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Old 12-28-2011, 12:56 PM   #1
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any thoughts on using plywood for trunk lines?


I am thinking of using cabinet grade (smooth) plywood for a run of 20x12" supply and returns in a conditioned space for cost and finishing reasons. I am planning to seal the interior of these ducts with polyurethane or a high quality gloss paint. The first 8' off the supply and return plenums would be traditional sheet metal but then I would transition to the wood after that. Any thoughts?
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Old 12-28-2011, 01:08 PM   #2
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Hello and welcome Mike to the best darn DIY'r site on the web.

You will get many responses to this post, not many of which will agree with your thought process.

This kind of thinking is exactly why there are building codes, and in some areas a home owner can't even pull a permit to do their own HVAC work. There are reasons why supply ducts are to be a minimum of 1" from combustible materials let alone be made of them.

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Old 12-28-2011, 01:24 PM   #3
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any thoughts on using plywood for trunk lines?


It's bad enough that our duct systems can transport the incendary heat of a fire unseen anywhere throughout a domicile, but to line it with a fuel source ??????
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Old 12-28-2011, 02:05 PM   #4
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any thoughts on using plywood for trunk lines?


dont do it............
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Old 12-28-2011, 04:59 PM   #5
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any thoughts on using plywood for trunk lines?


It is completely against code!
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Old 12-28-2011, 05:05 PM   #6
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any thoughts on using plywood for trunk lines?



Measuring/Monitoring Moisture Levels

http://www.woodweb.com/knowledge_bas...re_Levels.html

The hows and whys of monitoring moisture content in lumber. 1998.
By Dr. Eugene Wengert
I. Moisture and RH
Wood is a hygroscopic material, meaning that it is continually trying to achieve an equilibrium moisture condition with its environment. As the relative humidity (RH) changes, so does the moisture content (MC) of the wood. The following tabulation expresses the relationship between MC and RH.
MCRHEMC%%%519 to 255625 to 326732 to 397839 to 468946 to 5291052 to 58101158 to 6411
Note that temperature is not an important factor; it is just the RH. Also, these numbers hold true for all species of wood.
II. Moisture Effects on Wood-Shrinkage
So, what is the big deal about the MC of wood? The truth is that the properties of wood change with changes in MC--most noteworthy is that wood shrinks and swells substantially with MC changes. plus, with the property changes come changes in processing--gluing, machining, fastening, and finishing. Let's briefly look at a few of these changes in order to establish how closely we need to measure and control moisture levels.
Of all the changes, certainly the most troublesome change has to be shrinkage and swelling--wood shrinks when the MC decreases and swells when the MC increases. And to complicate matters, wood shrinks differently in the three directions--lengthwise shrinkage is usually quite close to zero; across the grain, parallel to the rings (width of a flatsawn piece) wood shrinks up to 1% for a 3% to 4% MC change; and across the grain, perpendicular to the rings (thickness of a flatsawn piece) wood shrinks about 1% for a 7% MC change. Of course, there is variation from piece to piece of the same species, as well as variation from species to species. The following tabulation gives the average change in size for a 3 inch wide, flatsawn piece of lumber for a 2% MC change for several important species. Shrinkage is essentially a linear function of MC, so double the values for a 4% MC change, triple them for 6% MC change, and so on.
SPECIESSIZE CHANGESPECIESSIZE CHANGEInchesInchesApitong0.031 Mahogany0.014Ash, white0.016Maple, hard0.021Basswood0.020Maple, soft0.015Banak0.019Oak, red0.022Birch, yellow0.020Oak, white0.020Cherry0.015Pine, Southern0.016Hackberry0.019Pine, white0.012Hemlock, Eastern0.014Teak0.011Hickory0.019Yellow-poplar0.017In summary, we are looking at a 3 inch wide piece changing 1/100 to 3/100 inches in width for a 2% MC change--a very small size change. But such a change is quite large when it comes to edge gluing and to flatness in a high gloss finish, as is discussed later.
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Old 12-28-2011, 05:22 PM   #7
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any thoughts on using plywood for trunk lines?


No flammable material is allowed inside any duct that can have supply temps that exceed 120F.

Quote:
Polyurethanes are a good example of a traditional organic polymer system that has useful structural and mechanical properties, but is limited by its low thermo-oxidative stability. Although many varieties of this polymer are widely used in aerospace applications and home construction, they are highly combustible.
Quoted from PA State University.
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Old 12-28-2011, 09:14 PM   #8
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Don't do it...period.....
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Old 12-28-2011, 09:24 PM   #9
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any thoughts on using plywood for trunk lines?


