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Old 01-10-2018, 11:12 AM   #1
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Suburban NT furnace / heater ducting question

I would like my NT-30SP heater to sit on a plenum that has heat ducts coming off it. I was thinking of building a wood plenum and lining it with sheet metal. Does anyone see any issues with this? I found some photos online that show this closely to the way I was thinking. See photos.
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Old 01-10-2018, 04:46 PM   #2
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Any suggestions?
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Old 01-10-2018, 05:18 PM   #3
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No problems as long as the sheetmetal liner is sealed well to protect the wood from moisture from the intake air when the heater isn't working.
But a 20-24 guage sheetmetal plenum insulated will support that unit just fine.
I don't like using the duct liner made to insulate the inside of the duct even though it's common in a residential application.
I have replaced many pieces of duct with the internal liner and the liner itself almost every time was covered in mold,mildew and caught a lot of lint/fibers in the air.
And the government jobs I work on have almost entirely stopped its use except for in basically small section like air transfer from room to room registers. But your plan to use the lumber on the outside is a form of insulation.
I would check out the heater air output temp and make sure that they could never get hot enough over long term use to set those pine boards on fire? Not likely unless you use a board that has a knot and is still oozing pine sap onto the heated metal but that is a worst case scenario that I always try to build by. Maybe for that application use a hardwood.
Hope this helps.
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Old 01-10-2018, 05:18 PM   #4
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Why not stick with all metal and insulate it? Wood contributes to combustion should the heater malfunction. Insurance not gonna like that idea imho.

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Old 01-10-2018, 05:28 PM   #5
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Well there is no risk of the wood combusting from the temperature inside, in the short term.

What happens over the long term is that the constant heat dessicates the wood until all that is left is a lightweight shell.

This can happen too to the 2x4 or 2x6 wood joists in a wall, even behind sheetrock, which is why woodstoves need a specific gap between them and the wall, even when the wall feels cool to the touch.
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Old 01-10-2018, 05:38 PM   #6
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Thanks for the advice TWIGG.
Hopefully the OP will be back soon?
I am a steel and concrete construction man myself.
I knew about the flue pipe spacing and have never thought about wrapping a duct in lumber? But if a low 120-140 deg. Unit then a good hardwood could be feasible?
But I didn't like it so I proposed a sheetmetal insulated plenum.
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Old 01-10-2018, 07:45 PM   #7
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Thanks for the replies. My thought was that just making a wood box and lining it with a metal plenum would be easier and nicer than making a separate wood box and making a separate metal plenum to fit inside the wood box. No mater how I do it, wood will contact metal somewhere. The furnace requires zero clearance to the bottom too. I'm undecided as to how to proceed. I wont do much on it for a few days, so I thought I would see what you all think. Ill take some photos tomorrow and post them and see what kind of ideas we can come up with.
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Old 01-11-2018, 03:01 PM   #8
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I attached a couple photos. One shows the corner the furnace will be located in. Another shows the bottom of the furnace. The bottom of the furnace has a 3.25x10 inch cutout for a duct. I plan to use that to feed the plenum that I will build. The wood cabinet that will house the furnace will sit in that corner. You can see there is a duct in the wall that will need to be connected to the plenum. Then there will be two more vents coming off the plenum one to the left and the other strait out. In total including the ducting in the wall there will be 3 vents total.

I should add that the cabinet to house the furnace will be a only for the furnace. And will need to be as small as I can make it. There will be a table and chairs next to it.
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