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  1. #1

    Smoke detectors / rtu units

    Can someone please explain to me how smoke detectors interact with roof top units and how they disable the unit..??

  2. #2
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    Quote Originally Posted by Acmaster111 View Post
    Can someone please explain to me how smoke detectors interact with roof top units and how they disable the unit..??
    Imagine a smoke detector with a long sensing probe with a bunch of wholes inserted into the return or return and supply. The detector will have multiple contacts, so that when it senses smoke it will break control voltage to the unit and stop it, along with sending alarm signals.

  3. #3
    If the smoke detector contact opens up does this stop the 24 volt flow to R from the transformer.?

  4. #4
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    Quote Originally Posted by Acmaster111 View Post
    If the smoke detector contact opens up does this stop the 24 volt flow to R from the transformer.?
    It depends on how the unit is configured.

  5. #5
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    Smoke detectors are ALWAYS a life safety device and should ALWAYS break the control power to any device immediatly including any time delays. Some manufactures have built in control logic for shut down-but you must never trust control logic as a primary source of life safety shutdown as controls do fail and go bad. For your own liability-always wire in smoke detectors to completly shut down any air moving device unless the system is designed for building smoke purge. Smoke purge requires a fan to run and remove smoke from an evacuation area to allow a safe means of escape for the occupants. You must check the classification of the building before you wire any of this stuff in or consult the fire protection company of the building. This is not something that should be guessed at.
    Last edited by Beacon Tech; 05-02-2012 at 09:26 PM. Reason: More info needed

  6. #6
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    ACmaster,
    To answer your question...
    Yes, the easiest way too tie a smoke detector in for fan shutdown is to take the transformer control voltage (R) straight to the detector contacts and then to the thermostat.
    No control voltage, No run.
    NFPA may require either the same detector or an additional one to report to the FACP to sound a local alarm but this depends on occupancy of the building and other local codes.
    Another way to achieve fan shutdown is to use relays (in series with control voltage again) tied to the FACP and shut down all AHU's in a building or on a floor of a building upon the receipt of any general alarm.

  7. #7
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    Quote Originally Posted by blabath View Post
    ACmaster,
    To answer your question...
    Yes, the easiest way too tie a smoke detector in for fan shutdown is to take the transformer control voltage (R) straight to the detector contacts and then to the thermostat.
    No control voltage, No run.
    WRONG!!!

    On some units this won't guarantee that the blower will shut down right away. For example, for most units, if it is in heating and you break the "R" wire between the unit and the thermostat, the fan will keep running for a period of time to blow the extra heat off of the heat exchanger.

    The correct way (if the unit's control board does not have terminals specifically designed for a smoke detector) is to break the "R" wire coming right off of the transformer, before it goes to the control board or anything else.

  8. #8
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    "The correct way (if the unit's control board does not have terminals specifically designed for a smoke detector) is to break the "R" wire coming right off of the transformer, before it goes to the control board or anything else."

    "take the transformer control voltage (R) straight to the detector contacts and then to the thermostat.
    No control voltage, No run.

    And...
    How is this different from what you said?

  9. #9
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    Quote Originally Posted by blabath View Post
    "The correct way (if the unit's control board does not have terminals specifically designed for a smoke detector) is to break the "R" wire coming right off of the transformer, before it goes to the control board or anything else."

    "take the transformer control voltage (R) straight to the detector contacts and then to the thermostat.
    No control voltage, No run.

    And...
    How is this different from what you said?
    The difference is his way shuts the unit down immediately but you don't really mention a control board so it essentially could be the same depending

  10. #10
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    Ahhhh I see!!
    We meant the same thing. Ammoniadog just said it better than I.
    I guess i assumed the OP would understand what I meant.
    To be fair though, I DID say to take control voltage straight to the detector contacts. If one did that it would achieve the desired result no matter how poorly I said it.
    Bill

  11. #11
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    Oct 2002
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    doesn't some air handlers and gas heat control blower with high voltage switchin through bonnet fan control or heat sequencer? 24 volts gone would do nothing.

  12. #12
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    Shouldn't the fan starter or VFD have contacts for shutdown by the FAS? The controls either keep power and are aware the FAS has shut down the fan or could also lose power and shut the unit down hard.

    The OP may be curious about what this is for. On everything above 2000 cfm (there are a lot of 1950 cfm units out there), the smoke detector on the supply figures out if the unit is blowing smoke and shuts it down to protect the spaces served and reduce fire spread. On each return tap larger than 15000 cfm you detect smoke coming away from a space and toward the unit and shut down there. IMC based codes require detectors at each smoke damper for additional protection. In earlier codes the FAS would shut FSDs in a fire/smoke zone based on the FAS sequence of operations.
    I hope I helped!

    Feel free to contact me and other users here for more detailed assistance within the context of a contract. Advice and opinions are provided here "as-is" without any warranties, guarantees or acceptance of liability for use, misuse, or accuracy of advice given. (Gotta keep my legal team off my back at work.)

  13. #13
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    ARTICHILL

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