# Plenum Mechanical closets in R2 units.



## swerve (Aug 27, 2021)

I would love to hear everyone's take on this common site. We have a situation here in our jurisdiction where mechanical contractor has installed a mechanical closet and is pulling the return air through a wooden louvered door on the mechanical closet. He is adamant that its not classed a a plenum therefore the wooden door does not have to meet the 25/50 smoke/flame spread rating. If you read the commentary it seems to make it more confusing.
  I was able to find an advisory memo from Dade county in Florida that backed me up totally saying its a plenum and must meet the 25/50 smoke/flame spread requirements for construction material surfaces, see attached memo
  Thanks for any valuable feedback, as usual he likes to say other jurisdictions around here allow it.


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## TheCommish (Aug 27, 2021)

The Florida ruling only apply to Florida, has the Virginia Board given any guidance or adjugated an appeal?

It the  return air lover was through the side wall of the  mechanical space with a short piece of ductwork, would  that be any different than the  door lover?


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## cda (Aug 27, 2021)

Not my area,,,

But to me appears they got the code stretcher tool out.


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## cda (Aug 27, 2021)

If the door is on fire,,, you might just have bigger redneck problems


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## swerve (Aug 27, 2021)

Well if there was a metal grill and a connector then it would not be a plenum. I understand it seem crazy to worry about this when the whole apartment is full of flammable stuff but I'm just trying to understand why they changed the code 2012/ 2015 to include construction materials of a plenum and not just what's is inside a plenum with regards to the smoke/ flame spread rating.
  Regards to appeal ruling, it not been appealed yet, that would at least give us a definitive answer.


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## cda (Aug 27, 2021)

swerve said:


> Well if there was a metal grill and a connector then it would not be a plenum. I understand it seem crazy to worry about this when the whole apartment is full of flammable stuff but I'm just trying to understand why they changed the code 2012/ 2015 to include construction materials of a plenum and not just what's is inside a plenum with regards to the smoke/ flame spread rating.
> Regards to appeal ruling, it not been appealed yet, that would at least give us a definitive answer.




So what was the change, can you point to it in either 2012 or 2015


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## swerve (Aug 27, 2021)

Certainly sir. The section for construction was added for 2015, 2012 only stated that plenums shall constructed of materials permitted for the type of construction, no mention of 25/50 smoke flame spread compliance, only material within a plenum, so I think in the 2015 code they made it clearer ( as mud  ) Hope this helps.
   I have not been on here much but I think its hugely valuable forum, not sure why it doesn't show me as a sawhorse, maybe complete lack of activity. Thanks again for your thoughts on this, the last thing I want is to over enforce the code but I do believe in doing it to the best of my ability.

*2015    602.2 Construction*

_Plenum _enclosure construction materials that are exposed to the airflow shall comply with the requirements of Section 703.5 of the _International Building Code _or such materials shall have a flame spread index of not more than 25 and a smoke-developed index of not more than 50 when tested in accordance with ASTM E 84 or UL 723.

*2012      602.2 Construction*

Plenum enclosures shall be constructed of materials permitted for the type of construction classification of the building.

The use of gypsum boards to form plenums shall be limited to systems where the air temperatures do not exceed 125°F (52°C) and the building and mechanical system design conditions are such that the gypsum board surface temperature will be maintained above the airstream dew-point temperature. Air plenums formed by gypsum boards shall not be incorporated in air-handling systems utilizing evaporative coolers.


*602.2.1 Materials Within Plenums*


Except as required by Sections 602.2.1.1 through 602.2.1.5, materials within plenums shall be noncombustible or shall be listed and labeled as having a flame spread index of not more than 25 and a smoke-developed index of not more than 50 when tested in accordance with ASTM E 84 or UL 723.


*Exceptions:*

Rigid and flexible ducts and connectors shall conform to Section 603.
Duct coverings, linings, tape and connectors shall conform to Sections 603 and 604.
This section shall not apply to materials exposed within plenums in one- and two-family dwellings.
This section shall not apply to smoke detectors.

Combustible materials fully enclosed within one of the following:
5.1. Continuous noncombustible raceways or enclosures.
5.2. Approved gypsum board assemblies.
5.3. Materials listed and labeled for installation within a plenum.


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## steveray (Aug 27, 2021)

Seems very foolish with a central return and all of the rooms are the "plenum"....But hey....


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## cda (Aug 27, 2021)

But other parts of the system can be wood???

*602.3 Stud Cavity and Joist Space Plenums*

Stud wall cavities and the spaces between solid floor joists to be utilized as air plenums shall comply with the following conditions: 

Such cavities or spaces shall not be utilized as a _plenum_ for supply air.
Such cavities or spaces shall not be part of a required fire-resistance-rated assembly.
Stud wall cavities shall not convey air from more than one floor level.
Stud wall cavities and joist space plenums shall comply with the floor penetration protection requirements of the _International Building Code_.
Stud wall cavities and joist space plenums shall be isolated from adjacent concealed spaces by _approved _fireblocking as required in the _International Building Code_.
Studwall cavities in the outside walls of building envelope assemblies shall not


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## cda (Aug 27, 2021)

There was a recent discussion on PLENUMS


*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.


