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  1. #1
    Join Date
    Nov 2004
    Posts
    35

    Register outlet temperature

    What is the general rule for an acceptable outlet temperature for A/C? I thought I remembered reading somewhere that 30 degrees below the outside temperature is acceptable. Any thoughts?

  2. #2
    Join Date
    Sep 2003
    Location
    Portland.OR
    Posts
    313

    Red face how hot?? how cold??

    20 degrees across the inside coil is more realistic. 30 degrees for a great AUTOMOTIVE AC..Often on older systems, 15 is all you get...

    just measure the cold air return and at the registers and do the math.

    Terrible installs or many variables, like long metal ducts suck up lots of heat. Ranch style homes with 60-80 feet of duct can hold a lot of heat...
    Insulation is our best friend nowadays with energy costs going into outer space.
    leaky ducts,not properly sized, uninsulated [helps in both heating and cooling seasons.]

  3. #3
    Join Date
    Nov 2004
    Posts
    35
    Thanks, so if it's 78 inside, the air coming out of the registers should be roughly 58?

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,064
    There is no one set temp difference.
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  5. #5
    Join Date
    Mar 2006
    Location
    The South
    Posts
    2,195
    bm

    Just did a PM on my system back in May and took readings with digital temp gun at several supply registers in my home. time of day mid afternoon. outside temp was in the low 80s. Thermostat set at 75 degree. Gun was registering in high 50s.

    IMO

  6. #6
    Join Date
    Jun 2007
    Location
    Round Rock
    Posts
    3,559
    The standard in the HVAC industry is 15-20 degree drop across the coil. Most will achieve 18 fairly easily. Lower than that and there is usually a problem. You can't really go by that if you have variable speed. It could be dehumidifying, in which case it is anybody's guess.

  7. #7
    Join Date
    Jan 2007
    Location
    N.E. Ok.
    Posts
    1,370
    Is that a I.R. temp gun?
    They don't measure air temp., probably reading register temp or surrounding area which may be close to air temp.
    If you want to measure the temp of a reflective surface or glass you can use a piece of black tape or paper mounted in contact with surface.

    Quote Originally Posted by tigerdunes View Post
    bm
    Just did a PM on my system back in May and took readings with digital temp gun at several supply registers in my home. time of day mid afternoon. outside temp was in the low 80s. Thermostat set at 75 degree. Gun was registering in high 50s.

    IMO

  8. #8
    so my system sounds like its doing ok. I placed a thermometer near my return air duct and then near my floor vents and got about 14-16 cooler air coming out of the vents, so it sounds like i'm not doing too bad, thats a relief to hear. I always thought 14-16 difference was really low, but my house is on a slab and the ducts are within the slab work as in the ducts consist of just areas within the concrete slab that are open, no metal ducts or anything, (which really really sucks) but i guess if i'm coming in very close to the lower end of normal than I won't worry about it.

  9. #9
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,286

    Thumbs up R.H. and Diffuser Temps are good comparative performance indicators

    Quote Originally Posted by bmathews View Post
    The standard in the HVAC industry is 15-20 degree drop across the coil. Most will achieve 18 fairly easily.

    Lower than that and there is usually a problem.
    18'F across the coil is fair-good.
    If your attic is hot enough & significantly affecting the duct losses during the day,
    your diffuser temperature may be only 14'F less than the room temperature.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  10. #10
    always heard 15 to 20 degrees as rule of thumb

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,064
    Quote Originally Posted by uncfan29575 View Post
    always heard 15 to 20 degrees as rule of thumb
    So how do you know which it is.
    How do you know if 17° is what it should be when your at that call?
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  12. #12
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,286

    Hmm R.H. versus coil dT ?

    Quote Originally Posted by uncfan29575 View Post
    always heard 15 to 20 degrees as rule of thumb
    Where does 15'F across the coil get you in terms of Relative Humiidty?
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    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

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