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  1. #1
    I live in southern AZ. my house is about 6 years old. During the summertime, when the outside temp gets up around 110 or so, the unit runs all day and the temp indside never gets below 80. this is our first summer in the house. Two HVAC techs have checked out the unit (carrier split system) and found nothing wrong with it.

    Airflow hasn't been tested but feels a little weak to me. One tech though our flex ductwork may be reducing airflow.

    1400 sf
    3.5 ton unit

    any help much appreciated

    thanks

  2. #2
    Join Date
    Nov 2004
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    SW FL
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    Air flow measure & ESP's Return + Supply

    Originally posted by bennydeez
    Southern AZ
    outside temp ... 110 or so, the unit runs all day and the temp indside never gets below 80.

    One tech though our flex ductwork may be reducing airflow.

    1400 sf ... 3.5 ton unit
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    ESP Return 0. ?"
    ESP Supply 0.___
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  3. #3
    Join Date
    Jan 2004
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    PA
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    30° cooler then outdoor temp when its 110° outside.

    Sounds like its doing its job.
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  4. #4
    Join Date
    Aug 2003
    Location
    Henderson, NV
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    209
    Yes its 30 degrees below ambient outside which is correct, but it still should be able to get below 80 in the home.
    Things to check:

    Air Filter clean
    Are the pressures recorded on any of the Techs invoices
    Split system, evap section in attic or Garage
    Has the techs visually checked the Evap coil to see if clean
    Are the split temps recorded on invoices also, if not take them your self, easy: Return air temp_____/supply air temp out of register___________, there should be at least a 20-23 degree difference or split, example:80 degree return/59 degree supply air temp out of register.Check all air vent temps, if there is a conciderable difference in temps out of different vents , lets say a 8-10 degree diff between 2 or 3 registers, there may be a possible air leak in attic sucking attic air in, remember the attic air is 125-140 degrees out here in the Desert.
    Hope this helps.




  5. #5
    Join Date
    Aug 2003
    Location
    Henderson, NV
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    And just to explain, Beenthere, you are correct about the 30 degree below ambient outside temp rule of thumb, but out in the Desert such as Phoenix and Vegas, if that rule of thumb applied to everyone there t- stats would be set at 85+ degrees, and we all know that isnt comfortable for anyone, so everything is over worked triple time in the desert enviroment, You dont know how many times a day I explain "the way the system is designed speech about the 30 degree below ambient temp" to all my customers,lol, they all expect it to be 70 degrees in there homes so they can Hang Meat or something, crazy!!!!

  6. #6
    Join Date
    Sep 2002
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    3,708
    Originally posted by beenthere
    30° cooler then outdoor temp when its 110° outside.

    Sounds like its doing its job.
    I'm with Been on this one.

  7. #7
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    PA
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    Originally posted by steamer
    Yes its 30 degrees below ambient outside which is correct, but it still should be able to get below 80 in the home.




    Few a/c units are sized to do a 35° difference from the outdoor temp, exspecially at 110° outdoor.

    Just a guess, but that would take about another 12,000 btu's for most units, as there capacity drops as the ambient temp increases.

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  8. #8
    Join Date
    Aug 2003
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    Henderson, NV
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    Well I guess it comes down to Location , Location, Location,, That is probably why I have been reading in other posts in regards to sizing of units, tonage vs sqft, and out here in the Dry desert heat the norm for sizing is 350-400 sq ft per ton, according to posts made by others in other parts of the country, they claim they are sized up to 800sq ft per ton, which coinsides with your theory as to being oversized for the 35 plus calculations.

  9. #9
    Thanks for all your feedback. Here is some additional information that may be helpful.

    The condenser is outside against the north wall of the house (shaded) and the coil/air handler is in the attic.

    The filter is clean and the tech visually inspected the coil which was also clean.

    I don't have the numbers but the pressures were good according to the tech. One tech made the comment that the subcooling temp was intermittently fluctuating. he then verified that the correct sized piston was installed and was free of any debris. The subcooling temp leveled out after the unit ran for a little while.

    According to the tech, I have a 15 degree split at the vent and 20 degree at the coil so I’m losing 5 degrees somewhere along the way. Unfortunately this tech only checked the temp at one vent. The other tech checked all the vents but he was using a laser thermometer which was on the frtz.

    Even on mild summer days, the unit struggles to keep the house cool.

    To my untrained senses, I believe the airflow is poor (ducting problem/leak?)

    Also, the tech who inspected the coil initially thought it was installed backwards (the pointed side of the “A” was facing the incoming air) but then backed off that theory after conferring with a colleague over the phone.







  10. #10
    Join Date
    Dec 2002
    Location
    Houston,Tx.
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    Originally posted by beenthere
    30° cooler then outdoor temp when its 110° outside.

    Sounds like its doing its job.
    I have to go with the been man, just because he has beenthere.
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  11. #11
    Join Date
    Jun 2005
    Posts
    83

    Air Conditioner Running all the time

    Your unit is doing pretty good. 30 degrees below ambient.
    I strongly recommend insulating the liquid line if it is run in that hot attic the liquid could flash before the metering device. Normally flashing or boiling the freon occures after the metering device. This is more a problem on low SEER units, say 5 years ago.
    Now when you go to replace the air conditioner, install the next size larger unit. Have a two stage air conditioner so it will be able to cool the house on those extreme days, and be able to run on low speed at night.

  12. #12
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    Dec 2004
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    Bad advice when you start talking about "rule of thumb" which we all know shouldn't be applied to any system today. I also agree with been. 30 degree delta T is above and beyond. Any more than that, and I don't belive you're talking about simple a/c.

    Keep in mind, that if you were to size the system to keep indoor temperatures 72 degrees when it was 110 outside, you would have an entirely new set of problems the remainder of the time. Never, never size an air conditioning system for the exception, rather size it for the type of weather that reflects the majority of the time. IMHO.

    Again, don't see any reference to load calculations.
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