Exhaust system

[ssn61]

We are connecting all the exhaust ducts in a three story office building to a main duct and connect the main duct to an exhaust fan located in penthouse/mechanical room, then discharge it through a discharge duct to above roof louver/vent. Is it acceptable to have the discharge duct at the discharge side of the exhaust fan, inside mechanical room, where air handling units are located? I believe there are codes saying that no part of exhaust duct, which is under pressure can be within an occupied space. Is that true regarding mechanical room? Do you know of any code that talks about this issue?

 

[Joesf35]

You can't have a positively pressurized code required exhaust duct located in a plenum. If the units are fully ducted you are ok. If the mechanical room is a plenum then you have to enclose the positively pressurized ductwork, to isolate it from the mechanical room plenum.

 

[ssn61]

Thanks Joesf35 for the reply. Two questions, one, how do we enclose the ductwork? Two, in what code book and what section can I find this code reference?

 

[cry22]

Use welded ductwork on the discharge side, and you're done with it.
Check Air distribution Chapter in IMC.

 

[MECHJAMIECTA]

Are the AHUs in the penthouse direct ducted to an outside air louver or to a DOA or are there general OA louvers and the AHUs are pulling in "OA" from within the penthouse?

 

[ssn61]

We are using the penthouse as plenum. Please let me know what code or standard are you referring to?

 

[317069]

check with the city hall about the code you have to use.

 

[Wareagl487]

Joesf35 (Mechanical)
3 Nov 11 23:36
You can't have a positively pressurized code required exhaust duct located in a plenum. If the units are fully ducted you are ok. If the mechanical room is a plenum then you have to enclose the positively pressurized ductwork, to isolate it from the mechanical room plenum.

Yep, you are correct - been buried in the codes since I can remember - but rarely seen it enforced (I can only recall getting "hit" by it once in my career).

Here is IMC 2003 take on it:

601.3 Contamination prevention.

Exhaust ducts under positive pressure, chimneys, and vents shall not extend into or pass through ducts or plenums.

I am not a big fan of this code requirement where general exhaust is concerned.

Andy W.

 

[ssn61]

Thank you very much Andy. Your comment was very helpful.

SSN