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  1. #1
    Join Date
    Jan 2006
    Posts
    8
    First - thanks everyone for answering most of my questions in the thread:
    http://www.hvac-talk.com/vbb/showthr...threadid=94509

    My first contractor (estimator) specified a 90kBTU Carrier (58CVA090), based on my 2200sq.ft. home.

    My second contractor (estimator) specified a 70kBTU Carrier (58CVA070), because the existing duct work above the furnace is a 14" cylindrical opening, and won't allow the full 16k cu.ft./min that the bigger 4-ton unit can produce. He said if the duct work were bigger (or if I spent a jillion dollars redoing the duct in the home), he "would" have specified 90kBTU, but not in this case. Furthermore, he said the bigger unit would initially come on full blast at 16k cu.ft/min. and then cycle back to about 11k cu.ft/min once the furance detects the backpressure. So he felt the bigger unit would be a waste for me to get.

    Question1: From what I've read on this board, it's better to get the smaller one in this case (70kBTU) because it will run steady, and more efficiently, than the larger unit, under the above circumstances. It is likely to cycle on/off/on less.

    Question2: The kicker: Both contractors estimated the same $price within about 50$. I would have figured the smaller unit would be less, but my guess on this is that the profit from the unit is not much, and the real work involves installation costs. My gut feel from meeting both contractors is go with #2, especially his knowledge and attention to detail.

    Thanks
    -Dave


  2. #2
    Join Date
    Jun 2000
    Location
    N. W., IN
    Posts
    1,017
    "attention to detail"

    that's good.

  3. #3
    Join Date
    Jan 2006
    Posts
    214
    did either contractor do load calculation?sounds like #2 is in the ballpark.

  4. #4
    Join Date
    Jan 2006
    Posts
    8
    No one did a load calc.

    It was more rule-of-thumb, and experience with "these kind of houses" etc., along with the fact that I am going to replace all the single-pane leaky windows with dual-pane (more energy efficient) Andersons (either new construction windows or replacements).

    I have to imagine that the 14" cylindrical duct limitation is really a fact: that it does indeed limit me to 12.5k cubic feet-per-minute. I have no calculations for that.

  5. #5
    Join Date
    Aug 2002
    Location
    Southold.calm
    Posts
    5,685
    Dave

    Only Manual J can determine the size of your unit.

    Click on the red tab above and do your own.
    Changing the windows will greatly reduce your heat loss so factor this in.

    Once you have the size down then and only then will you know if the existing ducts can handle it.

  6. #6
    Join Date
    Jun 2000
    Location
    WYO
    Posts
    2,019
    A lot of older homes have insuficient ductwork to carry air required by new furnaces @ A/C systems.
    On changeouts,a heat load is a good start.It may not be accurate tho due to insulation settling,actual air infiltration,construction anomalies, and unseen material(or lack of in walls and such.Along with upgrades.
    One figure I can give you is I.M.C.s code requirement of 2 sq.in of ductwork (supply and return) and openings, per 1000 Btu,s. Your 14" round duct has about 153" of area.
    Under this guideline,a 70K furnace would require 140 sq".
    The 90K would require 180sq"s.
    I would do the heat load,and ask the contractor if you need any duct modifications after that,some duct changes can be done at the furnace relatively easy. Manual D will size it correctly.
    never say never

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