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Dryer Ducts in a Shaft-A Case Study

June 21st, 2009

Recently, I performed a preliminary inspection on an eight story building whose upper 5 floors were being converted into residential lofts. Framing was near completion on one of the upper floors and trade work had been started. One of the first things I look at any residential use in a multistory building are:

  1. Where does the bathroom exhaust go?
  2. Are there clothes dryers and if so where do they vent? And…
  3. Where does the heating and cooling come from?

I am looking to see if shaft requirements have been met.
On this occasion I found both the dryer duct and the bathroom exhaust entering into a single shaft.  The bathroom exhaust was protected by a fire damper at its penetration to the shaft.  The 4’ dryer duct was only firestopped at its penetration.  The shaft and ducts terminated at the 7th floor where an undampered 10” duct penetrated the shaft and terminated at an exterior wall of the building. The building was of type 1B construction and was fully sprinklered.  The design was reviewed under the 2003 Michigan Existing Building Code.

Dryer Ducts in a shaft-c3

Smoke and Fire Dampers Required

Section 716.5.3.1 is very specific in requiring both fire and smoke dampers where ducts penetrate a shaft. There are many exceptions but none that allow either penetrations to be installed without a smoke damper.  Although the building code does not address dryer ducts, I know that dryer ducts cannot be dampered in any way. This would create surfaces for lint to accumulate and create a fire hazard.

Catch 22

While the State of Michigan had not adopted the 2006 International Building Code at the time, it had adopted the 2006 International Mechanical Code. Under the ICC, chapter 6 of the Mechanical Code and chapter 7 of the Building code have been nearly identical for the section on “Ducts and Air Transfer Openings”.  The 2006 now allows an exception for smoke dampers for dryer ducts, kitchen and bathroom exhaust in Groups R and B (previously this exception had only been for bathroom exhausts in B uses only). This exception is allowed under the following conditions:

  1. The building is sprinkled with an NFPA13 system.
  2. The openings into the shafts are installed with steel subducts at least .019 inch thick.
  3. The subducts must extend at least 22 inches vertically into the shaft.
  4. An exhaust fan is installed at the top of the shaft.  This fan must continuously run and be connected to an approved “standby” power source.

With some modifications (subducts and exhaust fan at top of shaft) this installation can bypass both the fire and smoke dampers. (There is an exception for fire dampers in any use group if you install subducts-provided there is a continuous airflow upward to the outside.  The only foreseeable problem is the 10 inch duct at the upper floor.  Subducting would be not be practical because of constraints of air flow, lint accumulation and height. The shaft could be continued horizontally to the exterior wall of the building bypassing any fire/smoke damper requirements.  The final issue is the renovation was reviewed under 2003 IBC and these changes do not exist in that code. The builder would have to apply for administrative relief through the local Authority Having Jurisdiction.

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