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» ICC Bulletin Board » Code Chat » Plumbing, Mechanical and Fuel Gas Codes » Duct Smoke Detectors

   
Author Topic: Duct Smoke Detectors
doughill3
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Building type VB-ll, all walls CMU, ceilings either grid/tile, or gyp. No walls/ceilings fire rated. 8 FCU, ducted supply return, all but one less than 2000CFM. Local fire marshall claiming that since no rated separations, the entire building is a plenum, and all units need duct smoke detectors. ICC reviewed plans and required detectors only at the one unit.
Is there any ammo out there for my owner to use to fight this interpretation?

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cda
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IMC, edtion ahj is under

have inspector site section and than review said section to see if it valid or not

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doughill3
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The following citation was transmitted:
909.19.2 Duct Smoke Detectors.
Where multiple air-handling systems share common supply or return air ducts or plenums with a combined design capacity greater then 2000 cfm, the return air system sha11 be provided with smoke detectors. (No title to code reference given.)
I believe the AHJ is defining each "fire area" as a supply/return "plenum". Hence the requirement that all my HVAC must have duct detection.

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Paul Sweet
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Sounds like he's trying to stretch the code way beyond its intent. I think this section was intended to apply to a store or large meeting room served by mulitple units, or to buildings where walls stop at the ceiling, and the ceiling return plenum serves more than one unit.

Chapter 2 of the 2003 IMC defines 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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ken12000
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The following is a copy of the text I recieved from an NFPA staff member regarding this subject:

You state that you've got two or more HVAC systems (less than 2,000 cfm each) are serving a common open air room. Your first question is; "Would this section of the current NFPA 90A require that we treat all of the units serving this common open air area as one system and calculate the sum total of all units in this area to obtain the system cfm?" And your second question is; "If so, would the duct smoke detectors require to be interlocked so that when one detector went into alarm all of the common area units shutdown as well?"



The answer to the first question is "no". By definition, an "Air Distribution System" is "a continuous passageway for the transmission of air that, in addition to air ducts, can include air connectors, air duct fittings, dampers, plenums, fans, and accessory air-handling equipment but that does not include conditioned spaces.(emphasis added)" It is not unusual for a large conditioned (occupied) space to have multiple air distribution systems. The key is the path the air follows once it leaves a conditioned (occupied) space until it is returned to a conditioned (occupied) space. That entire path is one air distribution system.



The exact wording in 90A that requires detectors and "shutdown" is: "Downstream of the air filters and ahead of any branch connections in air supply systems having a capacity greater than 944 L/sec (2000 ft3/min)". The words "having a capacity greater than 944 L/sec (2000 ft3/min)" modify the term "air supply systems". Any individual air distribution system that's less than 2000 cfm, regardless of how many other air distribution systems serve the same conditioned (occupied) space, doesn't need "isolation" smoke dampers with smoke detector initiation and unit shutdown.

I hope this clarifies the requirement for you.



Sincerely,



Allan B. Fraser, CBI, CPCA

Senior Building Code Specialist

Building Fire Protection and Life Safety



ABF/lc#5074



Important Notice: This correspondence is not a Formal Interpretation issued pursuant to NFPA Regulations. Any opinion expressed is the personal opinion of the author, and does not necessarily represent the official position of the NFPA or its Technical Committees. In addition, this correspondence is neither intended, nor should be relied upon, to provide consultation or services.

--------------------
Kenneth Locke, CBO
Chief Mechanical Inspector
City of Tallahassee, FL

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cda
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Appears the congressman from the great state of Florida is correct, ken12000

not sure where he is gettin the reference "909.19.2 Duct Smoke Detectors" maybe there is something in there that we do not know

[ 03-03-2008, 11:10 AM: Message edited by: cda ]

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Dr. J
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It is not clear to me from the OP if there are multiple FC units in a common plenum, or if the non-rated walls extend to the structure without any transfer between FC units.

For ammo, please confirm code and year being enforced. The 909 citation is not valid for IMC (909 is for Vented Wall Furnaces). It is also not valid for NFPA 90A (it only has 7 chapters). NFPA 90A is only valid if the AHJ has adopted it, or has adopted NFPA 101 and by extension NFPA 90A.

