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  1. #1
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    Default HVAC suspended from rafters. What is the correct method?

    Regarding the steel straps that are used to connect some attic HVAC units to the rafters,I see varying amounts of fasteners holding the steel straps to the rafters. Sometimes I see 3 nails, sometimes 2 nails, sometimes 1, and some time sheet metal screws (See the photos). Also I sometimes see only one sheet metal screw holding the HVAC cabinet to the straps. I recently heard about an attic unit pullling loose from the rafters. My question is there a guideline for the correct amount of fasterers connecting to the rafters and also connnecting to the cabinet? I so far have not located a guideline.

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  2. #2
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    Default Re: HVAC suspended from rafters. What is the correct method?

    Gene,

    The answer is: It depends.

    It depends on the species of wood.

    It depends on how thick the metal is and if aluminum or steel.

    It depends on if a nail or a screw.

    It depends on the size of the nail or screw.

    It depends on if you are referring to pull out or shear.

    I did not find a shear/pull out chart like I was searching for, but did find this one:
    - Common wire nails

    Also, while searching, and makes good reading, are these, and they show one of the connections I described on another thread - the split-ring connector:
    - http://www.fpl.fs.fed.us/documnts/fp...chapter_08.pdf (scroll down to page 8-19)
    - http://www.woodweb.com/Resources/woo...dbook/Ch07.pdf (different chapter, same connector on page 7-19)

    -http://www.awc.org/pdf/NDSCommentaryCompressed/Part12NailsandSpikespp140to149.pdf
    - http://www.most.gov.mm/techuni/media...014_chap2a.pdf

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

  3. #3
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    Default Re: HVAC suspended from rafters. What is the correct method?

    Hey Jerry, thanks. I will check out those links. Gene


  4. #4
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    Default Re: HVAC suspended from rafters. What is the correct method?

    Gene,

    Here is an interesting, and maybe helpful, online calculator for loads:
    - Connection Calculator

    If you select the following:
    a) Allowable Stress Design (ASD)
    b) Lateral Loading (for shear)
    c) Nail (or wood screw, your choice)
    d) Single Shear (only choice)

    Then click 'Submit Initial Values'

    The select the following:
    a) Southern Pine
    b) 1.5 in (the default)
    c) Steel (the default)
    d) 20 gage (the metal straps are probably 26 gage, there is no thinner choice than 20 gage)
    e) Common Wire
    leave all the remaining choices at 1.0 unless you know differently, for example Load Duration Factor for 10 years after the nail was driven is 1.0, then click 'Calculate Connection Capacity',

    The Adjusted Stress Design Capacity (ASD) is shown as 76 pounds.

    Note that 1 nail in each of of the 4 straps would be 76 x 4 = 304 lbs.

    If there were 2 nails in each of the 4 straps, the capacity does not double as nail spacing affects the additional capacity, as does the strength of the metal. With a 26 gage strap the strength of the metal is not going to allow much increase for additional nails as the strap capacity will be exceeded.

    How much does that air handler weigh?

    When I repeat the above but start with 'Wood Screw' and select a #8 screw, 1" long (which is likely the length used), then click to calculate, the answer is "Inadequate penetration length of the wood screw in the main member. Try selecting a longer wood screw, or a thicker main member, or a thinner side member."

    When I go to a 1-1/2" long screw, which has likely not been used, the answer is ASD = 97 lbs.

    Now, I wonder what the screw into the metal housing is ... (there is no option on this calculator for metal to metal) ...

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

  5. #5
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    Default Re: HVAC suspended from rafters. What is the correct method?

    I think support should be from floor joists. Insulate the attic floor, including space under a platform for the HVAC unit. Rest ducts on the insulation, and insulate ducts with tucked-in over-wrap.

    Like this with flexible ducts.

    Like this with solid ducts.

    I don't think 2x4 roof joists are meant to be point-loaded this way. Why would you?


  6. #6
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    Default Re: HVAC suspended from rafters. What is the correct method?

    I recently had a new furnace installed in my attic and the installation manual specifies 3/8" threaded rod and 2x2x1/8 angle iron under the cabinet. See page 9 of this pdf.
    http://www.alpinehomeair.com/_viewresource.cfm?ID=2252

    Based on this I would think that what you have pictured is woefully inadequate. In any case, if nails into the rafters are sufficient, I think those straps should go all the way around the furnace.


  7. #7
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    Default Re: HVAC suspended from rafters. What is the correct method?

