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  1. #1

    Strange infrared signature on outside A/C unit


    I'm a residential owner in Houston, TX. I had a thermal infrared energy audit of my home this weekend, and in addition to insufficient insulation, some failed door seals, and a few duct/vents not getting a good seal, there was a weird thermal image of one of my 2 outside A/C units.

    The first, presumably working unit looked much the same in infrared as visible light; the radiator was a fairly solid color, indicating good heat transfer throughout the radiator.

    The second had a relatively cold radiator, surrounding a band of hotness that was right around the horizontal center of the unit. I should get pictures sometime this week, if so I will post them.

    Basically if you imagine the AC unit, and wrapped a red water hose around the middle of it, that's what it looked like.

    Any idea if this represents a problem, or if so what it is?

    The thermal inspector said he'd never seen anything like that before...

  2. #2
    Join Date
    May 2008
    My initial guess would be that the unit is undercharged with freon. The condensor coils are not completely being used due to this undercharge. Are both these units the same age?

  3. #3

    units same age?

    Not sure if the 2 units are the same age. They look to be about the same, I'll check. The house is about 7 years old, we've been in it about 5 months.

  4. #4
    Join Date
    May 2008
    I'd recommend you have both units serviced, if it's been over a year since they were looked at.

  5. #5
    Join Date
    Sep 2009
    Arnold mo
    I'm wondering about non-condensables and how that would show up on infra-red.
    An answer without a question is meaningless.
    Information without understanding is useless.
    You can lead a horse to water............

  6. #6
    Join Date
    Aug 2003
    Fort Worth, TX
    An undercharged system will have hotter discharge off the compressor due to insufficient internal cooling of the compressor motor, oil, and parts from the suction gas, not to mention heat of compression.

    In the OP's case it's possible the refrigerant from the compressor is introduced midway in the coil and distributes out from there, which would explain the indication of high temperature in the unit's midsection vs. above and below it.

    Non condensables elevate condenser head pressure. In this case, even if non condensables were present in the system, a low charge would return the same symptoms the IR scan revealed.

    Hmm...maybe we've stumbled across a quick way to assess refrigeration circuit problems prior to gauging up to a system?
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.

    A building is NOT beautiful unless it is also comfortable.

  7. #7
    Join Date
    Feb 2010
    In a boiler room
    Are both units the same model?

    Would love to see some pics.

  8. #8
    Join Date
    Oct 2010
    SW Michigan, near Battle Creek
    how about a blockeed distributer to the top circut of the coil? that makes the top cold. now we overcharge to get some subcool that makes the bottom cool?

    or under preforming compresser?

    The none condensables idea is the best I think. again with a overcharge or maybe a correct charge crowed ito the bottom of the coil with lots of subcooling.

  9. #9
    Join Date
    Aug 2007
    Northern VA 38 degrees N by 76 degrees W
    It works great for finding restrictions, air flow bypass etc.

  10. #10

    Images showing outdoor AC unit

    Here's the pic. You can see the suction line is not well insulated either.

    Attached Images Attached Images  

  11. #11
    Join Date
    Jul 2000
    Northern Wisconsin
    I'd guess the hot spots on the outdoor coil is where the compressor discharge is entering two seperate circuits of the outdoor coil. By the time the gas gets to the next row it is already cooled down to the discharge minimum temp. From training, the drawn pictures usually always show this happening evenly along the entire coil.

    A guess (from a great distance) is that possibly the AC is undercharged, BUT, that is only a guess. Only way to tell is by having the units (inside and out) serviced.

    If you use the reference chart on the side of each picture the majority of the coil and the small line going back into the house are the same temperature. Never having seen a radiant picture taken of a condenser before I can definately see where they could be helpful in troubleshooting.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  12. #12
    Join Date
    Jun 2011
    whatever that black strap or whatever is you have on the insulation, matches the signature of the photo.
    "I am for doing good to the poor, but I differ in opinion of the means. I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it."
    Benjamin Franklin, 1766

  13. #13
    Yeah, on the top picture, which I think is the other unit (not the one with the hot zone), it's pretty neat, there's a crack & gap in the black insulation over the suction line, and you can see where the insulation is missing, the crack, the connection, fitting, etc., clearly!

    Here's another pic of some ducting issues and other stuff.

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