Question regarding dryer exhaust penetrating a frie rated exterior wall.

[ericl0706]

Hi everyone:

im new to this forum and have a question regarding the dryer exhaust duct penetrate the fire rated exterior wall of the building. By NYC code if the duct penetrates a fire rated wall it requires a fire damper. However, the code states that fire damper, combination fire/smoke damper and any similar devices that will obstruct the exhaust flow shall be prohibited in clothes dryer exhaust ducts. In this situation can we wrap the exhaust duct with 3m fire barrier duct wrap 615+ to avoid the fire damper? Thank you.

 

[MintJulep]

I would interpret as:

In general, ducts that penetrate a fire rated wall need dampers, but dryer exhausts are a special case and dampers are prohibited for this special case.

 

[ericl0706]

Hi thanks for the reply. So in this case, instead of putting a damper do i need to wrap the duct with a fire barrier duct wrap? Or do what the code states construct the duct with aluminum gage 26 or galvanized gage 30?

 

[Drazen]

that is tricky.

i never find reasonable explanation for fire-rated outside walls, but the fact is that they exist.

if that is the only fire barrier your duct penetrates, than any kind of wrapping does not make sense, as you need to break fire advancement through exterior wall, which means duct has to be closed during fire situation, not isolated in whole length.

can you make more investigation of your code, what does it exactly means "no any flow obstruction"? outside louvers are the same kind of obstruction as fire dampers are, so you should not have them as well.

or, does code allow some alternative, like interface with fire alarm system that will switch off drier when dampers are triggered?

i see no logical reason why dampers would be totally and unconditionally banned. such reason does exist for instance in case of kitchen exhaust, there flammable grease could deposit in dead corners of dampers.

of course, there are some elements of each and every code that do are not fully aligned with logic, yet they are mandatory.

 

[DrRTU]

Your answer depends on the rating of the exterior wall and the type of dryer exhaust system. If the exterior wall is built to 1hr, then 26G or better thru both membranes will meet UL details. Review the various fire stopping guides such as Hilti or 3m for external fire stopping details. IMC -2009 is recommending reading for dryer duct. If the exterior wall exceeds 1 hr, you will need to extend a rated shaft thru the exterior wall. Shaft & wall intersection will need fire stopping.

 

[MintJulep]

To be fair, the code is not very clear.

I can guess that the intent in prohibiting dampers or other obstructions in a dryer exhaust is to not give lint a place to collect and start a fire.

But then again, the code REQUIRES a backdraft damper at the outside wall. So I'll have to guess again that a lint fire on the outside of an exterior fire rated wall is somehow acceptable.

Regardless, you are certainly not the first person to ever design a dryer exhaust in NYC. So find out what it is that the inspector wants to see and do that.