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Thread: HVAC Trunk Sizing

  1. #1
    DIY Junior Member
    Join Date
    Jul 2007
    Southeast PA

    Default HVAC Trunk Sizing

    Looking for some adivce on what size trunks to run and when to reduce.
    I am looking to frame in a wall that will house the trunks so I need to know the sizes prior to framing.
    I am not ready to purchase the acutal hvac unit at this time and will leave this area open.

    Do I need additional supply reigsters for the bedrooms?
    How many returns? ( I was thinking 1 large hallway return)

    FYI becasue the floor was finished by previuos owner I could only run 6inch flex duct for all my supply branches under the floor. Ceiling is not an option for running ducts because of rafters and flat roof.
    Totatl space is apprx 800sf. This is the second floor of a 1500sf house (total will be 2300sf)
    First floor has hvac from oil burning f.h.a. Essentialy I am creating a second zone on second floor.
    Attachments show cfm per room as well as distances.
    Yes this will all be inspected. Not looking to cut any corners.
    Any advice would be appreciated.
    Break down
    Bedroom #3 228cfm
    Laundry room 64cfm
    hallway 120 cfm
    bathroom 70cfm
    bedroom#4 168 cfm
    Thank You!
    Attached Images Attached Images   

  2. #2
    DIY Member Lightwave's Avatar
    Join Date
    Aug 2007
    Vancouver, BC


    Looking for some adivce on what size trunks to run and when to reduce.
    That's a very big question that could take several hours worth of work to figure out. If you want to do it yourself, you'll need to get a copy of ACCA Manual D. Some duct sizing information is on the web, but you'll need the fitting loss tables from Manual D to make much headway.


    Your best bet is probably to hire a mechanical engineer to do the calculations for you. Duct design is not easy.

    Do I need additional supply reigsters for the bedrooms?
    There's no way to know this without having the load calc data.

    How many returns? ( I was thinking 1 large hallway return)
    One for every room except for bathrooms and kitchens.

  3. #3
    DIY Senior Member Hube's Avatar
    Join Date
    Sep 2004


    The ONLY proper way is to use a Manual "D" in order to obtain correct duct sizes. Any other means would just be a guess and could result in a waste of money/heat. You also have to consider the size of the windows and the amount of "R" value in each of these rooms that require heating/cooling.

  4. #4
    Retired prof. engr. gator37's Avatar
    Join Date
    Nov 2009


    Assuming the bottom sketch is the proposed new addition. How did you come up with you HVAC load? (The total CFM indicates about a 1.5 ton unit, but you should really calculate the load if you haven't.) Orientation can make a big difference especially when you are considering the glass areas.
    Are you talking about getting a furnace with a cooling coil or a Air Handling unit with electric heat. Furnace vs. AHU makes a difference on how you size your duct. Warning, flex duct has a large pressure drop.


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