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  1. #1
    Join Date
    Aug 2012
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    Katy, TX
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    50

    LENNOX 3-ton marginal delta between return & register temps

    I have a 5-yr old 1,600 s.f. (single story) townhome in Houston, built in 07. The builder-grade LENNOX A/C is a 3-ton unit that until recently had been working fine (conventional box/A-Coil w/TXV, not a slab). (R410A) A couple weeks ago we noticed it not cooling well & unable to achieve 75-degree set point (wife going through 'the change' so greenies pls don't lecture me). Before doing anything I checked the return air (80 deg) and register temp (72 deg) nearest the plenum. Outside temp 95. Obviously something wrong. Amprobe showing 9A which seemed about normal, perhaps a little low, data plate shows RLA is 14A.

    We called the same co. we've used & trusted for years tho they's recently changed ownership. (they also come every spring and fall for seasonal inspection, media filter changes, etc). The tech says low refrigerant, performs leak check but initially finds nothing, later discovers small leak at needle valve where manifold gauges attach. He evacuates the system & replaces an o-ring seal, pumps it down & recharges, adding 2 lbs of coolant. However, only minimal improvement. 80 deg return, 68 at register. Tech says should be at least 15 deg w/18~20 optimum. (Amprobe now showing 12A). System pressure showing 400 psi high side and 140 low side (indoor temp 76). Tech trying to talk me into replacing the TXV with a piston. I think I'm ready for a second opinion. Thanks.

  2. #2
    Join Date
    Jan 2004
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    PA
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    68,759
    Possible slight restriction before metering device?TXV. Did he change the liquid line filter drier when he fixed the leak.
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  3. #3
    Join Date
    Aug 2012
    Location
    Katy, TX
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    No, and I did ask. He said the drier was fine, tho dunno how he determined that. I'm a retired commercial broadcast engr, & former industrial electrician (John Deere) so I know just enuf about A/C to keep my hands in my pocket and watch.

    The high side pressure seems awfully high to me tho and I'm not happy to see the substantial current increase. How likely is it that a TXV might go bad? If it does crap out would it be better to replace it with another TXV or put in a piston instead? Tech said if he put in a piston that I'd also need a hard-start kit.

  4. #4
    Join Date
    Jun 2005
    Location
    SW Wisconsin
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    Okay, the mfg'ers data on a Goodman 13-SEER, 3-Ton, R-410A, 95-F ambient, 80F indoors:

    At a high 1351-CFM of airflow; @67F Wet Bulb or around 50% indoor RH, temp-split is 18F; @71F WB or, 64% RH split is 15F.

    At 1200-CFM 67-WB, split is 19F; @71F WB, or, 64% RH split is 15F.

    Of course that 8-F indoor airflow temp-split is unacceptable.

    We called the same co. we've used & trusted for years tho they's recently changed ownership. (they also come every spring and fall for seasonal inspection, media filter changes, etc). The tech says low refrigerant, performs leak check but initially finds nothing, later discovers small leak at needle valve where manifold gauges attach. He evacuates the system & replaces an o-ring seal, pumps it down & recharges, adding 2 lbs of coolant. However, only minimal improvement. 80 deg return, 68 at register. Tech says should be at least 15 deg w/18~20 optimum. (Amprobe now showing 12A). System pressure showing 400 psi high side and 140 low side (indoor temp 76). Tech trying to talk me into replacing the TXV with a piston. I think I'm ready for a second opinion. Thanks.
    Well, that is a 12-F split which might not be too far off if there was a very high indoor humidity; you have to know the indoor humidity & should also know the approximate airflow CFM.

    A 400-psig is 117-F condenser discharge, @ 95F ambient, that is a fairly high 22F condenser temp-split; which could mean a high indoor humidity or drawing hot air into the Return-Air from somewhere, plus other causes...

    Do NOT replace that TXV; he is missing some other important factors!
    Last edited by udarrell; 08-19-2012 at 12:03 PM. Reason: Left outother factors...

  5. #5
    Join Date
    Dec 2002
    Location
    Houston,Tx.
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    16,121
    Quote Originally Posted by udarrell View Post
    A 400-psig is 117-F condenser discharge, @ 95F ambient, that is a fairly high 22F condenser temp-split; which could mean a high indoor humidity or drawing hot air into the Return-Air from somewhere, plus other causes...

    Do NOT replace that TXV; he is missing some other important factors!
    Darrell, that's because he has it overcharged, from here all I saw was, he might have just needed the tech to check the SC and adjust the charge accordingly, but I have been wrong once before.
    __________________________________________________ _______________________
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  6. #6
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    Jun 2005
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    Quote Originally Posted by Mr Bill View Post
    Darrell, that's because he has it overcharged, from here all I saw was, he might have just needed the tech to check the SC and adjust the charge accordingly, but I have been wrong once before.
    Mr. Bill, you are on target; I was just getting ready to post that quite obvious point to a good troubleshooter.

