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Thread: hvac and duct work DIY install

  1. #1
    DIY Junior Member rocinante's Avatar
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    Default hvac and duct work DIY install

    /Bought goodman furnace and ac. want to install my new system. How do I decide on duct work size and reduction for adequate air flow? Will install units myself and get pro to do hookups. Wonder if gas company will hookup instead. Figure 60k btu and 2 ton a/c for second floor addition (3br, 1 bath added) about 900 sq. ft including hall and stairway. All four rooms on same back side of house, hall on front side as access to them, i.e. straight run 40 feet long.
    duct work in hall feeding all 4 rooms. Can fit 8x16 duct to start and reduce as necessary. What to do?

  2. #2
    DIY Junior Member TheycallMeDave's Avatar
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    Quote Originally Posted by rocinante View Post
    /Bought goodman furnace and ac. want to install my new system. How do I decide on duct work size and reduction for adequate air flow? Will install units myself and get pro to do hookups. Wonder if gas company will hookup instead. Figure 60k btu and 2 ton a/c for second floor addition (3br, 1 bath added) about 900 sq. ft including hall and stairway. All four rooms on same back side of house, hall on front side as access to them, i.e. straight run 40 feet long.
    duct work in hall feeding all 4 rooms. Can fit 8x16 duct to start and reduce as necessary. What to do?
    If you have an electric furnace or an 80% efficient gas furnace, I would install it in the attic and run flexible insulated round ducts from an insulated supply plenum mounted to the furnace ... to each room , with a central return in the hallway providing your doors are either undercut or you can put in transfer grilles for each room so the air can escape back into the hallway. However, if you are putting your furnace in a closet in the hallway and ducting up into the attic using horizontal ductwork, then you need to use whats called 'The Equal Friction Method' of duct sizing so you get enough air to the last branch run from the furnace . Youll be able to find alot of info online for this method or you can buy a book . A 2 ton cooling system for 900 sq. feet in N.J. will work nicely so long as the ductwork is generously sized . I would also consider a roof mounted attic fan on an automatic thermostat to rid the attic of superheated air during the summer months or else alot of that hot air will penetrate thru the ceilings into your second story rooms and make the a/c system work longer ; provide sufficient intake air for the attic if you dont have it already.

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    Moderator & Master Plumber hj's Avatar
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    You are trying to learn in one e-mail stuff that AC guys take months to learn. There is more to it than just starting a piece of ductwork and making periodic reductions in size, because you have to know HOW bit to make the branches based on the room's requirements and the distance from the main. You MIGHT be able to get an AC person to make your calculations and design, assuming he has enough work so he didn't care if you did it yourself.

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    In the Trades Tom Sawyer's Avatar
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    Yes indeed, the duct design manul is over 200 pages long and that's just the residential section.

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