    The HVAC installs I generally see have the airhandlers suspended on all-thread
    bolted to the rafters and connected to two pieces of unistrut as a saddle. System sits on rubber/cork vibration isolators. This is a much more robust method. This doesn't really address the OP's question but the system pictured does look pretty flimsy. I would be concerned about the sheet metal screws vibrating out.


  8. #8
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    Default Re: HVAC suspended from rafters. What is the correct method?

    What do you think of this installation?

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  9. #9
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    Default Re: HVAC suspended from rafters. What is the correct method?

    Can the sheetmetal screws or nails pull out, Yes. So far I have not seen a unit where straps with screws and nails have totally failed. However, I have seen units where the nails clearly were pulling out of the rafters and the sheetmetal screws were pulling out from the cabinet.
    That type of install used to be more common around here but has faded away. At least I don't see it much anymore. Most HVAC guys have gone to hanging the units with rod and unistrut. This allows for easier servicing and the easy installation of a pan under the unit.
    I write up what you have as a poor install, consider changing it, and installing a pan.

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  10. #10
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    Default Re: HVAC suspended from rafters. What is the correct method?

    Quote Originally Posted by Rich Goeken View Post
    What do you think of this installation?
    Well ... the flexible ducts are required to be strapped within 18" of the metal collar they are attached to where the duct is still straight out from the attachment collar and *before* the duct bends and goes in another direction.

    Also, when one reads the installation manual for the flex ducts, it seems to imply that there should be a support within 18" of each side of the bend.

    The straps are required to be at least 1-1/2" wide, and look like they are.

    I doubt that the spray foam insulation is deep enough to meet the required minimum R-value (I keep a piece of #14 AWG solid copper wire in my clip board so I can poke it into the insulation to see how deep it is - it is seldom, if ever, applied as thick as required for the required R-value - I measure the thickest part so I can state how thick that is, the thinnest part, and the 'average' thickness - if the 'thickest part' is too thin ... ). Is there an ignition barrier sprayed on over the spray foam insulation?

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

  11. #11
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    Default Re: HVAC suspended from rafters. What is the correct method?

    Quote Originally Posted by Jerry Peck View Post
    Well ... the flexible ducts are required to be strapped within 18" of the metal collar they are attached to where the duct is still straight out from the attachment collar and *before* the duct bends and goes in another direction.

    Also, when one reads the installation manual for the flex ducts, it seems to imply that there should be a support within 18" of each side of the bend.

    The straps are required to be at least 1-1/2" wide, and look like they are.

    I doubt that the spray foam insulation is deep enough to meet the required minimum R-value (I keep a piece of #14 AWG solid copper wire in my clip board so I can poke it into the insulation to see how deep it is - it is seldom, if ever, applied as thick as required for the required R-value - I measure the thickest part so I can state how thick that is, the thinnest part, and the 'average' thickness - if the 'thickest part' is too thin ... ). Is there an ignition barrier sprayed on over the spray foam insulation?
    JP.. You missed the substantial support provided under the HVAC distribution box.


  12. #12
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    Cool Re: HVAC suspended from rafters. What is the correct method?

    Those rafters were sized for the expected live and dead loads. Now, you add additional dead load without any reinforcement. Ideally, those rafters should be doubled up but this is seldom done.

    There is plenty of strapping available that is less than 20 gauge. You can hardly cut it as it twists in your shears.

    Most pros I know use 3/8" all thread rod either directly through-bolted to the cabinets or better yet, to Uni-strut trapezes.

    Keep the fire in the fireplace.

  13. #13
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    Default Re: HVAC suspended from rafters. What is the correct method?

    Obviously follow the manufacturer's instructions, but we may be over thinking this a bit.
    Just for fun, I just googled the weight of a 3 ton attic heat pump unit commonly used here.
    At the listed shipping weight of 115 lbs. I doubt there is a need to loose sleep over the weight placed on the trusses.
    Say there are 4 straps, all-thread or struts, that is less than 30 lbs. per corner. Even at double the weight (doubtful) for a gas furnace we are still not talking about anything that will really tax the structure of the average house.

    Jim Luttrall
    www.MrInspector.net
    Plano, Texas

  14. #14
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    Default Re: HVAC suspended from rafters. What is the correct method?

    Jim has a good point about the weight.

    One thing I always look for is minimum three screws at every connection or joint. If strap is used I'd prefer to see it wrapped over the top edge of the cord but if it is the top cord that's going to be hard to do. So the strap should have at least three 1-1/4" screws into the wood and the strap should continue down to underside of the duct with at least three 1/2" screws into the metal.

    Never allow sheet metal screws into the cabinet except at the duct and coil flange connections.

    Truth is a 20 gauge strap can hold up a lot of weight.


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