    If the Return-Air is still at 80F the Ambient at 95F; the condenser temp-rise should be around 17F not 22-F; the suction should be around 130-psig not 140.

    He says the indoor temp is at 76-F; if that were the return-air temp the head should be around 351-psig or 107-F or only a 12F temp-rise; suction 129-psig or 44F.

    Find out what the subcooling (SC) should be & check it; I'm betting it has too much SC.

    Further tests might also reveal some Non-condensibles in the refrigerant system...
    Last edited by udarrell; 08-19-2012 at 12:52 PM. Reason: Missing word...

  7. #7
    Join Date
    Aug 2012
    Location
    Katy, TX
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    50
    Thanks. That's something else he mentioned, possibly slowing down the blower but I nixed that idea because of the wife's hormone issues in my original post. I understand the fan speed logic, but it's worked fine for 4˝ yrs so I'm reluctant to allow someone to alter the mfr's original design (swap TXV for piston). I also don't want to pay out the wazoo for someone to shotgun the system without knowing what the problem is. In this area there's no shortage of Bozos with a license but not a lot of experience. There's also 3 mo. remaining on the mfr. warranty, so maybe time to call someone else.

  8. #8
    Join Date
    Dec 2002
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    Houston,Tx.
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    Quote Originally Posted by Cap'n Preshoot View Post
    In this area there's no shortage of Bozos with a license but not a lot of experience.
    But I thought they downloaded all the experience and knowledge you will ever need, into your head upon passing the test.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  9. #9
    Join Date
    Aug 2012
    Location
    Katy, TX
    Posts
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    I was not there at the time. He told me he pumped the system down to place a vacuum on the line, coil, etc. supposedly as part of his leak check, and held the vacuum for 30 mins w/no loss, eliminating suspicion of the coil. No obvious oil showing up anywhere.

    The system also has an outside "fresh air" return but is electronically damper-controlled. I was there at the time he inspected the fresh-air damper. It was closed. Apparently the T-stat controls it, though I don't know under what circumstances it calls for fresh air. This fresh air thing was one of the "selling points" of the home, stating that it helped hold a positive air pressure and would thus help eliminate dust. ANyway it is closed and temporarily disconnected at the wire nut to ensure it stays closed.

    There are two inside returns, a 20" line coming from a 20x30 return in the hallway (below the stat) and a 12" line coming from a 12 x 12 return in the ceiling of the MBR. Coincdentally the MBR is the farthest run but is historically the coldest room in the house.

  10. #10
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    Quote Originally Posted by Cap'n Preshoot View Post
    I was not there at the time. He told me he pumped the system down to place a vacuum on the line, coil, etc. supposedly as part of his leak check, and held the vacuum for 30 mins w/no loss, eliminating suspicion of the coil. No obvious oil showing up anywhere.
    So he check an existing system with a charge on it, by pulling a vacuum? I wonder why he did not just use an electronic leak detector like most do.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  11. #11
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    Dec 2002
    Location
    Houston,Tx.
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    Quote Originally Posted by Cap'n Preshoot View Post
    This fresh air thing was one of the "selling points" of the home, stating that it helped hold a positive air pressure and would thus help eliminate dust. Anyway it is closed and temporarily disconnected at the wire nut to ensure it stays closed.
    Ok I am not Mr. Rocket Scientist here as for as air flow, but how does it act as a "fresh air" duct and also help to hold a positive air pressure in your home? Maybe one of the air flow experts here can explain this one to me.
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

    "Never let success go to your head, and never let failure go to your heart". - Unknown

  12. #12
    Join Date
    Aug 2012
    Location
    Katy, TX
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    Quote Originally Posted by udarrell View Post
    A 400-psig is 117-F condenser discharge, @ 95F ambient, that is a fairly high 22F condenser temp-split; which could mean a high indoor humidity or drawing hot air into the Return-Air from somewhere
    You may have hit on something here... We recently had an alarm guy in the attic adding glass-break sensors in 2 bedroom ceilings. I just wonder what the chances are he might have accidentally gotten tangled up in a return air line? I guess an easy way to check that would be to see if we have good suction at both return air registers (MBR ceiling and hallway)

  13. #13
    Join Date
    Sep 2009
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    Arnold mo
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    Quote Originally Posted by Cap'n Preshoot View Post
    You may have hit on something here... We recently had an alarm guy in the attic adding glass-break sensors in 2 bedroom ceilings. I just wonder what the chances are he might have accidentally gotten tangled up in a return air line? I guess an easy way to check that would be to see if we have good suction at both return air registers (MBR ceiling and hallway)
    These sensors are installed by cutting a hole in the ceiling? How much insulation was disturbed/pushed aside? Cutting holes in the ceiling and pushing insulation aside will increase your heat load. Some estimate that as little as 7% voids in insulation can reduce its R-value by as much as 50%. A sudden change in a/c performance and someone also happening to be in the attic is definetly worth checking to see if maybe one of the returns have been kicked off the main trunk line.
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