Limit for Duct Detectors on the Same Zone?

11/1/2022

How many duct detectors are allowed on a single zone?

The maximum number of flow switches allowed is five, but are there any requirements or restrictions for duct detectors beyond the equipment capability?

Thanks in advance.

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4 Comments

Chad

11/1/2022 08:52:36 am

As an AHJ, it’s per unit, you cannot have us guessing which RTU it is. We only require them to be an alarm in critical facilities and healthcare occupancies (I realize that can be controversial). But I can’t imagine having to get on the roof and guessing from one of five or even four units being in alarm.

I’m not in front of my copy of 72 but I’m sure I could wrap a code section allowing me to define that In regards to efficient response and annunciation

Darrell Rich

11/1/2022 11:01:10 am

i totally agree with. not only are you dealing with possible false alarms, but you also have to monitor the trouble contacts as well. trying to figure out a ground fault or open circuit can be a HUGE HEADACHE!

Tim D

11/1/2022 09:40:04 am

NFPA 72 states maximum size of zone is 22,500 sq ft. Only limitiations for amount of devices on zones is for waterflow and supervisory signals for sprinkler systems in protected premise. Assuming this is a conventional system, if you are using two wire duct detectors then you are going to be limited on how many you can use because of the voltage draw. If you are using four wire duct detectors then you are only monitoring a set of conacts which will drastically change the amount of detectors you can monitor. When it comes to identification of which duct detector is in alarm, that is covered by NFPA 72 requirement for remote indicators for smoke / duct detecors in concealed locations. If this is an addressable system the same rules apply. Theoretically you could use one monitor module to supervise multiple duct detectors but doesn't make practical sense.

Doug link

11/1/2022 05:09:34 pm

What one person means by the word “Zone” is one definition; what another person means by the word “Zone” is often a different definition.

Firefighting Zone: Firefighters, when trying to control a fire in a building are using the word “Zone” as an area of a building that limits the spread of a fire. Usually, zone is enclosed by firewalls or other restraints for the fire. This zone is restricted in size to 22,500 square feet by the NFPA 72.

Fire Alarm Zone: Fire alarm people, when connecting wires to a fire alarm panel refer the word “Zone” to mean the input screws of the fire alarm control panel. Many times, I have seen Zone 1, Zone 2, Zone 3, etc. silkscreened next to the screw terminals of quite a few UL Listed fire alarm control panels to specify the Input 1, Input 2, Input 3, etc.

I have also seen references to the input modules on an addressable fire alarm system, in fire alarm product literature, as “Zone Inputs”. The zone inputs for an addressable fire alarm system are really “Point Inputs”, and are often used individually for each duct detector, so anyone can easily identify which duct detector is in alarm, supervisory, or trouble.

Remote indicators are helpful, but when one Roof-Top Unit (RTU) air handler is covering several different firefighting zones, even finding the location of the remote indicator, and on which ceiling, may be difficult. It’s also confusing, when the remote indicator is on the ceiling, and there are 10 closely spaced RTUs on the roof in the area above the remote indicator.

The fire alarm zone may have been considered to be the same as a firefighting zone, half a century ago, but the meaning of the fire alarm zone has since been changed to be “Input Zone Number x”.

Both of the zone meanings have been used in the above question and answers.

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Limit for Duct Detectors on the Same Zone?

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