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Thread: one smoke detector or two

  1. #1
    IT Consultant / Network Engineer beekerc's Avatar
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    Default one smoke detector or two

    in the process of remodelling my basement, i'm upgrading the alarm system as well, primarily to increase the number of monitored zones because of added points of entry, and since they'll be out anyway, i'm looking at adding smoke/heat detectors. currently i have one smoke detector, centrally located upstairs and one heat detector in the basement laundry room. i'm adding one detector in each bedroom upstairs (not required but it seems prudent), one in the basement office (back half of the house), a heat detector in the garage and one in the basement den. the den is 13' x 24' and the ceiling is 8' high. running down the middle of room (cutting the 24' dimension in half) is a heating duct (approx 18" wide and 10" high) that sits below the bottom of the joist line. this duct has been framed and will be sheetrocked while the rest of the den will be a drop ceiling, about 2" below the joist line.

    here's my question. do I need to have one smoke detector on each side of the duct or would one, centrally located, on the bottom of the heating duct chase be sufficient? if smoke were travelling up and across the duct area it would be detected right away, but if the fire started in one of corners and went straight up, it would have to fill that 10" cavity space before crossing the duct to set it off. and since i'm using a drop ceiling, if i did put on on each side, i'd have to do a veritcal mount on a wall, which means if a fire starts on an opposite walll, there would still be a necessity to fill the cavity, although not as much.

    Thanks
    B

  2. #2
    Homeowner Thatguy's Avatar
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    Default as long as you bring it up

    Don't know if it's code, but wouldn't a smoke detector in the duct detect smoke anywhere (as long as the blower is running), in addition to detecting a cracked heat exchanger?

    This would be OR'ed with the others.
    Last edited by Thatguy; 12-11-2008 at 01:55 PM.

  3. #3
    IT Consultant / Network Engineer beekerc's Avatar
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    Default smoke detector

    perhaps this will help.
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    Electrical Contractor/Instructor jwelectric's Avatar
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    Default

    This is a building code issue and not an electrical

    Open one of the alarms and read what the instructions say

  5. #5

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    Two smoke detectors - one on either side of the duct would be better. On the ceiling is better than lower down. (Smoke and heat rises.)

    In my area, interconnected smoke detectors are required inside and outside of every bedroom.

    As to smoke detectors inside of a duct, there are special "duct smoke detectors" which have a rod which inserts into the ducts with holes in it and this "samples" the air. They are typically wired to shut down the air flow and also trip the alarm. (I've only seen them in commercial installations.)

    Here is one...
    http://www.ap-c.com/products/duct_sm...ies/SL2000.asp

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    Plumber jimbo's Avatar
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    CHeck the installation instructions. They show proper locations. In a nutshell, you need to avoid being too close to where a horizontal and vertical plane meet, because you can get a "dead zone" that smoke travels around.

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    IT Consultant / Network Engineer beekerc's Avatar
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    Quote Originally Posted by Billy_Bob View Post
    Two smoke detectors - one on either side of the duct would be better. On the ceiling is better than lower down. (Smoke and heat rises.)
    two would be better than one, but both sides of the hvac chase are drop ceiling and i believe i've only ever seen wall mounted detectors when a drop ceiling is present. I'll talk to the ceiling contractor when they're out and see what they have to say about that. agreed two is better, but as I'm already incurring the cost of putting 3 in the bedrooms, this becomes an issue of prudent spending. little costs do add up.

    Quote Originally Posted by Billy_Bob View Post
    In my area, interconnected smoke detectors are required inside and outside of every bedroom.
    I believe that is the case here, but for new constructions only. if I opt to put in additional detectors, i believe the only requirement is that the are interconnected. Since my detectors are part of my alarm system, the interconnection is already there.

    Quote Originally Posted by Billy_Bob View Post
    As to smoke detectors inside of a duct, there are special "duct smoke detectors" which have a rod which inserts into the ducts with holes in it and this "samples" the air. They are typically wired to shut down the air flow and also trip the alarm. (I've only seen them in commercial installations.)

    Here is one...
    http://www.ap-c.com/products/duct_sm...ies/SL2000.asp
    [/QUOTE]

    a very cool concept, i do wonder how much it costs, however, I never said i iwas interested in putting a detector in the hvac duct. I mentioned the heating duct because that what the chase is for. I should just said "a sheetrocked chase 18-inches wide that drops 10-inches down from the drop ceiling line".


    Quote Originally Posted by jimbo View Post
    CHeck the installation instructions. They show proper locations. In a nutshell, you need to avoid being too close to where a horizontal and vertical plane meet, because you can get a "dead zone" that smoke travels around.
    I am aware of the deadzone when mounting vertically, but just not sure what the actual measurements are. Ideally, I'd like to mount them horizontally on a ceiling line as is traditional. i guess the $64 question is, can that be done with a drop ceiling. I've already asked my ADT guy but for as long as he's professed to be in the business he says that no one has ever asked before - I was the first, and he didn't know - go figure.

  8. #8
    DIY Member B2CHR's Avatar
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    Sure, mount them to the 2x4 drop panel with holler wall anchors or toggle bolts. You now can buy smoke detectors cheap so I would use 3 in the room. One on each side in the middle and one above the drop ceiling mounted to the bottom of the floor framing. Get a fire from something above your drop in ceiling and might be a while before the smoke makes it through the ceiling to set off the ceiling alarms. Just my two cents.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    HD sells a contractor pack of smoke detectors - you get six in the box that essentially gives you 6 for the price of 2.5 if you buy them in pretty retail packages. Can't vouch for their quality, but they do work. I'd look for contractor packs if I was going to need a few.

    Keep in mind also that the recommendation is to replace smoke detectors every 10-years, or if they fail the self-test that I know you perform regularly.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    IT Consultant / Network Engineer beekerc's Avatar
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    Default

    i'll have to look into the drop panel. i'm assuming that in instaed of the flexible fiberglass panel, it's made of a firm material that can hold screws and anchors, but not as heavy as plywood.

    the detector above in the joist space makes sense, however, the joists are 4 feet apart and run the entire width of the basement, plus there's only 2 inches between the bottom of each one and the drop ceiling, so to effectively monitor that space, i'd really need one detector for each joist bay.

    and while regular smoke detectors are cheap, these are designed for the alarm system so these are not cheap. each one probably costs twice as much as the contract 6 pack at HD. If i didn't have a monitored alarm system, i'd be all over that one, but (to me) it doesn't make any sense to have an unmonitored detector when i have a system that will automaticlly call the fire department.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Some manufacturers make available interfaces that will enable them to trip others trigger line...I'm guessing that as long as one monitored one goes off, it calls the fire department. This may still be doable with less expensive items.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  12. #12
    IT Consultant / Network Engineer beekerc's Avatar
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    Default smoke detectors

    Quote Originally Posted by jadnashua View Post
    Some manufacturers make available interfaces that will enable them to trip others trigger line...I'm guessing that as long as one monitored one goes off, it calls the fire department. This may still be doable with less expensive items.

    good thought. i'll ask my rep when I talk to him again. although i have a hunch the answer would be no, after all, that just means less smoke detector sales for them.
    thx

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