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  1. #14
    It is hard to say if what you are doing is ok. We need to hear more details about how you intend to construct this duct system. Do you intend on just enclosing the three sides of the 18" trusses with ductboard, cut out your openings for the registers, and direct all the air back to the furnace. If this is the case, and it is all that you have specified so far, then no it won't work. Because the air ducts need to be sized properly to get the correct air volume and velocity to each room. This is calculated by heat loss and heat gain factors. I have worked on condos that didn't use an enclosed duct system and even on a small scale the performance of the system was very poor. Of course now that it's done to change it to a ducted system would be very expensive. So the tennants have to deal with low levels of comfort, mostly in the A/C season. If you do not size the ducts or enclosurer the system will not even come close to working right. So now that you know the ducts need to be sized, is it worth your time to mess around cutting and securing ductboard to the plywood? I have installed many systems and I just don't see how what you intend on doing could work and/or be worth the time.

  2. #15
    thank you.

    under duct systems in the 2003 Interntional Residential Building Code it says it is ok to use floor joists for supply inside of conditioned spaces.

    Does the National code not allow this?

  3. #16
    Your welcome.

    To answer your question, yes, use of under-floor space as supply plenum is ok, as long as you follow code when making the installation.

  4. #17
    Join Date
    Feb 2004
    chicago suburbs
    take pics and post.

  5. #18
    thank you.

    under duct systems in the 2003 Interntional Residential Building Code it says it is ok to use floor joists for supply inside of conditioned spaces.

    Does the National code not allow this?

  6. #19
    sorry for the triple post. That's what I get for using my cell phone to check this website.

    Thanks for everyones advice, lots a work todo.

  7. #20
    Join Date
    Jul 2002
    Zelienople, Pa
    If your A/C doesn't maintain a RH below 50%, expect growth problem immediatly in the near future.
    How tall are you Private???!!!!

  8. #21
    Join Date
    Nov 2001
    Be sure to install a drop ceiling on the 1st level.
    This way when your system falls apart, you can go beck in with metal and install a reducing trunk line system.

    1 question, Why would you consider this ?
    The cost of the 4th side is less then nothing compared to the cost of having the duct come apart.
    Which if it does you not only just lost air to the area that came apart, you lost it to the whole system.
    And not knowiing where it seperated, your looking at removing the ceiling.
    Besides, with duct board you'll loose 4" of valiable duct space. Well 3" your way.
    If you try to fail, and succeed.
    Which have you done ?

  9. #22
    Join Date
    Oct 2004
    The idea of using the floor joists is similar to the way they duct out mobile homes.... With no reducing trunk line... there's lot's of air at the begining of the duct and hardly any at the end...

    Doing nothing to account for static pressure loss=big mistake...

    It's your house...If you think there's lots todo now... wait till later when you see how well it works....

    Lots more todo later to rectify the problem....

    Like I said.... "Good Luck" the man

  10. #23
    Join Date
    Jul 2003
    I read right through supply air. Look up UMC 602.1
    stud/partition cavities not OK for supply air.
    NCBolton is correct you would get mold if above 50% R.H. The supply air is very cold and water condenses, this help to lower the R.H. of your home. Fire code also comes in play as flame spread of your ducts also need to conform.
    If it were my house I would use sheet metal, but duct board seems to be taking over. I would not want fiberglass duct board coming apart and blowing around in my house. Duct cleaning companies here offer to spray the inside of the duct board to protect it. Also if you add a UV light that could break down the duct board. The Sheet metal guy I use is very knowledgeable with air flow. He makes what I ask for and offers ideas too. I only have a small brake so it easier to get what I need made.

  11. #24
    thanks binford,

    I misunderstood "supply" air for combustion air.

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