Date
    Feb 2007
    Location
    Fremont, Indiana
    Posts
    1,554
    I remember those days of not fully understanding the concept of SH & SC and its place in troubleshooting!!!

    sent from my Samsung Galaxy Note
    Member of the "Work Exchange Program"
    "Will work for knowledge"

    "Everyone is a genius. But if you judge a fish on its ability to climb a tree, it will live its whole life believing that it is stupid"
    A Einstein

  11. #11
    Join Date
    Nov 2011
    Location
    Texas
    Posts
    316
    Quote Originally Posted by hvacrmedic View Post
    I have to wonder how many systems you've left running with a partial restriction, and how many you've left running with an undercharge or overcharge. Both are common problems, especially the latter. How do you know the TXV is operating correctly without a SH reading?
    I would like to put my gauges on a system after you leave.....It will tell you Low charge,restriction, Restriction with overcharge, overcharge, low outdoor airflow, low indoor airflow and much more if you know how to use the values you are looking at. If you dont check SH or SC your not doing your job. Beer can cold is not acceptable anymore. When i went to school they had gauges with white out over the pressures for those beer can cold guys who were stuck in their pressure readings. I hardly look at pressure, just saturation temps. Only time i look at pressure is when i pressure up for a leak search, head pressure, certian occasions. always SH and Sc. It will make your life alot easier, unless you dont have any pride in your work. It really irritates me when a co worker calls asking for help, I ask what his SH and SC are, and the answer i get is, uhhhh i dont know i didnt check.... Im not knocking you man, obviously you are on here asking that question for a reason, your on the right track. Alot of guys dont even care to know why good techs use SH an SC in their diagnostics.
    (((PEACE IS THAT BRIEF MOMENT IN HISTORY WHERE EVERYBODY STANDS AROUND RELOADING)))

  12. #12
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,671
    Lemme guess...

    You're "Old Skool" and you don't need those measurements.

    Probably don't measure airflow, drybulb and wetbulb change across the evaporator, either.


    While you CAN make equipment function by using the "touchy-feely" method, measuring pressure and touching lines, it is not the proper and correct way to do it.

    If it takes you a lot of extra time to measure SH/SC then maybe you need to evaluate your techniques because it doesn't take a lot of time to do, really.

    Once you fully understand what those measurements are telling you, you won't work on a piece of equipment without taking them.

  13. #13
    Join Date
    Mar 2009
    Location
    Mid-Mo
    Posts
    3,595
    Beer can cold and 70 pounds, all day everyday.

    Good example for you....40 ton Mcquay a vintage chiller...our guys have put in 8 or so compressors in over the past two years. I get the call for no ac. I get over there and it's in alarm on freezestat. I start to check things out, reset it, put a set of gauges on and let it rip. It starts to pull down, its running like a 80/250. Both compressors are sweating worse than I was. Sounds good based on your method? Lay the thermometer on it and it's running a 35 degree superheat and a 55 degree subcooling. I think I'm onto something here. Try I adjust expansion valve, no dice. Btw, the outdoor wet bulb was around 70. I get so tired of hearing "well it was sweating back good".

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