Duct code

What is the code for the distance between a smoke detector and a return from a forced hot air system?
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NFPA 72 does not really differentiate between supply diiffusers and return grilles or heat ing and cooling.
They want a three foot clearance.
The main reason being a supply in cooling could cool of the detector. When the AC cycles off mositsure could condense inside of the detector. Mositure can deflect light imilar to particles of smoke and cause false alarms.
Glenn wrote:

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typically smoke detectors are installed in supply air plenums as close to the ac unit as possible, and in some cases in the return air compartment on package units. Some situations require SD's in both supply and return air plenums, for instance sometimes the fans are ran 24/7. typically sd's are not required on systems of 2000 cfm or less.
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snipped-for-privacy@gonefishin.net wrote:

Those are duct mounted smoke detectors, designed to shut a unit down when they begin to recirculate smoke. Some jurisdictions will want them on fan coils under 2000 CFM if the fan coil serves an exit corridor.
I assumed the original poster was refering to ceiling mounted devices montoring the space.
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In Ohio that is true unless the chase is serving more than one unit and the chase experiences a combined total of 2000 CFM's or more.
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24" to 48"...which puts it in the 36" range

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5.7.4.1 of the 2002 NFPA 72 says 3 foot minimum. In the accompanying hand book, they go on about the return grille 'diluting' the smoke. I doubt a return has much influence at that far away. If anything I would guess the smoke would migrate towards a return, get sucked in hence the requirement of the duct mounted detectors.
daytona wrote:

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