# plenums and fire areas 2006 codes



## Plans Approver (Aug 12, 2010)

1. IMC 602.1 limits a plenum to one (1) fire area.

2. IMC 607.5.1 and 607.5.2 (as well as IBC 716.5.1 and 716.5.2) permit ducts and *air transfer openings* in firewalls and fire barriers respectively.

Size of all openings meet the limitations of 705.8 and meet the requirements of IBC 705.11 Exception and 712.

I have always enforced 602.1, but, in discussion with an M.E., he got me to doubting my position.

The question is: do the specifics of 607.5.1 and 607.5.2 override the general 602.1 for an *air transfer opening* between plenums on each side of a firewall or fire barrier?


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## TJacobs (Aug 12, 2010)

If the ducts air transfer openings are protected by fire dampers as required by IBC 716, I would say that it is permitted.


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## mtlogcabin (Aug 12, 2010)

From the 2006 IMC Commentary

Limiting the location of air distribution system ple­nums reduces the potential for the spread of fire and smoke through the air distribution system. To prevent the rapid spread of smoke through occupied areas, plenums are restricted to concealed and uninhabited spaces within a building. Plenums are limited to one fire area to allow for the containment of fire and smoke, thereby reducing the risk of it spreading to other areas of the building. A "fire area" is a floor area that is en­closed and bounded by fire walls, fire barriers, fire-re­sistance-rated horizontal assemblies or exterior walls of the building. Plenums must be designed to maintain the integrity of the fire area enclosure.

If return air ceiling plenums over two fire areas are linked together by transfer openings, thereby main­taining the integrity of the fire areas. This means that the two return plenums are discrete and are each limited to a single fire area. However, this begs the ques­tion, when would a plenum not be limited to a single fire area? This presents a puzzle in that the code ap­pears to restrict a plenum arrangement that could never exist because of other code provisions that reg­ulate fire areas and fire barriers.

The conservative interpretation of the restriction of plenums to a single fire area is that the code intends to prohibit plenums in different fire areas from being linked through protected transfer openings in fire barri­ers, thus requiring each fire area plenum to be individ­ually ducted back to the airhandler 

I also take this approach and limit plenums to one fire area.


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## TJacobs (Aug 12, 2010)

I stand corrected but not because of the Commentary.  Upon further review, IMC Section 602 is specific to plenums while Section 607 is not.  Since the 607.5.x sections do not modify 602 they cannot override 602.  My bad.  I could still see treating the situation as in my first response.

You could evaluate as a single fire area if the fire-rated assemblies are not there specifically to create separate fire areas and see if it works.


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## Coug Dad (Aug 12, 2010)

Rarely do we have fire areas in a building.  A design would only use fire areas to keep the area under the maximum where a sprinkler system would be required.  For example, Section 903 has requirements for sprinklers based both upon fire area size and buiding size.  Just because you may have rated walls and floors does not automatically create a fire area.  If you utilize the fire area provisions, then it is logical that the restriction to one fire area for plenums applies.


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## Mule (Aug 12, 2010)

Isn't the "fire area" the old "area separation" wall where if you constructed an area separation wall it was counted as a separate building?


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## Coug Dad (Aug 12, 2010)

The IBC fire wall is the old UBC area separation wall.  Yes, a fire wall can be used to create separate fire areas.


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## Plans Approver (Aug 12, 2010)

Misc. facts.

Original building constructed late 70's as a medical office building owned by a hospital. Subsequent additions for use as I-2, 3 hour separation then, 2 hour separation required today. Areas, separated or combined, will meet allowed area w/ open perimeter increase even though its not sprinklered. I-2 went away and replaced by more medical offices, but, the owner wants to maintain the fire separation in case it becomes I-2 again.  The question came up regarding an office tenant alteration which spanned across the fire separation which is masonry, and by my guess, an original, non-bearing exterior wall. Openings below the ceiling plenum plus the openings within the plenum meet all code limitations.

The problem is the air transfer openings cited in 607.5.1 & .2 neither specifically say they can or cannot be used within the plenum space.

Being cheap, I did not purchase the 06 IMC Commentary. My 03 Commentary does not include the last 3 paragraphs that mtlogcabin posted. That is very helpful. In fact, I told the M.E. to use return air ducts w/ the fire dampers back to the unit to maintain the separation. Or, submit documentation that it is one fire area, then they could do without the fire dampers altogether.


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## Dr. J (Aug 12, 2010)

Anywhere you have fire resistive rated construction, a fire area is created.

"_FIRE AREA. The aggregate floor area enclosed and bounded by fire walls, fire barriers, exterior walls or fire-resistance-rated horizontal assemblies of a building._"

Thus even an incidental use area bounded by fire barriers is a fire area.  You can transfer air through a fire damper below the ceiling, but if you do it above the celing everone will die unless it is ducted back to the unit.


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## Dr. J (Aug 12, 2010)

> but, the owner wants to maintain the fire separation in case it becomes I-2 again.


The owner is going to be hosed if they change occupancy to I-2 anyway.  The entire building will need to be sprinklered, and the only thing that creates separate buildings are structurally independent fire walls.


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## Coug Dad (Aug 12, 2010)

Dr J.  If the intent of the code is that strict, then how can you do a multi story building with a common air handler on the roof and return air plenums above the ceiling at each floor?  The plenums would be connected across the fire resistive rated floor through the return air shaft.


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## Dr. J (Aug 12, 2010)

The definition of a fire area is what it is - pretending it is only the equivalent of the old "area separation" does not change that.

The multistory RA riser does not make one plenum across multiple fire areas.  Each floor is ducted back to the air handler.   Thus in a 10 story building, there are 10 plenums (pleni??).  This is similar to a floor that has an incidental use area.  By ducting it back to the air handler instead of just transfering to the adjacent plenum, two pleni have been created.


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## mtlogcabin (Aug 12, 2010)

* FIRE AREA. **The aggregate floor area enclosed and bounded by **fire walls, fire barriers, exterior walls **or **horizontal assemblies **of a building. Areas of the building not provided with surrounding walls shall be included in the fire area if such areas are included within the horizontal projection of the roof or floor next above.*

*+ This term is used to **describe a specific and controlled area within a building** that may consist of a portion of the floor area within a single story, one entire story or the combined floor area of several stories, depending on how these areas are enclosed and separated from other floor areas. Where a fire barrier with a fire-resis­tance rating in accordance with Section 707.3.9 of the IBC divides the floor area of a one-story building, the floor area on each side of the wall would constitute a separate fire area. If a horizontal assembly separating the two stories in a two-story building is fire-resistance rated in accordance with Section 712.3 of the IBC, each story would be a separate fire area. In cases where mezzanines are present, the floor area of the mezzanine is included in the fire area calculations, even though the area of the mezzanine does not con­tribute to the building area calculations. See the com­mentary to Sections 707.3.9 and 712.3 of the IBC for further information.*

*Note that fire walls are one way of creating fire areas** but are typically used to create separate buildings.*

*Unless there is a code section requiring a fire area such as Section 903 I do not believe the intent of the code was to classify all areas of a building seperated by fire barriers or fire walls to be classified as fire areas. Although a horizontal exit creates different fire areas by definition what is the pupose of a fire area? To avoid sprinklers or H occupancies.*


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## Dr. J (Aug 12, 2010)

I started a rant, but will just leave it that this is a messed up part of the code.

I conceed that the intent was to subdivide a building to avoid sprinklers (I think it takes a "control area" to avoid H occupancies).  I certainly WANT it interpreted that any rating does not create fire areas.


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