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Thread: Rigid vs Flex

  1. #53
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    Quote Originally Posted by Tom Thermo View Post
    What I don't like about flex in crawlspaces is the DAMM RATS !!!
    That's why they keep Cats in the crawlspace.
    Hos 4:6 My people are destroyed for lack of knowledge. Because you have rejected knowledge, I will also reject you, that you may be no priest to me. Because you have forgotten your Gods law, I will also forget your children.


    "You've got to Stand for Something or You'll fall for anything" (A. Tippin)


    We are on a "Man-made Highway to Hell", Our so called "Leaders", Political & Religious, are encouraging The Mushroom like Sheeple to go faster.

    (AC-DC Lyrics)

  2. #54
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    Quote Originally Posted by Scoobie View Post
    I see this all the time, but dont think its a problem. Your intuition says the cold air gets pulled right back into the return, but it really doesnt. If a little supply gets sucked in there, so what...it will dehumidify better. I often get blank stares when I make this point, lol.
    As I see it an egg crate return has an influence of about 1' around it. The diffuser has a throw of around 4.5' so about a third of the air is going into the return. Let's say you have a space that needs 150 cfm of conditioned air in, it also needs 150 cfm of room air out. For every CFM of supply air that goes directly into the return you have 1 cfm of room air that is not leaving. The space gets warm, the return air gets cooled, and the thermostat gets lowered to compensate. Eventually you get to a 70 room temp with a 67 or lower return temp and the tech wonders why he gets a freezing coil and can't get the pressures up. There is a thread on this site within the last month of a tech with a 65 return air and can't figure out why his suction pressure is so low.

  3. #55
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    Quote Originally Posted by DavidDeBord View Post
    An HVAC System, Properly Sized, & Laid out, could easily installed in a Home with "Floor Joists" that are 16" O.C., with Flex Duct, or Hard Pipe.

    I've never, ever, installed Flex Pipe, in a Basement, ... It's always been, & will be Metal Duct.

    In Crawl Spaces, & Attics, the S/A & R/A Trunkline has always been metal, insulated on the outside, with all Supplies & Returns ran in Flex Pipe. Not Once, have I ever been red-tagged for using Flex.

    Now, .... "If" the Customer wanted Metal Duct, I gave them that option, but 95% of the time, when they saw the extra Labor/ material cost, ... They quickly went with Flex System.
    He is not talking about 16" OC floor joist but 16" truss that only have a 13" opening in them for a chase. Usually the equipment is located closer to one end so you need 10" deep duct to move enough air. That gives you 4.5" of space above the duct until you get enough runs off to reduce it. Since you return needs to be as big or bigger than supply you can't get a 6" heat run across the return until it reduces or an 8" return across the supply anywhere. If you look at a single return free air opening it equals a 8" pipe.

  4. #56
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    Where I am we predominantly run our systems in crawl spaces so we use 20x10 / 16x8 / 12x8 to get good back pressure in some situations which require long duct runs. In ranchers or two stories without a crawl we run it under the joists and the builder drops the ceilings in certain areas. We NEVER run duct in attic. Sounds like a giant pain in the ass. We never run flex. Its always rigid pipe for installs. It does make us on the higher end of the quotes in town but from what Im told flex isnt as good as rigid pipe in terms of delivery.

  5. #57
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    Quote Originally Posted by BNME8EZ View Post
    He is not talking about 16" OC floor joist but 16" truss that only have a 13" opening in them for a chase. Usually the equipment is located closer to one end so you need 10" deep duct to move enough air. That gives you 4.5" of space above the duct until you get enough runs off to reduce it. Since you return needs to be as big or bigger than supply you can't get a 6" heat run across the return until it reduces or an 8" return across the supply anywhere. If you look at a single return free air opening it equals a 8" pipe.
    BNM,

    Look at the prints in Post # 39, ... No "Trusses" are shown.

    Then look at Post #49, ... He stated "Floor trusses".

    That's why I questioned it.

    2" x 10" Floor joists offer ample room for Ductwork, be it Metal, or flex.

    As far as Trusses? It would depend upon what they were Specified to be, & accepted by the Building Department, ...2" x 4"? 2" x 6"? By 8"?
    It also depends upon the "Bearing load".

    But again, & I'm not "pickin' a Fight here"

    His prints, are about "Floor Joists", not Overhead "Trusses", or "Rafters".

    He's calling a "Floor Joist" a "Truss".
    Hos 4:6 My people are destroyed for lack of knowledge. Because you have rejected knowledge, I will also reject you, that you may be no priest to me. Because you have forgotten your Gods law, I will also forget your children.


    "You've got to Stand for Something or You'll fall for anything" (A. Tippin)


    We are on a "Man-made Highway to Hell", Our so called "Leaders", Political & Religious, are encouraging The Mushroom like Sheeple to go faster.

