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  1. #1
    Join Date
    Feb 2005


    I am a homeowner, but I have been frequenting this site long enough to know I wouldn't dare do the work myself. That would be ugly. I did work for a HVAC contractor once upon a time just out of high school, but all I really learned how to do is run flex ductwork. And from info I've gotten here, I was taught wrong.

    I hope my question will not be as complicated as brianmu's. I'm adding 625 sq.ft. to my 1250 sq. ft ranch(1875 total). In the end, the house will look like a perfect V from above. Sort of like a 50ftx50ft square with a 25ftx25ft section removed from one corner. Since my air handler is a 21 year old Trane and would soon have to be replaced soon, I suggested to both contractors that we just start fresh with a whole house heat pump system and they agreed. Both suggested that the air handler be placed at one end of the house. Seemed funny to me as that would mean a 50ft+ run from the handler to 2 or 3 ducts on other end of house. But, I don't know, I'm no pro. I have single pane, fairly small windows with storm windows throughout the older portion of the house and insulated large windows throughout the addition. Insulation is r-30,13, and 19. I live in NC, so fairly humid and lows in 20's winter, highs in 90's summer. Their differences are that first guy said I needed a 3ton ht pump and a 3.5-4 ton handler and the second guy recommended at most a 2.5ton system. First guy rec. we incorporate existing r4 flexduct in original house, second guy rec. complete removal of old ducts and installation of new r6 flexduct. House has crawlspace.

    Both seemed credible and seasoned, but drasically different sizes? Not trying to discredit either, just want to do the best thing and be comfortable on those hot summer days and cold winter nights. Any thoughts to help me go in the right direction would be appreciated. By the way, really enjoy the forum, extremely interesting for someone like me. Thanks.

    [Edited by robr823 on 02-04-2005 at 11:36 PM]

  2. #2
    Join Date
    Feb 2003
    so, what does your load calc show?

    do you have somewhere to put a unit at the center?

    after all you have read here, you still want flex?

    BTW, I would put storms over the new windows! Protects those frames, adds insulation.

    the smaller unit will remove the moisture better!

  3. #3
    Join Date
    Apr 2003
    I cannot believe neither of them did a heat loss calculation. One of the them should have..... 1825 square feet with single pain windows.... and north carolina heat..... that could be as high as 3 1/2 ton.... do a heat load on the house...
    Pay attention to outside unit not the inside.... inside all they are getting is SEER with the size.
    a unit to small will run constantly...... an over sized unit will not run long enough to remove humidity.
    Ducting? don't know what shape yours is in... flex duct? I prefer flex duct in attics only. I'd rather see steel in insulated flex duct wrapping.... but cheaper with flex.... old flex duct can be rotting... so check that.

  4. #4
    Join Date
    Feb 2005
    Good point about the calculation. Come to think of it, I don't remember the first guy doing one. Perhaps the second contractor is my best bet then. First guy said that 3.5-4 ton handler may be required for the long runs, however there is room in the center for it. Grade is just higher so a little harder to maneuver in the center, but that's where the old one is now, maybe they're both just lazy and want to put it in the tallest space, which just so happens to be right next to the crawlspace door. As far as flexduct is concerned, that's literally all I ever see here. Went on two jobsites last week here for houses $700,000 and up--all flexduct. One contractor said it had something to do with humidity here. Only time I've ever seen steel duct is at my in-laws in Vermont, never once in NC, except in commercial applications in conditioned space. Thanks for the replies, gives me something to go on.

    [Edited by robr823 on 02-05-2005 at 09:28 AM]

  5. #5
    Join Date
    Jun 2004
    4H: Hot, Humid Houston H.O.

    Manual J and D

    You have a lot at stake in doing this right, do you not? How could it *not* be worth $50 to download HVAC-CALC and get your own load calculation? If you don't do your own, then at least see some detailed load calcs using MANUAL J to size your system. There are some other sizing methods but none have the pedigree of Manual J.

    I would want an installation using ductwork sized via Manual D as well, if you do that then there should be nothing wrong with flex duct. At least that's what I have heard from the most knowledgeable people on this board.

    Of course you want all the joints well sealed with mastic.

    Best of luck -- P.Student

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