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  1. #1
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    Dec 2008
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    Bryant Legacy and Preferred A/C units

    Anyone know what % of stated capacity is available for sensible load? I'll be talking my my contractor tomorrow to review the results of my HVAC Calc work. His quote was based on a 3 ton down and 3.5 ton up to copy what I have now. I will probably be putting in 4 or 4.5 tons. HVAC Calc manual says A/C units typically have about 72% of stated capacity available for sensible load but I don't know how much this varies across various manufacturers.

    Also, I read somewhere that stated capacity is usually at some temperature and that capacity drops at higher temps. Is that true? Does anyone have this data for the Bryant Preferred and Legacy series?

    PS I thought this was an excellent article

    C:\Documents and Settings\HP_Administrator\My Documents\Welcome To Home Energy Magazine Online.mht

  2. #2
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    Dec 2008
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    Ok, not a very good question. I just read a "how to use" Manual S article. I now see that sizing A/C units is far more complicated than I thought based on the HVAC Calc user manual. So, I will ask a different question, is it possible to size an A/C unit based on Manual J results alone or do you have to do a detailed Manual S analysis?

  3. #3
    Join Date
    Mar 2010
    Location
    Saint Louis, MO
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    18
    Quote Originally Posted by fixitfast View Post
    Anyone know what % of stated capacity is available for sensible load?
    Hi,

    The answer is... it depends. Each product combination (condenser, coil, and furnace) can result in a different number for the sensible load.

    Bryant publishes detailed product data on this website: http://www.docs.hvacpartners.com/idc...ments/techlit/

    Once the entire directory loads, search for your model number. You'll find files beginning with ii (installation instructions; you can ignore them) and pds (product data sheet). You want the product data sheet for your air conditioner.

    I've found that the product data of the condenser will contain "Detailed Cooling Capacities" based on a number of factors... cfm, coil, entering wet bell temperature, outside temperature. There's enough data to make your head spin!


    Quote Originally Posted by fixitfast View Post
    C:\Documents and Settings\HP_Administrator\My Documents\Welcome To Home Energy Magazine Online.mht
    This file appears to be on your own computer.

  4. #4
    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 fixitfast View Post
    Ok, not a very good question. I just read a "how to use" Manual S article. I now see that sizing A/C units is far more complicated than I thought based on the HVAC Calc user manual. So, I will ask a different question, is it possible to size an A/C unit based on Manual J results alone or do you have to do a detailed Manual S analysis?
    Depends mainly on your indoor design temperature.

    Mfrs. rate mostly at 80°F indoors,if you are using 76°F or less ,I'd be sure to use Manual S.

  5. #5
    Join Date
    Jan 2004
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    With an outdoor design temp of 89 and an indoor design of 78. You don't need to use Manual S.
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  6. #6
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    Dec 2008
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    Quote Originally Posted by beenthere View Post
    With an outdoor design temp of 89 and an indoor design of 78. You don't need to use Manual S.
    What about inside temp of 75? I see that Manual S is now part of the residential Building Code and I am thinking about resale. I want to be sure my new systems are designed on an acceptable basis. We keep the house at 78 or higher, but normal people probably wouldn't go for that.

  7. #7
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    On most York units. The difference between an indoor temp of 80 with a RH of 50% and a indoor temp of 75 and a RH of 50%. Is less then 1,000 BTUs of sensible loss. At 95 outdoor temp.
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  8. #8
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    Quote Originally Posted by beenthere View Post
    On most York units. The difference between an indoor temp of 80 with a RH of 50% and a indoor temp of 75 and a RH of 50%. Is less then 1,000 BTUs of sensible loss. At 95 outdoor temp.
    Loads aren't too much difference. I was trying to understand why Manual S isn't needed at 89 out and 78 in design temps. I was wondering if the Manual S requirement would kick in at some lower design temp?

  9. #9
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    PA
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    Generally A/C rated capacity is at 95° outdoor dry bulb temp, and indoor 80° dry bulb at 67° wet bulb temp.

    At lower indoor temps but still at 95 outdoor temp. A/Cs lose some sensible capacity.
    The lower the indoor temp, the more capacity it loses.

    At 89 outdoor. Your A/C will maintain its rated capacity at a lower indoor temp.
    Need the extended rating to determine how much.
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  10. #10
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    Dec 2008
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    Well, looks like the joke is on me. Talked with my HVAC contractor tonight. We went over my Manual J results and he said they looked good and made sense. BUT, he said my ducts were way too big to replace 6.5 tons with 4 tons. He said there just would not be enough CFM to get to the second floor. He said the air would "get lost". So, I may drop the 3.5 ton upstairs unit down to 3, but probably won't do more than that. He did agree to install two 80 btuh input furnaces to replace a 110k and 88k input. He said duct work is almost always the problem on retrofits preventing the HO from installing properly sized units. No way am I going to rework my duct systems.

    I did have alot of fun learning how to do a Manual J calculation and learning about Manual S, at least a little bit. I have a much better appreciation for the technology. I do very much appreciate the help I received here.

  11. #11
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    He said the air would "get lost".
    Heard that many times. Its not true. How can the air get lost. Its not like its intelligent.
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  12. #12
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    Dec 2008
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    Quote Originally Posted by beenthere View Post
    Heard that many times. Its not true. How can the air get lost. Its not like its intelligent.

    What is the best/easiest way for me to confirm that my duct system isn't too big? Going from a 3.5 to 2.0 ton uint on my upstairs will reduce CFM by 43% assuming the same temp drop across the new coil? Will I have enough static pressure to distribute the flow? I was also skeptical but understand his point.

  13. #13
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    If, IF your 3.5 ton were actually moving 1400 CFM in your current duct system, at a .5' static pressure.
    Then a 2 ton would move 800 CFM at a static pressure of roughly .16"

    The lower the static pressure, the more air the blower can move.
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