    (AC-DC Lyrics)

  6. #58
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    Quote Originally Posted by BNME8EZ View Post
    As I see it an egg crate return has an influence of about 1' around it. The diffuser has a throw of around 4.5' so about a third of the air is going into the return. Let's say you have a space that needs 150 cfm of conditioned air in, it also needs 150 cfm of room air out. For every CFM of supply air that goes directly into the return you have 1 cfm of room air that is not leaving. The space gets warm, the return air gets cooled, and the thermostat gets lowered to compensate. Eventually you get to a 70 room temp with a 67 or lower return temp and the tech wonders why he gets a freezing coil and can't get the pressures up. There is a thread on this site within the last month of a tech with a 65 return air and can't figure out why his suction pressure is so low.
    If you have a four way diffuser only 25% of the air is going towards the return. So let’s say the return is 150 cfm also. If 100% of the supply air heading towards the return gets sucked in, you still have a lot of return air. The supply aircruising across the ceiling will not get pulled in for the most part. It goes against your intuition for sure.

  7. #59
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    If a return can sucks in supply air depends on if the supply has a velocity that is low enough. A supply grill/diffuser by design entraines room air and develops a plume. The design choice depends on the space being conditioned and how the plume is intended to mix with the room air. If the velocity is too low the air does things like clinging to the ceiling or not entraining enough room air before the energy of the fan is spent.

    If the grill terminal velocity is great enough, the air won't enter the return. It has to do with the inertia of the air from the fan energy and if there is enough energy so the air can't make an abrupt turn. A plume tends to stay a plume. Temperature also plays a part in that air of a certain temperature doesn't want to mix with other temperatures. We see this with tornadoes. Two separate air masses of different temperatures. I've tried to mix air inside large air handlers where I needed to mix two temperatures. They really resist.
    Think of an air plume as wanting to remain a plume due to temperature and fan energy.
    It's not that supply air can't enter a return, it's more about why. I've known office heat pumps that when the filters plug and the supply velocity falls, this can happen. In some cases the supply air is low enough to cause a high head condition. So keep the filters clean or increase fan speed, or move the return. Maybe do all 3.

    A third is move the return closer to the supply. I know that sounds counter-intuitive but when understanding that for air to enter the return, the return air has to overcome the kinetic energy of the fan. By moving the return closer, the air can't make the turn. Like driving a car and trying to make a sharp turn and going off road.
    I should have played the g'tar on the MTV. MK

    You can be anything you want......As long as you don't suck at it.

    USAF 98 Bomb Wing 1960-66 SMW Lu49

  8. Likes Scoobie, BBeerme liked this post
  9. #60
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    I didn't mention but another choice might be to replace the diffuser with a three way.
    I should have played the g'tar on the MTV. MK

    You can be anything you want......As long as you don't suck at it.

    USAF 98 Bomb Wing 1960-66 SMW Lu49

  10. #61
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    Thread Starter
    Oh my! Let me try and clarify. My illustrations are just napkin drawings, not take offs from a plan. Let me try to clarify. A 2nd floor can have timber joists, (ie 2x10 etc), or I-Joists, (top/bot cord with plywood in between with max. residential depth of 16"), or engineered flat floor trusses, (2x4 top/bot cord with 2x4 webbing), In my case the first 2 are out and so will be using floor trusses. In my drawing you see a stairwell, I am now changing the direction of the trusses to run parallel with the stairs, so that every 2' there will be an opening to run trunks for supply and return. This also means a 16" high, (depth), truss will allow a full 16" of use from bottom of bottom cord to bottom of plywood floor above. The webbing will have plenty of room for branches. It's up to myself to order out the truss depth, 16", 18" to 24", whoever is drawing the plan makes this call. Loads are not an issue here as they are standard. If I go metal, then no conversation needed. That probably isn't going to happen, so then back to flex, and with it the only problem I see now is enough room for the return, (say 18" plus 4 1/2" insulation, that's 22 1/2"). The supplies can go in with 2 mains, so don't a problem there. I think it's pretty assumable that it will be a 31/2 ton high seer split system on platform in the garage. Then going into the attic over the kitchen to a plenum there and from there mains directly in between the 2' oc floor trusses. Mostly the size of the return that stumbles me. If indeed I'd need 22 1/2" for the return, then all the trusses would have to be 24". I've added 2 links to charts on truss and mechanicals. Please open them and I think you'll see more clearly. Thanks

    Kenny

    https://www.selecttrusses.com/mechan...ce-clearances/

    https://www.selecttrusses.com/span-chart/

  11. #62
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    Quote Originally Posted by Kenny4 View Post
    Oh my! Let me try and clarify. My illustrations are just napkin drawings, not take offs from a plan. Let me try to clarify. A 2nd floor can have timber joists, (ie 2x10 etc), or I-Joists, (top/bot cord with plywood in between with max. residential depth of 16"), or engineered flat floor trusses, (2x4 top/bot cord with 2x4 webbing), In my case the first 2 are out and so will be using floor trusses. In my drawing you see a stairwell, I am now changing the direction of the trusses to run parallel with the stairs, so that every 2' there will be an opening to run trunks for supply and return. This also means a 16" high, (depth), truss will allow a full 16" of use from bottom of bottom cord to bottom of plywood floor above. The webbing will have plenty of room for branches. It's up to myself to order out the truss depth, 16", 18" to 24", whoever is drawing the plan makes this call. Loads are not an issue here as they are standard. If I go metal, then no conversation needed. That probably isn't going to happen, so then back to flex, and with it the only problem I see now is enough room for the return, (say 18" plus 4 1/2" insulation, that's 22 1/2"). The supplies can go in with 2 mains, so don't a problem there. I think it's pretty assumable that it will be a 31/2 ton high seer split system on platform in the garage. Then going into the attic over the kitchen to a plenum there and from there mains directly in between the 2' oc floor trusses. Mostly the size of the return that stumbles me. If indeed I'd need 22 1/2" for the return, then all the trusses would have to be 24". I've added 2 links to charts on truss and mechanicals. Please open them and I think you'll see more clearly. Thanks
    --------------

