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    mikect05 Posts: 48, Reputation: 1
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    Mar 5, 2014, 05:54 PM
    Duct sizing
    Hi, we recently moved to a new home. Right now we have forced hot air and a 2.5 ton AC unit. The duct coming off the furnace is 18x8 and then reduces to 14x8, off this there are 12- 6" flex duct supplies. A couple of these are very long runs through my unheated garage. Needless to say the system is not working very well for our needs.
    We are thinking about putting in a high efficiency modulating gas furnace with all new ductwork. I have had several people come out for estimates and none of them have done a load calculation.

    The house is about 1500 sq', one level Ranch with finished basement area. We live in CT. From what I can figure out I believe we need a 60,000 btu furnace. The one I am looking at is a coleman echelon rated at 1200 cfm.
    I have a few questions:
    1. With my current set up having 12- 6" flex ducts was the main duct undersized?
    2. Should the main duct for my new setup be sized for 1200 cfm?
    3. Can I have three zones, which would be about equal in size?
    4. If so can I split the main duct into 3 ducts sized for 400 cfm, and then have my supplies come off those?

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