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Author Topic: A word about wiring your AC-powered smoke alarms to your security system...  (Read 12756 times)

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Among the most common questions asked in the fire alarm section of the DIY forums are questions regarding hooking up an existing 120 VAC interconnected smoke alarm system to a security system control panel.  Before you made any decisions or posts, I just wanted to give you some facts on what you want to do, and to discuss the equipment a bit further.  Hopefully you can follow along without getting lost!

Yes, manufacturers of residential 120 VAC smoke alarms do make a tandem relay that closes when any detector goes into alarm, and opens when smoke clears. While it seems harmless to wire your smoke detectors to a security system, I urge you to reconsider doing this.

The FIRST and FOREMOST thing you need to do when considering to add any fire detection equipment to your security system is to talk to your fire marshal or Authority Having Jurisdiction (AHJ).  Sit down with him/her, and explain to them what you currently have, what you'd like to do, and what their recommendations are.  They're not out to make money, they're out to keep you and your family alive.  We'll briefly discuss some fire codes here, and it's important to keep in mind that codes such as NFPA are just a nationwide set of recommended codes -- they hold no legal weight.  The fire codes in your specific municipality are what you are mandated to follow, and ALWAYS have the absolute last say in anything you do.  It may vary by city or county, so, we can't answer any specific questions.  Your municipality may choose to accept NFPA as-is, or they may tweak it a bit to a set of codes more applicable to your location.  The only way to find out is to talk to your AHJ.

In order to have your 120 VAC detectors trip a security system zone, you'd have to connect the 12-volt fire alarm zone to the 120 VAC relay.  This is strictly forbidden in NFPA 70, National Electric Code, as you'd have to have 120 VAC and 12 VDC entering the same enclosure at some point.

With that said, I've copied some text from the Simplex publication, "4098 Detectors, Sensors, and Bases Application Manual", publication number 574-709, revision M.  This explains the code issues regarding, and problems with trying to rig your AC interconnected smokes to a security system.  I know that this book refers to a "fire alarm system", as Simplex equipment is typically installed in commercial establishments, however, your security system panel is classified as a residential system, so yes, this does apply.  As I work for Simplex, this is the only documentation I have on hand, but I'm sure that if you check with other detector manufacturers, you'll find the same information.

Page 1-7, "Where Not to Place Detectors and Sensors (continued)"

Improper Locations for Detectors and Sensors, (continued)

"...Underwriters Laboratories (UL) has three standards for smoke detectors:  one for duct detectors/sensors, UL 268A; one for single- and multiple-station detectors/sensors, UL 217; and one for system-type detectors/sensors, UL 268.  Detectors/sensors should only be used in the applications for which they are specifically listed.

The NFPA 101 Life Safety Code states that single-station smoke detectors shall sound an alarm only within an individual living unit or similar area and shall not actuate the building fire alarm system.  It also states, 'All systems and components shall be approved for the purpose which they were installed.'

In addition to possible code non-compliance, the following deficiencies would exist in a series of residential smoke detectors connected in a fire alarm system mode:

   - Since the fire alarm system is not supervised, vandals or others could disconnect a detector or the entire system, leaving a buildng without protection.  The residents would be unaware of the serious life-threatening condition.

   - Residential detectors do not latch in alarm.  In other words, the detector self-resets.  One detector in alarm sounds all the detectors connected together.  It would be difficult to identify or locate a specific detector that initally put the system into alarm after the alarm condition was cleared.

System detectors/sensors latch in alarm.  They do not reset until power is momentarily disconnected.  This makes it convenient to identify the location of the detector/sensor that caused the control panel to alarm.  In addition, system detectors are specifically designed to connect to a supervised control panel.  Two-wire detectors require a UL compatibility review to verify that the detector and panel operate together...."

For the purposes of discussing general codes, a security system in a home could very well constitute a "building fire alarm system" since it's listed by UL as a "residential fire warning system control unit".  NFPA also states that when detectors are installed, manufacturers warnings and directions must be followed.  If the documentation forbids this, then it will probably be a code violation.

