Duct smoke detector in return air shaft

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My electrical contractor is providing the duct smoke detector as it is in their contract. The contract drawing includes a duct smoke detector but did not indicate its location. The building is a 3 story w/ basement, two air handlers with a common plenum return. Return air is through a mechanical shaft w/ fire dampers on each floor. My electrical and his sub (FA contractor) want to install the duct smoke detector in the return plenum The return air for both units is CFM is 2700 each. The fact that we have fire dampers for each floor makes me a little reluctant to use return air plenum as the primary detection.

I would recommend installing it on the 2nd floor supply duct w/ remote LED/Test station w/ access door to shaft. My P.M.?s concern is that it opens up more liability such as ladder and fall arrest in the shaft.

Is the contractor off base with the return air installation?
 
The smoke detector in the duct is there only to turn the fan off. It has no responsibility with regards to fire detection and alarm.
 
[Is the contractor off base with the return air installation?[/QUOTE]


What does the directions tell you?usually with units that small only requiring 1-duct detector we install them in the supply duct,usually within 10' of the first take off,however you need to follow manufactures recommendations...
 
as the other posters said, the primary function is to turn off the unit to avoid transmitting the smoke. however, the function intended for the detector should have been engineered. sometimes they report to the FA, sometimes they interface with a bldg. management system, and as other poster stated, they need to be installed according to the directions (most are usually in/near return plenums)
 
Smoke detector alarm status shuts down the fan based on detection of smoke typically from fan motor or the area the fan serves. The intent is to shut down the fan whether it is a stand alone system or not. My concern is regarding optimum location of the detector to properly sample smoke for fan shut down.:confused:
 
The mechanical engineer did include duct smoke detector in his design, but the fire protection engineer has it in his drawings with no direction for location or function. The unit is not supplied with duct smoke detector and makes no mention of installing one in the installation manual. An DCVR was written asking for a location. The response was to install the duct detector per manufacturer's reccomendation. Pretty vague!
 
If the fire damper closes, then there is no air flow, in turn no smoke to be distributed, fan state will not matter. Any leakage past the damper will be picked up by detector, which will then shut down fan.
 
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"If the fire damper closes, then there is no air flow, in turn no smoke to be distributed, fan state will not matter."

True!

"Any leakage past the damper will be picked up by detector, which will then shut down fan."


Fire damper leakage rate is very low. The fan would still be fueling the fire if it is in the area behind the damper.

If the smoke cam from the intake ( outside air plenum) or fan motor, it would travel through the building back to the reutn air shaft before the sensor picks it up.
 
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Duct detector location

Duct detector location

IMO the missing piece here is the area appropriate fire/building code and referenced document (NFPA 72, IMC, etc.) These will likely answer the question, or at least tell you that it doesn't matter to those codes based on unit size, other detection provided in the occupancy, etc.

Just a couple examples: Highrise would require (I will paraphrase IFC 907.2.12.1(2)) .....detection in the main supply and return of AC units greater than 2000 cfm.............located in a servicable area downstream of the last duct inlet......... Also the opening paragraph says that these detectors "........shall operate the emergency voice/alarm communication system." Or in a prison where the detection can be hidden up in exhaust ducts to keep the little buggers from tampering with them.

On the idea of ladders and fall arrest, the detector should be located in a place easier to access, again, in my opinion. Plus the duct detection is typically in a housing that samples air from the duct, but is physically located outside the shaft itself. I have never tested, or been asked to accept, a detector inside a duct except plenum areas the size of large rooms that allowed walking in upright and had enclosed (grating) shafts entering.:smile:
 
The Alarm contractor instructed the electrical to mount the detector in the adjacent room above ceiling and run the sampling tubes in the return air plenum (shaft). They will install a remote LED test station on the ceiling.
 
I recently finished a job similar in size. The duct detector was included in the HVAC Package and was on the return. Code is vague about it and allows it in either place. From my point of view it doesn't matter where it is placed.
 
The mechanical engineer should have included it in the contract drawings making the mechanical contractor responsible for providing a package that includes duct smoke detector. Thanks your input MJJBEE !
 
Code is fairly clear here.

Code is fairly clear here.

MJJBEE said:
I recently finished a job similar in size. The duct detector was included in the HVAC Package and was on the return. Code is vague about it and allows it in either place. From my point of view it doesn't matter where it is placed.
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Where I am (CT) there is quite a bit of guidance, in the fire code and building code, on this and it does matter which side of the unit it goes on depending on configuration, design parameters, and intent. As I eluded to in my earlier post, there is guidance in many places including NFPA 72 (2002 5.14.4 and 5.14.5), as well as the International Mechanical Code (2003 section 606) including diagrams of various configurations.

Depending on how detailed your inspector is, you (or someone on the job) will need to satisfy all of these requirements to pass inspection.

:)
 
Refer to IMC Section 606 and NFPA 72. After looking at the photos and the sketch in your attachment, it appears that any smoke from the 3rd floor return may not be detected by the DD. This would not be acceptable.

The DD must be connected to the FA system and must shut down both AHUs.
 
jbt260 said:
Refer to IMC Section 606 and NFPA 72. After looking at the photos and the sketch in your attachment, it appears that any smoke from the 3rd floor return may not be detected by the DD. This would not be acceptable.

The DD must be connected to the FA system and must shut down both AHUs.
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Yes, the 3rd floor return air will bypass the sampling tubes. In addition to that the duct smoke detector might not be in the best possible location (1/3 of the duct width) to capture smoke accross the plenum. This is one of the reasons that I will not accept this installation.

"The DD must be connected to the FA system and must shut down both AHUs."

The FA matrix calls for fan shut down and supervisory signal to panel and FD.

Iv'e been doing this for 20 years!! Well.. You've been doing it wrong for 20 years.
Click to expand...
 
As an EC, I placed duct detectors in the return air duct, and I always had someone more educated than me locate them. My FA subs always designed our systems in accord with the specs, the NFPA, local requirements, and the AHJ. (are you the AHJ in this equation)?

Someone dropped the ball on your project. All details should have been clarified and approved during the submittal process.

We ECs are only glorified installers, with papers.

Best Wishes Everyone
 
HighWirey said:
As an EC, I placed duct detectors in the return air duct, and I always had someone more educated than me locate them. My FA subs always designed our systems in accord with the specs, the NFPA, local requirements, and the AHJ. (are you the AHJ in this equation)?

Someone dropped the ball on your project. All details should have been clarified and approved during the submittal process.

We ECs are only glorified installers, with papers.

Best Wishes Everyone
Click to expand...

I am the Quality Control representative for the prime contractor in this equation.

Most prudent designers include supply and return duct smoke detectors. Everyone is dropping the ball on this one ( Fire protection engineer and application engineer).
 
A new install just completed,I debated and concluded if you would have a motor burn out,a bearing seize up causing a belt burn,or other air handler problem,it makes more sense to have duct detector on supply side of air handler
 
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