Page 1 of 2 12 LastLast
Results 1 to 13 of 16
  1. #1

    Your use of SH on sytems with TXVs???

    Curious to know if any of you use superheat in any manner when diagnosing an HVAC? I hear more and more techs using SH on sytems with TXVs contrary to all the books and charts that state SH to be used on fixed and SC on TXVs.

    Is there a chart/table out there that sates target SH for a TXV????

    I am new to this trade and it has gotten very confusing - evryone seems to have many different answers to the same question. I know you cant have both target SC and target SH on the same system at same time. Target SH on a fixed will not be the same on a txv system.

    Ona side note - other than some commercial units - are there many fixed residential systems out there????

  2. #2
    Join Date
    Jun 2009
    Location
    Upstate SC
    Posts
    1,439
    If its TXV then set the charge using subcooling. Monitor the superheat to evaluate for proper operation. Under normal conditions a proper operating system would run 8-15 superheat... however some txvs are adjustable. You need to monitor both to know what's going on.

    Alot of residential stuff is fixed orifice.

    Sent from my ERIS using Tapatalk 2

  3. #3
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,411
    Quote Originally Posted by jdblack View Post
    If its TXV then set the charge using subcooling. Monitor the superheat to evaluate for proper operation. Under normal conditions a proper operating system would run 8-15 superheat... however some txvs are adjustable. You need to monitor both to know what's going on.

    Alot of residential stuff is fixed orifice.

    Sent from my ERIS using Tapatalk 2


    To get the full picture, you've got to measure both SH and SC, along with a few other things.

  4. #4
    Join Date
    Dec 2004
    Location
    Grand Rapids, MI
    Posts
    611
    Don't forget the indoor temperature split per the %Rh
    luck dan

  5. #5
    Join Date
    Jul 2010
    Location
    nebraska
    Posts
    1,629
    To expand on JP's post... The roll of a txv is to maintain a fairly constant superheat. Charge by SC but check the SH to make sure the txv is operating correctly. If it's not then you get to find out why,start with air flow.

  6. #6
    Join Date
    Dec 2002
    Location
    in a house, Appomattox, Va.
    Posts
    3,255
    always check sh and sc, regardless of metering device, otherwise not getting a full picture of what's going on.

    sc for txv, sh for fixed.
    Col 3:23


    questions asked, answers received, ignorance abated

  7. #7
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,411
    Quote Originally Posted by martyinlincoln View Post
    To expand on JP's post... The roll of a txv is to maintain a fairly constant superheat. Charge by SC but check the SH to make sure the txv is operating correctly. If it's not then you get to find out why,start with air flow.
    I would switch your sequence up a little...

    Air conditioning starts with AIR.

  8. #8
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    6,441
    Sub cooling is always a must to check. It tells you many things about the system whether txv, fixed, or cap tube. Air flow is usually overlooked and it is a very common problem. On another note, you are well on your way to being a good tech by stressing over these things. Don't worry too much, you will make mistakes but they are the best learning experiences you will ever have. One day the light bulb will go off and you will understand the refrigeration cycle, it happens to every good tech I have ever met. It seems simple on paper but it's a hard thing to grasp bc you can't see the refrigerant boiling at 38f. We are used to water boiling at 212f.

  9. #9
    Join Date
    Aug 2012
    Location
    Illinois
    Posts
    43
    I am new as well. I have yet to see a unit with a TXV. I work residential and in my climate there are hardly any heat pumps, or residential units with a TXV. 99% fixed orifice. SH still plays pretty strong here but we check and record both SH and SC on our units.

  10. #10
    Join Date
    Mar 2012
    Location
    New Hampshire
    Posts
    84
    Quote Originally Posted by jpsmith1cm View Post
    I would switch your sequence up a little...

    Air conditioning starts with AIR.
    ZING!!

  11. #11
    Join Date
    Jun 2011
    Posts
    229
    A TXV monitors SH as long as there is enough Refrigerant in the system. Always check SH when diagnosing.

  12. #12
    Join Date
    Dec 2006
    Location
    Goodyear, AZ
    Posts
    197
    An A/C with a Thermostatic Expansion Valve (TXV) is charged to the subcooling of the liquid line leaving the condenser because the superheat is fixed by the TXV. The superheat is specified by manufacturer and is normal between 14 degrees +- 4ΊF in most residential air conditioning systems. Subcooling is the amount of liquid held back in the condenser. This allows the liquid to give up more heat, below saturated pressure- temperature. For every one degree of subcooling at the same condensing+ pressure, capacity will increase .5 percent. Increasing subcooling with an increase of discharge pressure and compression ratio, decrease capacity. Add 5 degrees of subcooling for every 30 feet of liquid line lift to prevent the formation of bubbles, flash gas due to the pressure drop.

    Roger

  13. #13
    Join Date
    Dec 2002
    Location
    in a house, Appomattox, Va.
    Posts
    3,255
    on airflow, I've seen some systems that teh txv was set for only 6-8 sh. with low airflow you could never get sc, and the coil would just run at saturation.

    recheck airflow, correct issues and could get the readings good. not much wiggle room on that low a sh setting it seems.
    Col 3:23


    questions asked, answers received, ignorance abated

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event