Corridor damper question

[Sifu]

I will try to articulate this clearly enough to understand, I have two concerns:

2018 IBC/IMC, R2, VB, Sprinklered, 3 story, no smoke control system, no shutdown of the DOAS (dedicated outdoor air system) from alarm or flow sensor, the DOAS is always on.

1) . The corridor construction is a rated corridor with an unrated dropped acoustic tile ceiling below the rated lid. Within this space there are ducts from a DOAS that supply each unit. All ducts run directly to each unit, with fire dampers installed at the penetration of the fire partition corridor wall above that unrated lid...except two, which supply the corridor with outdoor air. These are the only ducts in that mechanical space. I believe that 717.5.4.1 will require those two supply registers to be protected with a smoke damper.

2) The space between the unrated acoustic ceiling and the rated lid is being used as a return air plenum, which would require the unrated acoustic tile ceiling to instead be of be fire rated construction per IBC 1020.5.1/IMC 601.2.1. This would then impact the previously mentioned smoke damper by requiring it to be a corridor damper per 716.5.4.1 #1 or a ceiling radiation damper (not sure which) per 716.5.4.1 #2, and also require the ducts that penetrate the units to be also be smoke dampers per 716.5.4.1 #3.

I see these codes as preventing smoke from one unit from entering the corridor plenum space, then entering the corridor through the unrated acoustic ceiling and DOAS registers, and/or any smoke from one unit to another via the plenum.

Any critique of my understanding and concerns is appreciated.

 

[klarenbeek]

I think corridor dampers are not an option for this design. The only ones I know of are listed for installation in the ceiling of a tunnel style corridor, and IMC 607.5.4 only allows for them there as well.

A rated lay in ceiling would allow the space above it to be used as a plenum, and any registers/grills in it would require a radiation damper. The problem is I would guess the lay in ceiling doesn't have a smoke rating though, so that issue needs to be addressed in a different way. Is the plenum the return/exhaust for the DOAS unit? If so, every corridor wall penetration will require a fire/smoke damper, not just a smoke damper. Even above the rated lay in ceiling the wall will still be a fire partition, and in order to meet exception 3 of IMC 607.5.3 to not require a fire damper, the system needs to be fully ducted, no plenum.

By their listing, smoke dampers are required to be either at or within 2 feet of the smoke barrier. If the two openings come directly off the trunk line in the corridor, all penetrations of that corridor wall by the duct system will need smoke dampers. A way around requiring all those dampers would be to route the two runs feeding the corridor out of the corridor and then back into it, so you would need only two dampers at those two openings. That way there's no openings off the trunk line or individual supply runs exiting the corridor, only on the two runs coming back in. That design would meet the requirements of 607.5.4 exception 3. This would only apply to the supply side if the return/exhaust is a plenum.

At least that's everything I can think of, there's a lot going on here

 

[Sifu]

I think corridor dampers are not an option for this design. The only ones I know of are listed for installation in the ceiling of a tunnel style corridor, and IMC 607.5.4 only allows for them there as well.

A rated lay in ceiling would allow the space above it to be used as a plenum, and any registers/grills in it would require a radiation damper. The problem is I would guess the lay in ceiling doesn't have a smoke rating though, so that issue needs to be addressed in a different way. Is the plenum the return/exhaust for the DOAS unit? If so, every corridor wall penetration will require a fire/smoke damper, not just a smoke damper. Even above the rated lay in ceiling the wall will still be a fire partition, and in order to meet exception 3 of IMC 607.5.3 to not require a fire damper, the system needs to be fully ducted, no plenum.

The lay in ceiling has no ratings. The return plenum is for the FCU supplies that feed the corridors. The corridors form an L shape, with each leg served by different FCU's through supply air diffusers. Each leg has a return grill in the acoustic ceiling to provide a return path back to the FCU through transfer grills where the FCU is outside of the corridor. The ducts and transfer grills are protected by fire dampers, but not smoke dampers. There is no return path for the DOAS air.

At least that's everything I can think of, there's a lot going on here

Yes there is. This is but one area of concern.

 

[klarenbeek]

The lay in ceiling has no ratings.

Right, but like you said if they want to do the plenum above it, it will have to be a rated ceiling per 601.2.1. The other option is duct the return to the ceiling and eliminate that issue.

Each leg has a return grill in the acoustic ceiling to provide a return path back to the FCU through transfer grills where the FCU is outside of the corridor. The ducts and transfer grills are protected by fire dampers, but not smoke dampers.

If the FCUs serving the corridor are outside the corridor, where those ducts/openings enter the corridor they need both fire and smoke protection, either a combo damper, or a fire damper at the wall and a smoke within 2 feet of the wall.

There is no return path for the DOAS air.

So where is the DOAS pulling air from? Unless its nothing more than a makeup air unit, every one I've ever seen is basically a very large HRV with some form of supplemental heat to further temper the fresh air coming in above what the HRV portion does if needed. When I see these in an apartment building or hotel, they usually are used for bathroom exhaust, and supplying fresh air to the apartments.