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  1. #1

    Question Return and supply temperature difference

    I recently purchased a condominum unit with a very old air conditioning system (1980's).

    A home inspection revealed a return/supply difference of only 13 degrees. We requested that a licensed HVAC contractor come to inspect the unit (in particular the return/supply temp difference) and were given a brief "it is fine" type receipt. Not very convincing to say the least.

    We had our own contractor come out this afternoon, and the return/supply difference was only 11.9. He said there is obviously a problem with the unit. The real estate contract required a functional unit.

    The HVAC contractor said the return and supply difference should be anywhere from 18 to 20 degrees, and we are way under that. I have also read as low as 15 degrees. I was wondering what the pros consider an acceptable difference? Also, do you have any links to this in writing since we may need to cite a difference?

    Thanks,
    Chris

  2. #2

    Thumbs up

    17-20 degrees

  3. #3
    Join Date
    Dec 2006
    Location
    Modesto CA
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    51
    18-20 is what we go by

  4. #4
    Join Date
    Mar 2005
    Location
    burlington county n.j.
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    9,754
    depends on the unit the humidity and where you are taking temp measurements.

    could be anywhere from 10-20 deg and still operating properly.

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,308
    As stated above.
    Where those readings taken at the coil, or at the registers.
    What were the conditons that day, when the readins where taken.
    How long was the unit running before the readings were taken.
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  6. #6
    Join Date
    Sep 2002
    Location
    Raleigh, NC
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    41
    Quote Originally Posted by t527ed View Post
    depends on the unit the humidity and where you are taking temp measurements.

    could be anywhere from 10-20 deg and still operating properly.
    Exactly.

    Depending on the location of the ducts and air handling equipment, the temperature at the grill and vents may vary a great deal.
    I have seen duct with R-6 insulation in a vented attic gain 2-3 degrees every 10 feet of run, especially if it runs near the peak of the roof. In this situation, I check the temp drop at the unit.

    If the ducts and air handling unit are in the crawl or a basement then the heat gain on the ducts are almost 0, and you can read at the grill and vent.

    Also, if there is a high and low return, it is best to check at the indoor unit to get an average temp on the return side.

    Another factor, as quoted above, is humidity. If there is a high amount of humidity in the home, usually from the unit being turned off or opened doors or windows, the temperature drop will be lower due to the refrigerents energy being used up to remove the humidity. The unit needs to run for 24 hours with the house closed up, except for normal traffic, and then tested.

    I always expect a minimum of 15 degrees difference, but prefer 17 to 20.

  7. #7
    thanks for the responses.

    the temp difference was taken at the unit.

    i am not sure how long the unit had been running, but it has been tested twice and the difference was 13 and 12 respectively. neither day was overwhelmingly hot or humid, typical summer days in the 80's.

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,308
    Might be a dirty coil.

    Did your hvac guy check any thing other then temp difference.
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  9. #9
    Join Date
    Jan 2007
    Location
    Indiana
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    We are taught (present student) 18 to 20 degrees is optimal.

  10. #10
    Join Date
    Aug 2002
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Quote Originally Posted by cd42 View Post
    thanks for the responses.

    the temp difference was taken at the unit.

    i am not sure how long the unit had been running, but it has been tested twice and the difference was 13 and 12 respectively. neither day was overwhelmingly hot or humid, typical summer days in the 80's.

    How much difference btween the return at the unit and the return grill in your home?

  11. #11
    i'm not sure what the difference is at the grill, don't think either HVAC contractor tested it there.

    is there anywhere in writing that states it should be 18 to 20 degrees?

    if the difference is not in that range, is it considered a non-functioning unit? i am asking because the contract said the HVAC must be functioning, and while it is theoretically running, the temperature difference is still too low.

  12. #12
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,308
    Functioning is a broad term.
    And your A/C is functioning. It may not be functioning 100% proper, but your contract may not specify that.
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  13. #13
    Join Date
    Apr 2005
    Location
    new jersey
    Posts
    752
    I am in the same situation with a home inspection. Inspector would run it for 1/2 hour and take reading diff of 65 out/73in 2nd floor main at registers with a infrared.only 8 degrees.I found return connected to a unfininsh area with out insulation in the addition all so very noisey returns. I sealed off return,sealed majer leaks at plenum,adjusted fan to medhigh, doublechecked condenser preasures and adjusted to specs.The realitor turned the systems on 3 days earler and set temp to 72degres. (( Out door temp 90 , indoor temp 72, temp at plenum 58, return 73 )) 15 degree differance. the masterbedroom zone was at 15 differance as well. My concern is the inspector will measure at the register and get below 14 degree differance and want more done to get better readings. 14 to 20 differance is what he wants. With it being 90 outdoors and 72 indoor even before i made repairs I would think was working properly the minute i walked in the door. this house has 5 tons down 3.5 ton and 2 up..

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