How accessible does HVAC ductwork need to be (new construction question)?


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Old 12-16-14, 09:52 AM
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How accessible does HVAC ductwork need to be (new construction question)?

I am working on a plan for a second story addition to my ranch home. The ductwork is rigid and run in the attic (long trunk line that runs the length of the house). One of the ways I've read that contractors simplify 2nd floor additions is to build a second floor platform over the existing ceiling joists (rather than sistering them):

Ranch To Two Story-how Do They Do The Tearoff/framing? - Building & Construction - DIY Chatroom Home Improvement Forum

This is typically done in order to get around the fact that there's wiring and all kinds of other stuff run over the joists with no slack. It also keeps the first floor relatively weather tight and allows the homeowner to stay in place while work is going on. I'd like to do something like this, but basically encapsulating the existing ductwork. Would that be permitted by code, etc...? It would look like this:

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I'll have to run all of these plans by the code official eventually anyway, but I'm trying to get an idea because if this can't work, I have to change the floorplan entirely.
 
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Old 12-16-14, 10:08 AM
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Personally I have never seen another deck built above the existing ceiling with a space between for duct work. I'm certain it can be done but it seems like it wastes a lot of space and would be an expensive method of construction. Adding a second floor is major work so re-doing duct work and wiring is just one little step in a big project.

Leaving the existing ducts, wiring and attic insulation does not keep the house weather tight. Once the roof is off you can forget much of the weather tight stuff. Stay in the home when adding a second floor has been done by a relative of mine. Their family was in that line of work and they were highly organized, had material and labor ready and waited for a good weather window and hit it hard. If you are relying on a contractor I would not count on staying in the home while the work is being done but if you do I would prepare for the possibility of water getting into the home.
 
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Old 12-16-14, 05:34 PM
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I get what you're saying about moving ductwork seeming a relatively minor part of a big job, but the fact of the matter is, there's nowhere to move it to that's practical. However, I think I found a solution to my problem-my local lumberyard can supply 12" deep open web floor trusses for about $4.25 per foot. However, the question remains-can I enclose my ductwork in that manner?
 
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Old 12-17-14, 05:00 AM
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What do you mean by "in that manner"?

Scissor trusses can allow ducts, water and electrical to pass across in many cases. You'll have to pay attention to your trunk line and what direction it runs. The trunk is too big to pass through the openings in a truss that size and depending on the size of your lateral duct lines it may be a squeeze.

You can also get custom, engineered trusses. Depending on how many you need they are not much more expensive but openings can be built into them for what you need.
 
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Old 12-17-14, 07:03 AM
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What do you mean by "in that manner"?
In other words, can a main trunk be located in an area that will be totally inaccessible after construction is completed (between floors-not suspended in an unfinished basement or attic)?

Scissor trusses can allow ducts, water and electrical to pass across in many cases. You'll have to pay attention to your trunk line and what direction it runs. The trunk is too big to pass through the openings in a truss that size and depending on the size of your lateral duct lines it may be a squeeze.
I'm still in the process of working all of this out, but the main problem is that the main trunk runs perpendicular to the joists for the entire length of the house currently. Since the trusses would allow 24" OC spacing it should be easy enough to move laterals around for the existing registers and locate the new ones conveniently.

You can also get custom, engineered trusses. Depending on how many you need they are not much more expensive but openings can be built into them for what you need.
Again, with 24" OC spacing I'll only need 14. If I go with 28' trusses that are "off the shelf" I'm looking at a little less than $1900 with tax and delivery. I'd gladly pay a little extra for a void in the web placed exactly where I need it. A quick check of Lowes online pricing for dimensional lumber (2x12x12s and 16s spaced 16" OC) yields a price of $1125 with tax and delivery. Double the price for none of the headaches I suppose.
 
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Old 12-17-14, 07:59 AM
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If it is all metal duct then yes it can be inclosed. If it is flex duct it will have to be replaced sometime down the road so no it should not be closed up in a floor.
 
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Old 12-17-14, 08:02 AM
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Yes, trunk lines can and are commonly put in inaccessible locations. Basically everything between your two floors will only be accessible by cutting through the floor or ceiling.

If your HVAC trunk line runs perpendicular to the joists then that is something you need to figure out up front. My house has 20" tall scissor trusses and even at that size the trunk could not run perpendicular through the standard zig-zag section. A custom truss could probably have an open bay area near a load bearing wall for your trunk line.

There can be trade offs with truss sizing spacing. In my house I could have done 24" ctr to ctr with 18" high trusses but I choose to go to the taller 20" trusses to minimize bounce in the floor and 19.2" ctr to ctr spacing to minimize sagging between joists. I'm also a big fan of the premium OSB products for subflooring like Advantech.
 
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Old 12-17-14, 08:32 AM
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If your HVAC trunk line runs perpendicular to the joists then that is something you need to figure out up front. My house has 20" tall scissor trusses and even at that size the trunk could not run perpendicular through the standard zig-zag section. A custom truss could probably have an open bay area near a load bearing wall for your trunk line.
Thanks-it's good to have someone with real world experience weighing in on this. With regard to spacing, I'm hoping that bounce won't be an issue because I'm really not spanning very far compared to what these things can do. My house is 24'-6" wide with a load bearing wall in the center, so a 28' truss would be set flush with the front wall, span about 12' center load bearing wall, another 12' to the back wall, and cantilever past it by about 3'-6". I'll need an engineer to sign off on that of course, but I'm hoping that applying plywood gussets to both sides of the cantilevered portion as per the truss manufacturer's recommendation will suffice. Also, the duct runs parallel to the load bearing wall about 2" off of the center, so I'm hoping the custom void shouldn't really be an issue.
 
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Old 12-17-14, 09:34 AM
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If you buy custom trusses the truss company generally does all the engineering and provide signed and stamped drawings for your Building Inspections Dept. It's all very standard stuff for the truss companies so it's not that expensive to go custom especially when you consider the engineering they provide in the cost.
 
 

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