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## cda (Aug 27, 2021)

steveray said:


> Seems very foolish with a central return and all of the rooms are the "plenum"....But hey....





All the world’s a stage, or plenum
And all the men, women, plenums merely players;
They have their exits and their entrances;


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## mtlogcabin (Aug 27, 2021)

The louvered door is the return grille and the access door to service the equipment and is not located within the return air plenum.
It is an HVAC system located within a single dwelling unit and the entire "Plenum" within that dwelling space probably consist of no more than 16 cubic feet.

I agree with the contractor the wood door is not an issue in regards to the plenum construction requirements


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## steveray (Aug 27, 2021)

Take the door off and put it back on after CO....


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## Paul Sweet (Aug 27, 2021)

Is this a residence under the Va. Residential Code, or multi-family under the Va. Construction Code?  Which edition - 2015 or 2018?


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## swerve (Aug 27, 2021)

Multifamily, VCC 2015
Louvered door/ grille/ access door is one and the same, have a read of the commentary, clearly states including the door surface has to meet the 25/50


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## Paul Sweet (Aug 27, 2021)

The 2012 Va. Mechanical Code allowed plenum enclosures to be constructed of materials permitted for the type of construction classification of the building, so I guess the contractor is right about it having been allowed.  Unfortunately the 2015 changed it to noncombustible.

An air handler closet in a residence or apartment doesn't create the same danger as a mechanical room in a larger commercial building.  I agree that there are bigger problems if the door is on fire.  Intumescent paint might be a reasonable compromise.


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## cda (Aug 27, 2021)

Funny in the old days, a certain thinkness wood door, was equal to a 20 minute door.


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## mp25 (Aug 27, 2021)

I have seen this set up many times and I don't believe it is the intent of the code to consider this a plenum in the same category as a plenum ceiling that can extend over several spaces/business, etc.


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## mtlogcabin (Aug 30, 2021)

The code has an exception for materials in a plenum in a one or two family dwelling. The "plenum" you describe is for a single dwelling unit. I would have no problem under Section 105.1 to apply the same exception to a single dwelling unit in a multi family building. The reason for the restriction of material within a plenum is due to fire and/or smoke starting within the plenum. Is there anything that would create smoke or start a fire within this plenum?.Is the return located in the bottom of the unit? If so what is the air handler sitting on? A metal stand or a wood platform?


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## swerve (Aug 30, 2021)

*The return is the wooden louvered door. What code book are you pulling 105.1 from. I should say that by boss has told them its a plenum and if they want to dispute that then the would need to get a code interpretation for the state.*​*Thanks to all contributing to this topic.*


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## mtlogcabin (Aug 30, 2021)

The IMC which is the same section and language in the IBC, IRC, IFGC, & IPC.
That is the difference between your state and mine.

(29) The building official may waive minor building code violations that do not constitute an imminent threat to property or to the health, safety, or welfare of any person.

I agree you have to follow what your boss has determined and an appeals board is the only option left for the contractor. IMHO what you describe is not a plenum when in a dwelling unit.


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## mtlogcabin (Aug 30, 2021)

I agree the code change is better because in a Type V building it allowed wood to create a plenum

2009 IMC
602.2 Construction.
Plenum enclosures shall be constructed of materials permitted for the type of construction classification of the building.

2018 IMC
602.2 Construction.
Plenum enclosure construction materials that are exposed to the airflow shall comply with the requirements of Section 703.5 of the International Building Code or such materials shall have a flame spread index of not more than 25 and a smoke-developed index of not more than 50 when tested in accordance with ASTM E84 or UL 723.


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## Paul Sweet (Aug 30, 2021)

IBC 105.1 requires a permit.  IBC 104.10 & 11 allow for modifications and alternative methods.  

Virginia Construction Code 106.3 allows the building official to approve modifications.


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## jar546 (Aug 31, 2021)

mtlogcabin said:


> The code has an exception for materials in a plenum in a one or two family dwelling. The "plenum" you describe is for a single dwelling unit. I would have no problem under Section 105.1 to apply the same exception to a single dwelling unit in a multi family building. The reason for the restriction of material within a plenum is due to fire and/or smoke starting within the plenum. Is there anything that would create smoke or start a fire within this plenum?.Is the return located in the bottom of the unit? If so what is the air handler sitting on? A metal stand or a wood platform?


In some instances, the AC unit is only accessible through a closet in the common hallways of R2 buildings, the return is not ducted, just an open grill from inside the dwelling unit (they are not considered SFRs, they are dwelling units).  In this case when accessible through the common hallway closet, we ensure that the plenum space meets the 25/50 rule.  If the AC unit, however, is only accessible from inside the dwelling unit, we do not.


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