    Kenny

    https://www.selecttrusses.com/mechan...ce-clearances/

    https://www.selecttrusses.com/span-chart/
    Keeny4,

    Floors are built upon "Joists".

    "A joist is a horizontal structural member used in framing to span an open space, often between beams that subsequently transfer loads to vertical members. When incorporated into a floor framing system, joists serve to provide stiffness to the subfloor sheathing, allowing it to function as a horizontal diaphragm. Joists are often doubled or tripled, placed side by side, where conditions warrant, such as where wall partitions require support.

    Joists are either made of wood, engineered wood, or steel, each of which have unique characteristics. Typically, wood joists have the cross section of a plank with the longer faces positioned vertically. However, engineered wood joists may have a cross section resembling the Roman capital letter "I"; these joists are referred to as I-joists. Steel joists can take on various shapes, resembling the Roman capital letters "C", "I", "L" and "S". "

    https://en.wikipedia.org/wiki/Joist

    https://www.lowes.com/pl/Joists-Lumb...ies/2334369394
    ----------------


    Roofs are built upon "Trusses".

    "For other uses, see Truss (disambiguation).
    Truss bridge for a single-track railway, converted to pedestrian use and pipeline support
    An Egyptian ship with a rope truss, the oldest known use of trusses. Trusses did not come into common use until the Roman era.
    Typical detail of a steel truss, which is considered as a revolute joint
    Historical detail of a steel truss with an actual revolute joint

    In engineering, a truss is a structure that "consists of two-force members only, where the members are organized so that the assemblage as a whole behaves as a single object".[1] A "two-force member" is a structural component where force is applied to only two points. Although this rigorous definition allows the members to have any shape connected in any stable configuration, trusses typically comprise five or more triangular units constructed with straight members whose ends are connected at joints referred to as nodes.

    In this typical context, external forces and reactions to those forces are considered to act only at the nodes and result in forces in the members that are either tensile or compressive. For straight members, moments (torques) are explicitly excluded because, and only because, all the joints in a truss are treated as revolutes, as is necessary for the links to be two-force members.

    A planar truss is one where all members and nodes lie within a two dimensional plane, while a space truss has members and nodes that extend into three dimensions. The top beams in a truss are called top chords and are typically in compression, the bottom beams are called bottom chords, and are typically in tension. The interior beams are called webs, and the areas inside the webs are called panels.[2] "

    https://en.wikipedia.org/wiki/Truss

    https://www.menards.com/main/buildin...ses/c-5658.htm
    Hos 4:6 My people are destroyed for lack of knowledge. Because you have rejected knowledge, I will also reject you, that you may be no priest to me. Because you have forgotten your Gods law, I will also forget your children.


    "You've got to Stand for Something or You'll fall for anything" (A. Tippin)


    We are on a "Man-made Highway to Hell", Our so called "Leaders", Political & Religious, are encouraging The Mushroom like Sheeple to go faster.

    (AC-DC Lyrics)

  12. #63
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    Thread Starter
    For the sake of this thread, legal structural definitions aren't necessary. If you are an HVAC guy, why not just help a guy out with what he is asking for vs a lot of legal mumbo jumbo. As I said, I've already ruled out I-Joists and 2x's, so that leaves floor trusses. And the only question would be, what would be the minimal depth/height to accommodate flex ducts. Thanks

    Kenny

  13. #64
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    Yeah, that was kind of gay. Two guys forcing their members on each other. But, since you are new here, the bigger point here is:

    Although there is just about every answer to every problem on this forum [because we are just totally awesome], you do need a bit of discretion and/or thick skin. You'll get primadonna's here telling you how to wipe your butt, then there are others who have been drinking a bit too much, and go a bit off the handle.

    Personally, I'm not really into the flame wars. I have been sucked into one or another in the past, now there is a point when I just stop responding. Usually much sooner than later.

    Your choice how you want to react or behave in the future.


    Quote Originally Posted by Kenny4 View Post
    For the sake of this thread, legal structural definitions aren't necessary. If you are an HVAC guy, why not just help a guy out with what he is asking for vs a lot of legal mumbo jumbo. As I said, I've already ruled out I-Joists and 2x's, so that leaves floor trusses. And the only question would be, what would be the minimal depth/height to accommodate flex ducts. Thanks

    Kenny
    If you were a real tech, you'd solder a relay on that board and call it good to go.

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