Houston: 3.5 tons for 5,200 sq ft house?
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  1. #1
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    This morning I built a 5,200 SF virtual house in Houston. My cooling-load calculation indicates this house needs only a 3.5-ton AC system. Based on various comments I have read on this site, there is no way I could cool this house with less than 10 tons. What have I done wrong?

    The house has two stories of equal square footage. Inside dimensions are 35 x 70. The first floor has 10-foot ceilings, and the second, 9-foot ceilings. Five bedrooms. Roof is tile. SF of windows and sliding glass doors: N - 213, E - 75, S - 250, W - 75. All glass is low e and has a SHGC of 0.33. The house is well built and very tight with a measured ACH of 0.3. It is assumed the owners will close the blinds on the east and west windows during hot weather. All glass on the south side is shaded by eaves.

  2. #2
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    Well, in my personal opinion being in the a/c business for over 27 years in Houston trust me you will never cool a 5,200 SF home in Houston, Tx. with 3.5 tons of air unless it's 3.5 tons of ice you have poured on top of your home. Heat load, Manual J,manual D whatever heat load you use to calc. you will be warm in a home that big here with that little a/c I can promise you that.
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  3. #3
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    Why do you think you have done anything wrong?

  4. #4
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    Up here in Northern Wisconsin, we think that a well insulated house has about R60 insulation in the ceiling and R30 in the walls. Add an ERV, make sure the windows have whole unit U-value less than 0.32, energy efficient appliances and light bulbs. Plug those numbers into your virtual house, and you will need less than 3.5 tons. But don't plan on using a setback thermostat.


  5. #5
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    What indoor/oudoor conditions were used?

    What was the sensible and latent required?

  6. #6
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    Originally posted by dash
    What indoor/oudoor conditions were used?

    Per J7 -- 95 outside, 75 inside, 47 grains for 50% RH inside.

    What was the sensible and latent required?


    26,300 sensible, 6,750 latent.
    [Edited by Panama on 06-29-2005 at 01:41 PM]

  7. #7
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    Originally posted by Panama
    26,300 sensible, 7,750 latent.
    That looks more like 3.0 ton to me.

  8. #8
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    Originally posted by uktra
    Why do you think you have done anything wrong?
    Because I'm so far off from everyone else -- except maybe Hank Rutkowski, PE.

  9. #9
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    I know Hank--who is everyone else?

  10. #10
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    I'd say you are very likely have sized it correctly.

    Several things happen in the real world that account for conractors thinking that system would be too small,based on their experience.

    1.Man. J load come out exactly as yours,they look in ARI,and think a three ton will do it.Then when it won't the install a 4 ton.

    2. They select the proper equipment,but undersized the ducts ,sensible drops below rated perfromance,and they again install a 4 ton.

    3.correct size is installed ,has significant duct leaks,or is over/undercharged,wrong fan speed,etc.,again they install a 4 ton.

    So in "their" experience ,Man. J is wrong,cost them money and they always up the load on every job.No most peolple won't complain about overszed systems,they just run them at a lower temp to get comfortable.

    Additionally ,"by the square footers"fail to realize the difference in loads, between a 5000 sq. ft. single story and a 5000 sq ft two story.

  11. #11
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    Originally posted by travisfl
    Originally posted by Panama
    26,300 sensible, 7,750 latent.
    That looks more like 3.0 ton to me.
    I just corrected an error in the latent load I quoted. It is 6,750.

    The Lennox engineering data I used came up short on sensible capacity using a particular 3-ton system. It is so close, however, that some 3-ton systems might work OK. The 3.5-ton system I picked would be within the 15% excess sensible capacity recommended by J7. And then there is global warming --


  12. #12
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    Originally posted by uktra
    I know Hank--who is everyone else?
    mrbillpro, for one.

  13. #13
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    When you size for 75° indoor,most "rating" need to be adjusted ,as they are the capacity at 80° indoors ,not 75° indoors ,this can easily be a 1/2 ton difference.




    In my post above ,that's what I was refering to in #1.I thought Panama ,had made the adjustment,and selected a 3.5 ton based on the adjustment.

    I don't think you'll find a 3 ton that will handle the load as stated.

    [Edited by dash on 06-29-2005 at 03:38 PM]

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