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Thread: Global shutdown for duct detectors

  1. #1
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    Global shutdown for duct detectors

    There's a company I work with and there were a couple jobs where the AHJ specified they want global shutdown for the duct detectors. The contractor goes about this by using 1 control relay and tie-in in all the duct detectors into that 1 relay. There is a P.E. I also work with in a completely different state with a different AHJ and he wanted me to show global shutdown on a drawing but he specified a control relay at each duct detector. I'm just wondering how the majority of others go about this. Personally I would go the route of the P.E. who wanted a relay at each detector since this follows chapter 21 of NFPA 72 with the control relay being within 3' of the controlled device. Thoughts?

    Thanks,

  2. #2
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    My quick read of your question brings up the question ---

    Isn't a duct detector a controlling device and not a controlled device?

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  3. #3
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    Quote Originally Posted by gar View Post
    180328-0940 EDT

    My quick read of your question brings up the question ---

    Isn't a duct detector a controlling device and not a controlled device?

    .
    I had the same thought. The air handlers are what need to be shut down. I don't recall ever shutting down all air handlers for one detector in alarm.
    The more they overthink the plumbing, the easier it is to stop up the drain

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    While not always the case, duct detectors are frequently equipped with on-board form "C" relays. These relays are typically used to open one leg of the thermostat loop, shutting down the air handler. They can be daisy-chained so that if one duct detector is activated, all the duct detector relays are operated. For addressable systems, you can use an addressable relay to accomplish the same end for each air handler. The advantage with addressable relays is that you can zone air handlers together if you want fewer than all the units to shut down. You can also change which units are zoned together later by simple programming at the panel, not rewiring in the field.

  5. #5
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    Global shutdown is typically required in a case by case basis. For example: Los Angeles City Fire Department requires global shutdown for high riser buildings for all fans that are not part of a smoke control system.

    On projects with smoke/fire dampers, global shutdown is a good idea to be sure HVAC units are shutdown at the same time as smoke/fire dampers to avoid the dampers being back fed by the units. This could cause damage to the ducts if they do not have pressure shutdown switches.

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    Quote Originally Posted by ryant35 View Post
    Global shutdown is typically required in a case by case basis. For example: Los Angeles City Fire Department requires global shutdown for high riser buildings for all fans that are not part of a smoke control system.

    On projects with smoke/fire dampers, global shutdown is a good idea to be sure HVAC units are shutdown at the same time as smoke/fire dampers to avoid the dampers being back fed by the units. This could cause damage to the ducts if they do not have pressure shutdown switches.
    How to turn a square duct into a round duct without tools.

  7. #7
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    Quote Originally Posted by gadfly56 View Post
    While not always the case, duct detectors are frequently equipped with on-board form "C" relays. These relays are typically used to open one leg of the thermostat loop, shutting down the air handler. They can be daisy-chained so that if one duct detector is activated, all the duct detector relays are operated. For addressable systems, you can use an addressable relay to accomplish the same end for each air handler. The advantage with addressable relays is that you can zone air handlers together if you want fewer than all the units to shut down. You can also change which units are zoned together later by simple programming at the panel, not rewiring in the field.

    Yes! The contractor uses EST and their duct detector has the built in relay but there was a case where the mechanical contractor was supplying the detectors and they were just providing the relay for global shutdown. If this is the case though, how can you do this with the 3' minimum distance requirement? Maybe I'm confusing myself on this.

  8. #8
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    Quote Originally Posted by Fire Pro View Post
    Yes! The contractor uses EST and their duct detector has the built in relay but there was a case where the mechanical contractor was supplying the detectors and they were just providing the relay for global shutdown. If this is the case though, how can you do this with the 3' minimum distance requirement? Maybe I'm confusing myself on this.
    See post #3. The duct sensors are controlling devices, not controlled devices, so the 3' specification you cite does not apply.

  9. #9
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    Quote Originally Posted by GoldDigger View Post
    See post #3. The duct sensors are controlling devices, not controlled devices, so the 3' specification you cite does not apply.
    I was thinking it would be considered a controlled device if you added the control relay.

  10. #10
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    Fire Pro:

    Because something has a control relay (whatever that really is) in the something does not mean it is the controller or the controlled item.

    A controller is something that controls something, while a controlled item is something being controlled by a controller. So it matters where you draw the line between the two ends of a circuit.

    We are really talking about source and destination.

    .

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