So, assuming this is under IMC (since the OP referenced ICC), 606.2.2 states:

"606.2.2 Common supply and return air systems.
Where multiple air-handling systems share common supply or return air ducts or plenums with a combined design capacity greater than 2,000 cfm (0.9 m3/s), the return air system shall be provided with smoke detectors in accordance with Section 606.2.1.
Exception: Individual smoke detectors shall not be required for each fan-powered terminal unit, provided that such units do not have an individual design capacity greater than 2,000 cfm (0.9 m3/s) and will be shut down by activation of one of the following:
1. Smoke detectors required by Sections 606.2.1 and 606.2.3.
2. An approved area smoke detector system located in the return air plenum serving such units.
3. An area smoke detector system as prescribed in the exception to Section 606.2.1.
In all cases, the smoke detectors shall comply with Sections 606.4 and 606.4.1."


The operative words are "...common supply or return air ducts or plenums...". Use Paul's reference to the definition of a plenum, which does not mention any ratings, just "enclosed". This means that separate systems are delineated by any enclosure, such as sheet metal ducts or non rated construction.

If the individual units are not separated/enclosed, they form a common return system and you do indeed need to deal with smoke detection and shutdown for all units. You should review the entire section 606, because there are several exceptions to needing the shutdown (if units can't transfer smoke beyond the room of origin), and several methods to achieve the required shutdown besides a duct detector for each unit (plenum detectors, area detectors).

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doughill3
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Thank you gentlemen.

The AHJ is using IMC,IBC, and IFC 2006.

I have available to me IBC & NFPA-1 2003, and an older NFPA 90A.

Chapter 9, IBC 2003 does not list duct detection as a requirement for my occupancy "E" (day care). 909 is concerned with Smoke Control Systems, which we do not have.

The FCU are all ducted supply and return. There is no common plenum or duct between any FCU. The rooms are walls to structure with either gyp to structure, or grid/tile.

There is one instance in which two separate FCU supply a common room, (both are separately ducted), and only one FCU picks up return from the common room.

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cda
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if they use IMC than that is where the requirements are

they may have sited the wrong section

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Dr. J
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I missed the ducted return part of your OP. My bad (could have saved a bunch of typing). Individually ducted returns = not a common return systerm = do not need DD on FC < 2000 cfm.
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John Kopp
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It’s funny how they used 2,000 CFM as the threshold. One would think this means 5 ton sized A/C equipment or larger IE 400 CFM per ton X 5 = 2,000. Then you take into account static system pressures and review blower performance tables based on system static and A/C’s will work down to 350 CFM per ton etc. So one could argue their 5 ton system is really operating at 1,950 CFM and it probably is so this is not required.

They should have included the following text “and or nominal 5 ton sized systems or larger”. I have had some Inspectors look at 4 ton equipment and deside smokes were needed because blower tabels showed CFM’s up to 2,000.

I think code intent was for 5 ton equipment or larger.

John

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ken12000
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The issue with the 5 ton thing is that not all manufacturers 5 ton AHU/FC produce 2000 cfm. For years Goodman/Janitrol's 5 ton AHU/FC only produced 1800 cfm with no static pressures.

The main difference between the IMC and the NFPA 90A is in the wording of when the detector is required. The IMC states, "where the DESIGN capacity is GREATER than 2000 cfm". Whereas the the NFPA 90A states where "the CAPACITY is GREATER than 2000 cfm". The IMC defers to the actual design performance of the system where the NFPA seems to be more concerned with the unit's actual maximum ability. However, in either case, the requirement is not for a 5 ton air handler, but an air handler which either is designed or has the ability to produce MORE than 2000 cfm. So not all 5 ton AHU's would require a duct detector.

If a certified test and balance report indiated a unit was operating at 2000 cfm or less then what language would an inspector have to require a duct smoke detector to be installed? Additionally, if the manufacturer's fan curve chart showed there was no way the unit could produce greater than 2000 cfm, how could the inspectors require the detectors.

The agreement I have with our Fire Inspectors is if the unit has the ability to exceed 2000 cfm, we will both require the detector, but under the threshold, neither will require the detector.

--------------------
Kenneth Locke, CBO
Chief Mechanical Inspector
City of Tallahassee, FL

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doughill3
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More thanks.

My interpretation of "design capacity" is the system as designed, stamped, sealed by the Mechanical Engineer. Equipment capacity is important to the designer but not relevant post-design.
I have flexible duct rated to 5000 CFM, and this does not mean my design capacity is 5000 CFM.

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