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  1. #14
    Join Date
    Nov 2005
    Posts
    19
    We sign contract tomorrow morning (thursday morning).

    So...

    Any last minute input on the 80k vs 100k?

    thanks,
    yitzi
    (I'll check for the latest posts b4 the meeting tomorrow)

  2. #15
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,272
    Originally posted by yitzi
    To recap: My current sq ft is 1600-1700, but after all is said and done (with basement finished, attic, screen porch, etc) I should be right about 3000 sq ft.

    On the chance that 100k/btu might be too much...with a 2 stage and variable speed system, wouldn't the system almost naturally prevent "oversize" issues? or...the 80k???
    You'll need > 100 K
    to heat "SCREEN porch, etc." !
    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

  3. #16
    Join Date
    Nov 2005
    Posts
    19
    Dan;
    The screen porch will be closed in and insulated, and have viny/low-e/argon windows - along with the rest of the windows
    yitzi

  4. #17
    Join Date
    Mar 2004
    Posts
    1,042
    There's no point in having a two stage furnace if you never use the second stage, and that's what you get with an oversized two stage furnace.

    As long as he'll stand behind his work, take the smaller one. He's right in that there is an overwhelming tendency to oversize them.

  5. #18
    Join Date
    Nov 2005
    Posts
    19
    wyounger;
    That sounds like and excellent point. And, oversizing is real common and is a bad idea, agreed.
    But, in my situation is 80k/btu really going to be enough if/when I get up to 3000 sq ft??

  6. #19
    Join Date
    Mar 2004
    Posts
    1,042
    We can't really tell from here. It's certainly possible, but it depends on the structure. Square footage is just one factor.

    All you can really do is a load calculation on some hypothetical future addition plans, or trust your contractor. Anything you add on is going to be more energy efficient than the older parts of the building, so they won't need a proportional amount of heat.

    What is the age/size/efficiency of the furnace you're replacing? You have to keep in mind if you're replacing something old and inefficient that these BTU capacities are inputs; a 93% efficient 80k BTU furnace puts out 74.4k BTU- there's not much difference between the input and the output. If you're replacing a 70% efficient old beast with a 100k BTU input (70k output), the new furnace may actually put out *more* heat than the old one. So it's typically good practice when you're doing a furnace replacement to drop a size on input capacity, because every generation gets more heat out of the fuel than the last generation.

    The other concern is that 100k is WAY too much furnace for the current size of the house, unless you like to keep the windows wide open all through the winter. It's asking an awful lot to have a single system that can handle doubling the square footage at some point down the road. I'd suggest not trying to put so much excess capacity in. When you do the addition, if it turns out that what you've got isn't quite enough for an addition down the road (certainly get a load calculation done based on the plans before you start construction), some portion of the addition may be better off with a separate system anyway (master bedroom, sun room, etc.). You'll get more even temperature control and the bonus of zoned temperature control, so you can avoid heating spaces when they aren't in use.

    [Edited by wyounger on 11-10-2005 at 05:25 PM]

  7. #20
    Join Date
    Nov 2005
    Posts
    19
    wyounger;
    Your thinking sounds real solid, I like it, you've got me getting a better feeling about the 80k (why can't they just make 'em at 10k jumps and not 20k?!). Install happens monday/tuesday.
    thanks much,
    yitzi

  8. #21
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,272

    Design & Specs

    Originally posted by yitzi
    But, in my situation is 80k/btu really going to be enough if/when I get up to 3000 sq ft??
    I'll tell you haeting requeriment per ACCA Manual J
    IF you:
    1. Provide .DWG of house and
    2. House orientation
    3. Windows specs ( argon and low e is Good enough )
    ___SHGC= ~0.4 & U-value= ~0.3
    4. Insulation of wall ( R-11) and ceiling ( R-30)
    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

  9. #22
    Join Date
    Nov 2005
    Posts
    19
    Hey Dan,
    That'd be great. Not sure what you're asking for though, can you email me a little more detail? vienna18@hotmail.com
    (I'm sure you noticed over there in Naples, from earlier posts, that I moved from Miami...I sure don't miss being there for July and August).
    thanks yitzi

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