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  1. #1
    Join Date
    Jul 2006
    Location
    Las Vegas
    Posts
    34

    Hmm

    My AC sales guy said such a contractor estimate formula, "one ton of cooling for every 400 sq ft" was common for tract homes, especially single level. My 1165 sqft home is in Las Vegas with 100-115 degree summer temps. He sold me a 3 ton unit, saying it can be upsized slightly, and that the 2 ton would have been to small for needs. Also since I wanted two-stage, there were no 1/2 sizes, so 3 ton unit was proper size. Does this sound wrong? No Manual J load calc done unfortunately, but may still have one done out of curiosity by another contractor since reading this forum.
    -Ken

  2. #2
    Join Date
    May 2002
    Location
    Colorado
    Posts
    697
    A 2-ton is probably the correct size IF your house has:

    Good insulation in the walls and attic.
    Good modern dual-pane windows with a SHGC below 40.
    Window to floor ratio of no more than 15%.
    Not too much unshaded glazing on the southwest to northwest side.
    Tight construction (no leaky ceiling cans, etc.).
    Light colored curtains or blinds that can be closed.
    Adequate, tight ducts.
    Properly installed and charged AC with ample air flow for dry desert operating conditions. This means 900-1000 CFM.
    A tile roof would help, too.
    An owner who is willing to "set it and forget it." Too many homeowners like to turn the AC off when they go to work and expect it to cool the house down by dinner time when they get home.






  3. #3
    Join Date
    Oct 2004
    Posts
    3,400
    Sadly, that's pretty common.

    Most contractors know the proper way to size equipment, but then let this "rule of thumb" throw proper sizing out the window.

    He may be spot-on with his guesstimate, but I would like to get at least one second opinion.

    Don't make the mistake of telling the 2nd guy what the first one said.

    Just ask him to do a Manual J on your house.

  4. #4
    Join Date
    Jul 2006
    Location
    Lancaster,Ohio
    Posts
    464
    I've never heard that one, I have seen "companies" walk around placing registers and then counting up the cfms to determind equipment size. Then to beat all things they round everything up to next 50 cfms.
    IcyFlame

  5. #5
    Join Date
    Feb 2006
    Posts
    302
    "An owner who is willing to "set it and forget it." Too many homeowners like to turn the AC off when they go to work and expect it to cool the house down by dinner time when they get home" Very true.

    I have a 1200 sq ft house in Denver. It was 102 degrees outdoor on 7/16 and 73 degrees indoor (t-stat had maintained 71 except for maybe 2 hours of 73). I usually don't keep it this cold, but I had the overnight setting on "hold" on t-stat. I have a 2 ton outdoor unit (1 stage) with a 3 ton indoor coil, variable speed blower, txv. CORRECT SIZING MEANS AN OVERSIZED UNIT WASNT WASTED ON MY DUCTWORK.

    Use the HVAC Calc and tell your contractor to install the right size. Tell him he can keep the equipment cost difference for the smaller unit (not much, but it's something).

    Also, the adaptive recovery thermostats "learn" that when it's really hot out, the recovery starts sooner. So, there is really no benefit from oversizing.

    For a 115 degree outdoor maximum, why not consider a 2 stage 3 ton outdoor unit. You couldn't go wrong with that ???? Trane (probably other brands also) has 2.5 ton single stage outdoor units. But, don't let them guesstimate, you should use HVAC Calc.

  6. #6
    Join Date
    Dec 2005
    Location
    California/Nevada
    Posts
    3,607
    >>>>>>>>>
    Does this sound wrong?
    >>>>>>>>>


    no.


    your system is sized right,

    if you have any problems , it would be too small.

    if you have allot of windows on the west side, or anything else causeing a higher heat load,

    but , i wouldn't worry about it, you're system should be the proper size

  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,704
    XV80, he already said its 2 stage.

    At 115 OD design, that very well could be the size you need, depending on what your ID design is.

    Your not going to get 24,000 BTU's out of a nominal 2 ton unit when its 115 outside, and you want it 75 inside.
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  8. #8
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    do you buy your car with an engine sized 10hp for each seat?
    or, do you just buy the 10 passenger van so you can haul all your family & both parents to the Christmas parade ea year?

    it takes practice to make an educated guess -- nothing to ass/ u/ me = @1t/400sf.
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  9. #9
    Join Date
    Nov 2000
    Location
    Waco, Texas, USA
    Posts
    6,153
    Ken, There is no such thing as an undersized or oversized unit. You could install a 5 ton and have lots of fresh air intake or you could install a 1 ton and insulate & seal the hell out of the structure and only expect an 85F indoor temp.

    What DOES matter is that the system's ductwork be the correct size for the equipment.

    You load calcers & expert design contractors chew on that one for a while.
    "And remember my sentimental friend......that a heart is not judged by how much you love, but by how much you are loved by others" - Wizard of Oz.

  10. #10
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    And just when does one start being a troll?

  11. #11
    Join Date
    Jul 2005
    Posts
    435
    The only way to get an accurate size is to use the manual J method/program. Sizing by sq ft avg is a hack way of doing it!

  12. #12
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,283
    Originally posted by Steve Wiggins
    Ken, There is no such thing as an undersized or oversized unit. You could install a 5 ton and have lots of fresh air intake or you could install a 1 ton and insulate & seal the hell out of the structure and only expect an 85F indoor temp.

    What DOES matter is that the system's ductwork be the correct size for the equipment.

    You load calcers & expert design contractors chew on that one for a while.
    Hmm...by that logic why not go 40 tons? 250? As long as I give the thing enough heat load, WTH!

    You are spot on regarding ductwork...but the rest I find lacking (to be charitable). In an ideal world, the house WOULD be built to super high thermal envelope standards, and the HVAC would be merely for air quality control (ventilation and filtration) and modest temperature control, resulting in a lower than average need of capacity (tonnage).
    In the real world of slap-em-up-by-the-acre tract housing so many folks live in, the x-amount of tonnage per square feet still doesn't hold up due to each home being sited differently, and being occupied differently. Even if they're all cookie cutter look-a-likes.

    Personally I think two stage or variable capacity is the way to go with resi. Only during the worst heat waves will one need the full capacity of the system...the rest of the time it's to keep things comfy inside because it's still not nice enough outside to open up the house for natural ventilation.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  13. #13
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    Premise may be wrong

    >>..since I wanted two-stage, there were no 1/2 sizes, so 3 ton unit was proper size. Does this sound wrong?

    I am looking at Trane XL19i literature from 2003, and there most certainly is a 2.5 ton unit offered. Model number is 2TWZ-9030A. Is my info out of date? Has no other manufacturer offered a 2.5 ton 2-stage unit?

    Regards -- Pstu

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