I'd advise against it
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Old 12-29-2011, 06:08 AM   #10
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Noaaaaaaaaaaaaaaaaaaaaaaaaaaaaah.
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Old 12-29-2011, 01:48 PM   #11
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any thoughts on using plywood for trunk lines?


This is not intended to criticize and I don't know a lick about HVAC but my common sense would tell me that's a terrible idea. Wood burns, water damage, all that work to seal it and protect it from from water damage (why), not to mention possible insects/termites. Just sounds like a disaster idea.
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Old 12-30-2011, 06:43 PM   #12
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any thoughts on using plywood for trunk lines?


Quote:
Originally Posted by beenthere View Post
No flammable material is allowed inside any duct that can have supply temps that exceed 120F.
I don't think OP said whether he had a heat pump, an A/C or a furnace. If it was anything but a furnace, would the codes still apply? And if below, 120F would wood then be OK?

If the wood was covered with a material meeting the code required flame spread rating, would that allow wood to be used? For example a layer of FSK glued on? I was thinking about return air plenums that often pass by various building materials.
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Old 12-30-2011, 07:03 PM   #13
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any thoughts on using plywood for trunk lines?


A heat pump with electric aux heat, can still have an air discharge temp of 120.

Don't believe it would pass code, but can't say for sure. But it wouldn't prevent the wood from reaching high temp.
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Old 12-30-2011, 07:18 PM   #14
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any thoughts on using plywood for trunk lines?


Ive never seen a wooden trunk/supply line,nor wood i personally recommend it.
Cold air returns on the other hand have been made using panned off area between floor joists for years though without any serious issues im aware of though
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Old 12-30-2011, 07:21 PM   #15
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any thoughts on using plywood for trunk lines?


Quote:
Originally Posted by Doc Holliday View Post

Measuring/Monitoring Moisture Levels

http://www.woodweb.com/knowledge_bas...re_Levels.html

The hows and whys of monitoring moisture content in lumber. 1998.
By Dr. Eugene Wengert
I. Moisture and RH
Wood is a hygroscopic material, meaning that it is continually trying to achieve an equilibrium moisture condition with its environment. As the relative humidity (RH) changes, so does the moisture content (MC) of the wood. The following tabulation expresses the relationship between MC and RH.
MCRHEMC%%%519 to 255625 to 326732 to 397839 to 468946 to 5291052 to 58101158 to 6411
Note that temperature is not an important factor; it is just the RH. Also, these numbers hold true for all species of wood.
II. Moisture Effects on Wood-Shrinkage
So, what is the big deal about the MC of wood? The truth is that the properties of wood change with changes in MC--most noteworthy is that wood shrinks and swells substantially with MC changes. plus, with the property changes come changes in processing--gluing, machining, fastening, and finishing. Let's briefly look at a few of these changes in order to establish how closely we need to measure and control moisture levels.
Of all the changes, certainly the most troublesome change has to be shrinkage and swelling--wood shrinks when the MC decreases and swells when the MC increases. And to complicate matters, wood shrinks differently in the three directions--lengthwise shrinkage is usually quite close to zero; across the grain, parallel to the rings (width of a flatsawn piece) wood shrinks up to 1% for a 3% to 4% MC change; and across the grain, perpendicular to the rings (thickness of a flatsawn piece) wood shrinks about 1% for a 7% MC change. Of course, there is variation from piece to piece of the same species, as well as variation from species to species. The following tabulation gives the average change in size for a 3 inch wide, flatsawn piece of lumber for a 2% MC change for several important species. Shrinkage is essentially a linear function of MC, so double the values for a 4% MC change, triple them for 6% MC change, and so on.
SPECIESSIZE CHANGESPECIESSIZE CHANGEInchesInchesApitong0.031 Mahogany0.014Ash, white0.016Maple, hard0.021Basswood0.020Maple, soft0.015Banak0.019Oak, red0.022Birch, yellow0.020Oak, white0.020Cherry0.015Pine, Southern0.016Hackberry0.019Pine, white0.012Hemlock, Eastern0.014Teak0.011Hickory0.019Yellow-poplar0.017In summary, we are looking at a 3 inch wide piece changing 1/100 to 3/100 inches in width for a 2% MC change--a very small size change. But such a change is quite large when it comes to edge gluing and to flatness in a high gloss finish, as is discussed later.
I have actually dug up pieces of old wooden water mains in the old market area of omaha ne 20 plus feet in the ground,im guessing they were from late 1800s/early 1900s
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