No announcement yet.

Fitting ductwork under a mobile home floor.


Forum Top GA Ad Widget

  • Filter
  • Time
  • Show
Clear All
new posts

  • Fitting ductwork under a mobile home floor.

    I am trying to find the best and most cost-effective way to replace the ductwork under a 14X80 1980's mobile home in NE Oklahoma. The downflow furnace/blower unit is near one end of the house, with a single run of duct going the length of the floor, east to west, and feeding 6 floor registers in a straight line, 1 to each room. There are 2 registers to
    the east of the furnace and 4 to the west.

    The original ductwork was made of duct board, running directly beneath the vents, with rectangular metal boots going straight off the top side of the trunk to each 4X10 register. It suffered a rat infestation during the winter when it was unused (we use a wood heating stove in winter) and we removed it about two years ago, including the badly rusted register boots. My dad replaced the original duct with an uninsulated DIY painted plywood box that mildewed, and which I just removed. Because the original ductwork is gone, I don't know the exact dimensions of it, but I believe it was about 12" wide by 6" high.

    I want to purchase the components and install the duct myself to save costs. I expected to just run a 10-inch rigid metal duct the length of the house, with saddle collars on top connecting to the register boots, but I'm running into trouble with the space constraints created by the framing under the mobile home. The floor is on 2X6 joists, and the ductwork runs between 2 lengthwise 10-inch I-beams. Every 8 feet or so, these I-beams are tied together by an angle-iron cross-piece welded to the bottom edge, leaving only a ten-inch space between the joists and the cross-pieces to run my duct, unless I go below the angle-iron cross-pieces, in which case I would have to snake the main trunk over or around the mobile home axles. I thought this would be fine, until I went to purchase register boots and saddle collars and found that they won't fit into the 5-1/2 inches between the duct and the floor.

    I was advised to see if a mobile home supply company has special HVAC fittings for this kind of situation, but I can't seem to find any around here who will even answer the phone. This is likely a common enough setup that there must be a reasonably simple solution somewhere. Will a round duct even work for this application, or should I go
    with a rectangular duct? Either way, is there a source for low-profile fittings and boots to go straight up from the duct to the registers in a space of as little as 5-1/2 inches? The people I talked to assumed that I would be branching from the main duct to registers on the left and right, and seemed flummoxed by how one would connect straight up to the floor from the main duct. If I have to, I can drop the whole run down below the trailer framing, but this will require more fittings, expense, and avoiding other obstructions. Any ideas?

    I will be happy to answer any additional questions if someone needs clarification.

  • #2
    using 10X6 ducting would work, but it would be labor intensive if you don't have access to a press brake. you would have to remove the floor boots and run a length of 10X6 and see where the floor boot ends up - middle of run? at a joint? etc. if it is in the middle of a run then just mark out where the boot ends up, remove the section and cut the round hole for the boot - then put the duct back up and fit the boot into the round hole and go on to the next duct may not be able to use a full length 10X6 but shorter sections so the boot doesn't fall on a joint.


    • #3
      Thanks for that suggestion. I think that would probably work, as a 6X10 duct in that space would leave around 10 inches between the upper side of the duct and the upper side of the floor. I think 6X10 might be a little small, but I could go with a bigger size and probably do the same thing.


      • #4
        I have another question while I'm here: the bottom opening on my furnace/AC unit is 10X16, with metal fingers to attach to a duct board plenum. Am I likely to be able to find a prefab metal plenum that fits that, or should I plan on getting one custom made?


        • #5
          since this is a down flow unit with no plenum to speak of, is it at the middle of a run or the end of a trunk line? reason I'm asking this, is, I'll make up a diagram through microsoft "paint" for you to follow. take offs from a furnace almost always are a custom fit. upflow furnaces use factory bent plenums that fit the upflow ducting perfectly. also you mentioned that 10X6 is too small. OK, I'll agree - a 12X6 will give you more air flow.


          • #6
            The furnace opening is in the middle, toward one end, but with the main duct going both directions, east and west. There are two registers east of the furnace opening, with the most distant register about 5 or 6 feet from the furnace. Going west from the furnace, the duct feeds 4 registers over a stretch of about 60 feet. The downflow opening is positioned directly in line with the row of registers. I don't remember whether the original ducting that came out two years ago included any kind of plenum, but I think the main line just had an opening in the top connecting it to the furnace. Would some sort of larger space beneath the furnace provide a significant airflow improvement in this scenario?
            Last edited by Freewheel; 05-20-2015, 08:24 AM.


            • #7
              OK. From what you've listed, the furnace ducting opening is on the backside approximately center of the furnace, or the side of it at approx center. Coming off the furnace is a flexible transition fitting [ has a rectangular collar about 6 inches long with a rubber flex joint and another 6 inches of sheet metal duct.] This feeds into a 12X6 "turning elbow, downwards into a 12X6 trunk line. At the floor level the 12X6 goes into a Y fitting, traveling east and west into major trunk lines. The extreme ends are capped off. Floor registers are tapped into the trunk line on top of the line with a register boot. These could be 6 inches round to 10X6 rectangular to a floor box and steel register with or without a damper.


              • #8
                Here is a supplier of rectangular duct work and transitional fittings. []Ductwork | Ductwork Supplies | Sheet Metal Ductwork
                I checked Home Depot and all they advertise is round 6 & 8 inch duct.
                Ductwork consists of "drives" these are flattened out U-s, 6 or 8 inches long that fit over the joints of rectangular trunk line. each trunk line has a quarter inch folded back side wall of the trunk that will accfept a drive. the long section of the trunk uses what is called an S cleat, half of which slides over one top part of the trunk and the other half slides over the other top part of the adjoining trunk. I'll make up some diagrams to try to make this point clearer. I used to make all my own trunk lines and parts only because I had access to a press brake.


                • #9
                  HayZee, thanks for all your advice here. I think I didn't communicate very clearly, but the furnace duct opening faces downward, flush with the floor directly above the original trunk line. If I laid rectangular duct where the original line was, a flexible transition fitting like you describe might work fine to go directly from the downward-facing 10X16" furnace opening into the top of the trunk line. I have a very basic understanding of how rectangular duct goes together, but the connection to the furnace and the connections to the registers have had me a bit puzzled, as the restricted space between the top of the trunk line and the register openings makes it a bit hard to fit a standard collar and duct boot between the main duct and the floor register. Does that make sense?

                  It's getting warm here in OK and I was really hoping to get the AC up and running before my in-laws visit in 6 days, so I'm pressed for time. I don't know if I can wait for mail-order parts, unless they're fast shippers. Would a local HVAC contractor be likely to have these components on hand?


                  • #10
                    Click image for larger version

Views:	1
Size:	23.1 KB
ID:	87343Not sure if a local dealer or hvac contractor would have fittings on hand. anyways I'm sure he can make the trunk lines cheaper than pre-made ones. home depot shows one half of a trunk line costs $19.75 each so a complete trunk would be over 38 bucks. an hvac person can make a trunk in one piece with a pittsburgh joint.
                    ever hear the term "tin knocker?" if came about from the fact that trunk lines made in one piece had a pittsburgh joint, a half bent sheet metal part goes into a formed s joint and the flap would be hammered over the joint.