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  1. #1

    Quoted 3.5 ton by one company and 5 ton by another who is right?

    Brought in 3 reputable companies who performed heat load calculations that resulted in different outcomes. Here are some specs.

    Dallas, TX
    1850sqft
    single story
    current 5 ton system installed in 1996
    10' ceilings
    1 large 7'X7' skylight in kitchen

    I know you cant possibly do a calculation with that info (at least I don't think you can) but how do I know if a 5 ton is to large or a 4 ton is to small? The guy that came up with the 5 ton model took longer to make his calculation but also he said he did a more extensive process with involved measuring every room, ceiling....and some other things. The other guys ended up with 3.5 ton calculations but recommended a 4 ton.

    The 5 ton guy said I was at 297000 btu's I think. Any help would be appreciated.

    Also should I stay away from the allstyle coils all in ones, which is what I currently have or should I seperate the plenum and coil?

  2. #2
    Join Date
    Mar 2007
    Location
    NW AR
    Posts
    2,478
    Sounds like someone just SAID they did a load calc. Most likely he is just going by what is currently installed in the home.

    I dont know about your windows, doors, or insulation but 5 tons seems huge for that sq footage.

    You do have windows and doors correct?

  3. #3
    hehe, yeah and if providing all the info would help I would happily scan and post the schematic drawn up.

    I got a much better "feel" from the 5 ton guy. I am pretty handy and have been replacing parts in my A/C and furnace for several years now. I'm tired of doing this however and have decided to break down and get a new system installed. Anyways this guy really seemed like he knew his stuff. He spent twice as much time assessing my house and system. At first he said that the 5 ton was way to big but then he did the calculation and said I was right past the upper limit of 280000 btu's for the 4 ton system.

    He told me that the way he calculated, is recommended but not required. He called it something specific but I can remember what.

  4. #4
    room by room heat load calculation as suggested by the dep of energy, equipment manufacturers and ACCA

  5. #5
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    If your 1850sqft house in Dallas "needs" a 5 ton system, you are either missing a wall, have a couple of glass walls somewhere, and/or don't have any insulation to speak of...

    I'd be very highly suspicious of any "load calculation" that showed the need for a 5 ton system in that size house if it has even a moderate level of insulation and reasonable glass area.

    With the construction typical for the region, we regularly are pulling 4 and 5 ton systems out of that size house and going back with 3 ton systems(occasionally even a 2.5 ton system), new properly sized/routed R-8 flex duct supplies, returns for all the bedrooms, and increased attic insulation.
    If there are poor thermal performance skylights, unusually high amounts of glass, etc., maybe a 3.5 ton system.
    If more government is the answer, then it's a really stupid question.

  6. #6
    Join Date
    Jul 2002
    Location
    SE Texas
    Posts
    459
    As most of us know, you can't perform a load calculation on the Internet based on square footage alone. There are way too many variables. I suggest you talk to the contractors and have them explain to you how they came up with the numbers they did.
    With your chrome heart shining in the sun, long may you run.

  7. #7
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    5,025

    The Quoted Btuh numbers are way, way off base...

    I willing to bet the duct system is not sized to deliver near 5-Ton of airflow.

    Therefore, the 5-Ton condenser, if airflow & delivered Btuh were checked probably wouldn't be delivering 4-Ton of heat removal.

    At first he said that the 5 ton was way to big but then he did the calculation and said I was right past the upper limit of 280,000 Btuh for the 4 ton system.

    The 5 ton guy said I was at 297,000 Btuh I think. Any help would be appreciated.
    Where are they getting those Btuh numbers, both numbers are way, way off the mark & miles too high for the 5-Ton listed!

    If they did an honest load calc they should list all the R-factors & glass U-Factor numbers, plus the glass Solar Heat Gain Coefficient (SHGC) factor.

    You can tell if it can be cost-effectively made to use a smaller tonnage A/C to handle the new load numbers.

    In general, I agree with mark beiser's post; should be able to make a 3.5-Ton efficient system to handle the adjusted load.

    A new good article Apr 4, 2012 2:38 PM, By Rob 'Doc' Falke Why Some HVAC System Designs Don’t Work
    Last edited by udarrell; 04-04-2012 at 10:36 PM. Reason: Important Factors to consider...

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