# Use of wood in a plenum



## Sifu

I have a multi-family building, type V construction with each unit using an aquatherm unit located above the bathroom.  The plenum that encloses the units is limited to the smoke/flame spread requirements of IMC 602.2.1.  Got no issue there.  However the bottom of the plenum is constructed out of wood, which is now exposed inside the plenum.  First take is not permitted if no smoke/flame spread.  BUT, IMC 602.2 allows the plenum construction to be built of the same type of construction as the building.  Further, the commentary for the materials within section-IMC 602.2.1 says "this section addresses those items that are installed within plenums, not the materials that bound and create the plenum space".  So have any of you run up against this, and what is your take.  Seems a little strange to allow wood inside a plenum


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## steveray

Type V.....Allowed....


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## ICE

Dwelling.....allowed


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## Sifu

OK, some expansion on the answers?  Type V allowed...how?  Becuase of 602.2 materials classification of the building?  Does that mean we can build it out of anything the building is made of?  And Dwelling?  If you use the exception in 602.2.1, that is for one and two family dwellings, not R2 out of the IMC isn't it?


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## steveray

Yes....anything the building can be made of....602.2


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## mtlogcabin

PLENUM. An enclosed portion of the building structure, other than an occupiable space being conditioned, that is designed to allow air movement, and thereby serve as part of an air distribution system.

You rarely "construct" a plenum, I have some who will call a mechanical room with a free return a plenum and that is incorrect.


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## Sifu

This is a constructed plenum.  The unit hangs from the ceiling, they build an additional hard lid below it, cut transfer grills in to the sides of the space to pull return air from the adjacent spaces.  The more I have researched it I am inclined to agree with the answers.  Though I am still hung up on why they absolutely prohibit materials within the plenum but have no limitation on what they are made of.  DP's are fighting hard but my superiors don't want to allow it.  Seems to me if the worry is the exposure of the potentially problematic materials within the plenum they might also put some limits on the materials of which it is made.


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## ICE

> 602.2 Combustibles within Ducts or Plenums. Materialsexposed within ducts or plenums shall be noncombustible or
> 
> shall have a flame spread index not to exceed 25 and a smoke
> 
> developed index not to exceed 50, where tested as a
> 
> composite product in accordance with ASTM E 84 or UL 723,
> 
> except as indicated below.
> 
> Exceptions:
> 
> (1) Return-air and outside-air ducts, plenums, or concealed
> 
> spaces that serve a dwelling unit shall be permitted to be
> 
> of combustible construction.
> 
> (5) Electrical wiring in plenums shall comply with California
> 
> Electrical Code. Electrical wires and cables and optical
> 
> fiber cables shall be listed and labeled for use in plenums
> 
> and shall have a flame spread distance not exceeding 5 feet
> 
> (1524 mm), an average optical density not exceeding 0.15,
> 
> and a peak optical density not exceeding 0.5, where tested
> 
> in accordance with NFPA 262.


For my morning stretch.

I didn't pay close enough attention to the original post and only now realized that the furnace is in the plenum.

The furnace itself would need to be plenum rated.  The wire located within a plenum is either exposed or in a raceway.  The shell of a furnace is not a raceway.  Therefor the wire, grommets, zip ties, etc. is exposed.  As such all "_shall have a flame spread index not to exceed 25 and a smoke developed index not to exceed 50, where tested as a composite product in accordance with ASTM E 84 or UL 723"._

I'm still working on my first cup of coffee and I am usually full of $hit first thing in the morning but hey now, that'll get'em to thinking.

When they come back with, "It's the FURNACE, it HANDLES the air that we want to protect" .....ask them if they would have a problem with any other appliance....like maybe a toaster.  Is a table saw an appliance?

Wow, I'm extra full of it today, it's all those doughnuts.





That would probably be that the last dougnut you'd ever want to eat.  Only a Basset Hound would come back for more.  What do you wash that down with, a garden hose?

Geez I hope I don't smell thread drift here.


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## Sifu

No drift here cuz I'm not gonna touch it.  And yes, so far everything in the plenum is plenum rated.  I guess the two codes; 602.2 and 602.2.1 appear to argue with each other.  Is one more specific?  602.2.1 grows out of 6.2 and exception #5 seems to further the non-com requirement.  I do have an off-shoot question though.  602.2.1 exception #3, excepts "one and two family dwellings".  The individual apartments are one and two family dwellings are they not.  The building is an R2-IBC but would the apartments fit in this exception?


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## ICE

would the apartments fit in this exception?

no


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## Sifu

ICE said:
			
		

> would the apartments fit in this exception?no


I didn't think so either but that was one take on it I hadn't considered.  They have started building the new units with metal frame, though they seem to be holding out hope that we will forget about the 36 units already framed out of wood.


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## JBI

A one family dwelling is a detached building containing one dwelling unit. A two family dwelling is a detached building containing 2 dwelling units. Townhouses/townhomes are _attached_ single family dwellings.

An R-2 could contain only one or two dwelling units... a stroefront in downtown could be commercial on the first floor and residential above, the residential portion(s) would be an R-2 (if occupied long term) or possibly an R-1 (if for short term rental).

True one and two family dwellings limit the potential risk to only one or two families, where the downtown storefront exposes the residents to risk from the business, and the business is exposed to risk from the tenants. Hence the additional requirements/restrictions in the Codes.


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