Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Why the heck an installer would put air supply trunks by rim joist?

agurkas Member Posts: 238
I (and basically every contractor I have brought in) been cursing at decision made by original HVAC installer about 12 years ago. In the large basement room, where furnace is installed, supply air trunks were installed right at the perimeter blocking off all the access to rim joist. Literally the only two outside walls I have in the basement have those trunks running right in the last space bordering rim joist, so you can't insulate (there is no insulation there and ducts are thinly insulated from inside) around them or get to the electrical they are blocking.

Why would someone do that? I mean yes, there are registers there, but still.

I am thinking about taking the entire trunk and trying to move it by one floor joist bay.

Also, if say I wanted to go with round ducts (in case square ones are too nasty inside), what diameter do I need to use to replace 12X10 square duct? I am not even sure why that size is necessary, when the supply trunk is only 30 feet long and had 4 registers in it.


  • SWEI
    SWEI Member Posts: 7,356
    Not 100% sure what you're describing with the one bay over thing, but it sounds like you might be able to inject foam through holes drilled in the siding?

    12 x 10 is 120 square inches, which equates to a 12.36" diameter circle.
  • agurkas
    agurkas Member Posts: 238
    That is the thing, they placed the duct almost flush with the rim joist. I can't even get to the wiring behind it.

    Is there like a calculator I should look up for what diameter duct should be? I have 4 registers that are 15X5 being in that supply trunk. 30 feet total run, 40 feet, if I include from one end all the way to the furnace.

    Also, what is the disadvantage of higher air velocity? Right now I can barely feel the air coming out of those vents. Room gets heated very nicely with 2nd stage kicking in very rarely.
  • icesailor
    icesailor Member Posts: 7,265
    edited December 2014
    "" 12 x 10 is 120 square inches, which equates to a 12.36" diameter circle. ""

    Are you sure that it isn't 9.4249 Sq. In. in area? (12" X .7854)

    The OP needs to have someone familiar with heat loss and duct sizing to decide. It usually isn't as easy as just moving the ducting like it is to move some pipes.
  • Robert O'Brien
    Robert O'Brien Member Posts: 3,542
    SWEI is correct. "Pi times R squared
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    It could be that they were installed there to minimize soffits if basement got finished. Run into that all the time. But seldom do insulators come in before metal guys.
  • agurkas
    agurkas Member Posts: 238
    Basement isn't finished. That is the cooky part