The IFC does not reference NFPA 90a, but it does reference NFPA 72 which in turn references NFPA 90a. Is that how I require supply side smoke detection, and further, should I?
Are you an ahj??
or fire alarm company or other??
Notmally if you are using IFC as an adopted code, you use IMC.
So with those two documents the only time you would require ducts on supply side, is when the hvac is very large.
is there a reason you want them in the supply??
I am a fire marshal and am wondering why NFPA 90a requires it but not the IMC. Is it life safety?
As you know codes are a committe process, so they have decided it.
It has been required on the supply side, even before the I codes.
Not sure if it is because they want it before it hits the filters??
Does not really tell you why::
NFPA 90A and International Mechanical Code NFPA 90A, “Standard for Air Conditioning and Ventilating Systems”, requires that smoke detectors listed for duct installations be installed at a suitable location in the main supply duct on the downstream side of the filters to automatically stop the supply fans in systems over 2,000 cfm. For systems over 15,000 cfm an additional detector is required in the return duct or ple- num of each floor, at the point of entry into the common return, or a system of spot type smoke detectors is required to provide total area coverage. (Note: Users of this manual should refer to NFPA 90A for more informa- tion on smoke control requirements.) The International Mechanical Code requires a duct smoke detector in the return for units over 2000 cfm and requires a detector in the supply duct for systems over 15,000 cfm. Many designers require duct detectors in the supply and return in an attempt to meet both conflicting requirements. In order to obtain a representative sam- ple, hvac areas with stratification and dead air space should be avoided. Duct smoke detectors should be located in the hvac area between 6 and 10 duct-equivalent diameters of straight, uninterrupted run. The International Mechanical Code section, 606.2.2 also states that unless multiple air-han- dling systems share common supply or return air ducts or plenums duct detection is not required in the supply air system.
So why is there such a difference in requirements? There are two schools of thought concerning the purpose of duct detectors. There have been numerous unsuccessful proposals over the last couple of I-Code cycles to change the IMC requirements to match the NFPA 90A requirements. The primary purpose of the duct detector on the supply side is to shut down the air handler if fire is detected in the fan or filter. This would protect the equipment and keep smoke from being distributed to the air conditioned space. Some would argue that detectors on the supply side are susceptible to outside air or smoke, which could affect the operation of the supply side smoke detector. There also is the problem of false alarms due to dust accumulation on heat strips in the AHU burning off when heaters are turned on and activating the smoke detector. For this reason, both the IMC and NFPA 72 allow duct smoke detectors to initiate a supervisory signal rather than an alarm signal.
Duct detectors located on the return side and upstream of any filters or introduction of outside air detect smoke coming from an air conditioned space. This eliminates interference caused by outside smoke or heat-strip dust-accumulation burnoff. However, it takes time for smoke to travel through the ductwork in sufficient quantities to -activate the detector. If the purpose is to detect smoke coming from a room in the building, would it not make more sense to require a smoke detector to be installed in that area rather than waiting for enough smoke to reach a return duct smoke detector?
Where NFPA 90A Section 18.104.22.168 (1) indicates "air supply systems" is this referring to the supply side of the system? Or is it meaning in air supplying systems as a general?
NFPA 90A_2012: Chapter 6 Controls
In case of a conflict require both. The fuzzy part come in the inspection testing and maintenance required when the building has a fire alarm. With no fire alarm there is no enforcement on maintenance and testing of duct smoke detector s With a fire alarm all operation of the detector are tested including function, air sampling tubes and sensitivity
Putting aside the conflicting requirements of NFPA 90A and the IMC for a moment, the efficacy of duct smoke detectors is big concern for me. Is there any empirical data out there that proves the value of duct smoke detectors in actual use? The nearly universal perception from building owners and facilities managers is that duct detectors are no more than a maintenance headache.
Thanks for bringing back up. Yes research has been done. Please review this link:
Measurements of Smoke Characteristics in HVAC Ducts | SpringerLink
The purpose of a duct smoke detector is to provide for shut down of HVAC system in the event in the event of a fire to avoid actively re-circulation of smoke through a building or to activate a mechanical smoke management system.
Duct smoke detectors are typically considered mechanical equipment devices instead of life safety devices. Per NFPA 90A; The primary purpose of the duct detector on the supply side is to shut down the air handler if fire is detected in the fan or filter. This would protect the equipment and keep smoke from being distributed to the air conditioned space.
Information taken from link:
Duct Detectors | EC Mag
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