IMC Chapter 6

[ACFIXR]

This one is about smoke detectors and the 2000 cfm ratings.
Over the last 30 years I have heard many different interpretations of this code and it's requirements. I am hoping someone can shed some light or have more insight.
If you have 2 air handling units in a common area and their combined capacity is over 2000 cfm do they need smoke detectors?
Can a room be considered a common return air plenum?
Then there is the exception:Smoke detectors shall not be required where air distribution systems are incapable of spreading smoke beyond the enclosing walls, floors and ceilings of the room or space in which the smoke is generated.
I have had a state fire marshal explain to me that this "spreading of smoke" can be done by an isolated return in a room that is separated from the main service area.Therefore a detector is required.

?

[desert guy]

I have done Fire and HVAC since 81 and "My" interpretation is if the combined cfm of two or more AHU's or HP's into a room exceeds 2000 then then both or all get smokes. I have also seen Fan Powered VAV's that exceed 2000 get a DD with the exception of when multiple fan powered VAV's are feeding a large open area only the ones exceeding 2000 get them.
On a recent new construction project (in Tempe) the RTU HP's that all fed a big common area all had DD's in the return duct per the plans.
Depending on what the AHJ had for breakfast your mileage may vary.

[ACFIXR]

I guess I'll find out what the latest interpretation is at the final.............

The same aforementioned Fire marshal also explained that the exception applies no matter how much air.........His comment was generated because he noted that we had smoke detectors in units that all served the same room.

My experience has been that it doesn't matter what the fire marshals interpretation is though, it's "what the AHJ had for breakfast "
that really matters................