What does "continuous" mean?

[Sifu]

Pretty sure it has been covered, but it makes me uncomfortable so I'll ask again. What does "continuous" mean in 2018 IBC 717.5.2 exc. #3 mean? If taken in the common sense, if a 26g. steel duct ran from one side of a fire barrier back to the equipment then it is continuous. So unless it is an air transfer duct, it meets the exception. Some have claimed it means there can't be openings to that duct on each side of the fire barrier, but that is not articulated.

I have an electrical room that is required to be of 1-hr construction by the NEC. There is a very large supply duct from the AHU on the exterior building that has large supply diffusers on the inside and outside the room, crossing the fire barrier. It seems to meet exception #3 but I wonder about smoke entering the duct from inside the room and spreading to the other side of the fire barrier, and a fire damper wouldn't accomplish that anyway, so I am not sure. If it had a flexible connection installed somewhere in the duct, then it wouldn't seem to work since it is required to be continuous steel duct, but why would that be more hazardous than a giant opening? Frankly, I've never been too sure of this particular section.

 

[klarenbeek]

As long as the system is fully ducted (no plenums) and there's no flex duct in the system exception 3 is met, assuming the building is fully sprinkled. The idea is that the sheet metal provides a level of fire protection, either keeping the fire out of the duct or if it does enter the duct containing it inside the duct, at least for the required time. The way the exception reads, its not just from the fire barrier to the air handler, its the entire duct system from the farthest register to the air handler. Unless there's also a smoke barrier requirement, spread of smoke is not an issue as far as the code is concerned. If it is, you can still skip the fire damper and put in a smoke damper only.

The 2021 adjusted that exception to allow for a flexible connector at the air handler location (this would be the canvas connector for vibration elimination) and for flex duct for final register connections within the room where the register is located (typically for the last connection to a register in a lay-in ceiling).

 

[walker.t]

What does "continuous" mean in 2018 IBC 717.5.2 exc. #3 mean?

I know you are using the 2018 IBC but I’ll share what my thought is based on the 2021 IBC commentary.

Partial quote of 2021 IBC Commentary on 717.5.2 Fire Barriers (emphasis added)
“Exception 3 states that, in fully sprinklered buildings containing occupancies other than Group H, fire dampers may be omitted in duct penetrations of walls having a fire-resistance rating of 1 hour or less. Such duct systems must be 26-gage steel continuous from the air handler to the air outlet with no flex duct connectors.

In the 2021 IBC there are two sub-points to Exception 3 that state when “nonmetal flexible air connectors” may be used (the duct connection to the air handler and from the metal duct to a ceiling diffuser in the same room.” My interpretation of that is that by clarifying when flex duct can be used they are saying any other use of flex duct compromises the continuity of the metal duct.

So to answer you question, “continuous” means metal duct without sections of flex duct connecting the metal duct together (like maybe at a turn.)

 

[Sifu]

The 2021 adjusted that exception to allow for a flexible connector at the air handler location (this would be the canvas connector for vibration elimination) and for flex duct for final register connections within the room where the register is located (typically for the last connection to a register in a lay-in ceiling).

That makes sense. I read this as no FD required in this situation. The flexible connections I was curious about are the vibration boots you mention.

 

[steveray]

Sprinklers? This has gotten more relaxed in the last couple cycles....For better or worse....

 

[klarenbeek]

That makes sense. I read this as no FD required in this situation. The flexible connections I was curious about are the vibration boots you mention.

I would also. Even before the 21 code specifically allowed for them I never had an issue with the vibration isolator at the air handler connection.

Sprinklers? This has gotten more relaxed in the last couple cycles....For better or worse....

Building is still required to be sprinkled. Other than the two additions to the 2021 code mentioned above, this section has remained virtually unchanged for fire barriers since 2003. It was added to fire partitions also in 2012.

 

[Sifu]

Yes. 13 system in the house.