# Subduct exhaust system



## Codegeek (Aug 18, 2011)

I have an R-1 use group, four stories in height, Type VA construction, fully sprinklered.  They want to use a subduct system for the exhaust fans serving the bathrooms and eliminate the need for fire and smoke dampers at the shaft penetration.  I believe that in order to eliminate both the dampers they must provide all of the following four items:

•	Steel exhaust subducts extending at least 22 inches upward into the shaft

•	Have a minimum gage of 26

•	Have a fan at the top of the shaft which operates continuously

•	Provide a secondary power source for the fan

I shared this with the client.  They responded saying they’ve never had to provide the secondary power source and suggested I contact a codes consultant they’ve utilized on previous projects.  I called this codes consultant and was told that if I made the client provide all of these items I was wasting the client’s money as this was not a corridor and therefore smoke control didn’t apply.  I had to scratch my head on that one as I didn’t see the connection to corridors first of all, especially when I read the code language in Section 716.5.3 over the phone to them.  Secondly, in all my years of code enforcement, the second that fan shuts down, if there is any smoke in it, it will all go into the guestrooms, which goes against the entire purpose of helping to evacuate smoke.

Am I missing something here?


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## mtlogcabin (Aug 18, 2011)

I agree with you.

The charging languaqe is

716.5.3 Shaft enclosures.

Shaft enclosures that are permitted to be penetrated by ducts and air transfer openings shall be protected with approved fire and smoke dampers installed in accordance with their listing.

You can't use the exception for subducts except in a kitchen, dryer or bath exhaust. Where is the "Code Consultant" believes the secondary power requirement is limited to corridors?


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## RLGA (Aug 18, 2011)

I agree with you, too.  Corridors have their own requirements in 716.5.4.1.  How does this section for shafts apply to corridors?--beats me.


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## Coug Dad (Aug 18, 2011)

While it may be my opinion that the secondary power requirement is not warrented in a non high rise building, it is what the code requires.


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## Codegeek (Aug 18, 2011)

I wasn't quite sure where the corridor reference came from either and I asked.  I tried to tell this consultant that corridors did not apply as this was a shaft issue.  They continued to argue that the smoke control didn't apply.  I said that this was not about a smoke control system, but rather meeting the provisions for eliminating the requirements for smoke and fire dampers in a shaft, which include the secondary power supply.  I was politely told that I was incorrect and was encouraged to call ICC for an interpretation, with the code consultant telling me that ICC would agree with their interpretation, not mine.  So, I called ICC and the person handling code interpretations when I called agreed with me.

I wanted to see if anyone else agreed with the code consultant so I could better understand where they were coming from, so I appreciate all of your feedback.  Sounds like we're all scratching our heads on that one.


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## Paul Sweet (Aug 19, 2011)

I don't recall the secondary power requirement from BOCA, so it may have been added since the IBC replaced the old model codes.


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## Dr. J (Aug 19, 2011)

I agree with CD - it is also my opinion is that em power is not required, but it is in the code.  The 22" subduct will inhibit smoke from making a u-turn and head back out a grille.  There is a point (75'???) that stack effect could take over and force the smoke into a u-turn at the top.  However, I would have hoped the code would have encouraged a workable solution - perhaps a relief damper at the top to let out smoke in the event of power failure - similar to hoistway ventilation, and hoistways do not have the u-turn sub duct.  With the em power requirment, the building owner is faced with two bad choices -  emergency generator or dampers controlled by smoke detectors in a room/airstream we KNOW to be incompatible with smoke detectors.  After the first 3 times the  trucks roll from the shower setting off the smoke detector, they will get removed and there will be NO protection.

The charging statement for em power is in exception 2 to 716.5.3 "...is powered continuously in accordance with the provisions of Section

909.11".  If I were to try to say the code did not require em power, I might go to 909.11, and see the first sentence: "The smoke control system shall be supplied with two sources of power.", then say "If I had a smoke control system, I would certainly provide a generator, but I only have a toilet exhaust system, so I won't".  However, 716.5.3 ex 2 specifically sends me to 909.11, and 909 in general states "where required by other sections", so done deal - em power is required.


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## Codegeek (Aug 19, 2011)

Question for Dr J and CD since you both agree that em power is not required, but is in the code.  I read the language in 909.11 to say "an approved" standby source, which refers back to Chapter 27, which references NFPA 110 and 111.  I do not have either NFPA document, but referenced NFPA 70 for Emergency Systems in Article 700.  I found multiple options for sources of power, one of which is a generator.  Would you agree that the options listed in Article 700 would provide options to meet the em power?


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## Dr. J (Aug 19, 2011)

Yep.  Although it is Article 701 "Legally Required Standby Systems".  I used the generic term "em power", the proper term is "Standby Power".  But notice that both 909.11 and many of the options in 701 use the term "approved", so the answer may depend on if the AHJ's brother is a Caterpillar rep or not.  (One free "engineers gold plate thier designs to justify their fee" remark granted to the AHJ members).


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## Coug Dad (Aug 19, 2011)

I have seen non high rise buildings approved with the secondary power being from a different set of switchgear, similar to what is done for a fire pump.


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