In addition, one thing that's been covered on this forum, but not mentioned within detector documentation or Code is the performance differences between common household smoke alarms, and smoke detectors that are designed to be connected to a security or fire alarm control panel.  Residential AC-powered detectors are normally "ionization" smoke detectors, which use a small radioactive source to ionize an air chamber.  When "particles of combustion" (not necessarily smoke) enters the chamber, the current flow between the plates is affected, and the detector sounds.  Particles of combustion can be invisible to the human eye.  I remember as a kid, my dad's kerosene heater used to set off the smoke alarm all the time, even though there was no "smoke".  I'm sure everybody can relate to something similar.
Also, residential smoke alarms are designed to be replaced every 5-10 years, as the sensing chambers do get dirty and the radioactive material used will eventually start to lose strength.  Residential smoke alarms also tend to have a lower sensitivity level.  Smoke detector sensitivity is a detector's ability to detect a certain concentration of smoke per cubic foot of air.  Sensitivity is low so that the detector can err on the side of caution.  Because the amount of smoke required to cause a residential smoke alarm to sound is so low, you're very likely to get a false alarm from cooking, from a fireplace, from the shower, or from using an aerosol air freshener.  False alarms on a common residential smoke alarm typically aren't a problem, right?  We just fan some air into it, and it shuts up after a few beeps.  They're not considered a big deal, becase the detectors don't report a fire alarm off-site.  If these detectors are jerryrigged to an alarm system panel, you just dispatched the fire department because of those two beeps.

System-connected detectors are the "photoelectric" type.  They use a small beam of infrared light inside the detecting chamber to detect smoke by reading the amount of light bounced back into a receiver LED.  When smoke enters the chamber, light is reflected back at an angle, and the detector begins a verification period, where it conducts a series of algorhithms to determine if it caught a whif of actual smoke, or if it caught some shower steam.  Also, system-type detectors have the built-in capability to adjust their sensitivity levels if the detector chamber gets dusty or dirty to compensate for the effects the dust will have inside the detector chamber, while sending a maintenance signal back to the control panel to signal the need for cleaning.  Your residential smoke alarms can't do that -- shower steam or too much dust can mean an alarm.  The detector's verification period takes usually about 30 seconds if smoke is very light, but increasing or heavy smoke will cause one to trip quicker.

Furthermore, there is a difference in the UL listing.  Residential smoke alarms are UL listed as a "single or multiple station smoke alarm", NOT listed to trip a panel.  System-connected devices are UL listed as "Smoke-automatic fire detector for connection to a UL listed panel".  To connect any type of residential detector to an alarm system will be voiding the UL rating.

Tandem relays for AC-powered smoke detectors are really intended for auxiliary controls such as closing smoke doors, starting fans, or recalling elevators in buildings that don't require a central fire alarm system, but do require local smoke protection features.  Elevator recall, vent fans, and self-closing smoke doors are local smoke protection features.  Many elevator shafts have the same type of AC detectors used in homes, its sole purpose is to safely bring the elevator to the ground floor and lock it out in fireman's service when smoke is detected.

It's not uncommon for ionization detectors to fail and false alarm for no reason.  You seldom see ionization smoke detectors used in system-type fire alarm systems.  Less than one percent of the fire alarm systems I've seen (and I've seen a LOT) use ionization detectors.  They're used mostly on old systems where the detectors are between 15 and 30 years old.  And, I fail them usually by the dozen.

Please, just do the job right the first time around.  Leave your current AC interconnected system alone, let it do the only job it's intended to do - detect and wake you up.  Save yourself and your family, get out of the house and dial 911 from a neighbor's place.  If you want to supplement this with system-type smokes for automatic reporting when you're not home, that's great!  Follow the minimum guidelines of one detector on each habitable floor and one outside each sleeping area.

Annually, fire kills over 4,000 Americans, and that's more than all natural disasters combined!  When you think about how fast a fire can flash over in a room (as little as 30 seconds), it makes sense to keep your life safety systems code compliant and maintained properly.  

If you want to go the route of having a complete residential system with system-connected devices wired to the panel, that's great too!  BUT, you need to make sure your fire marshal will allow it.  You may be required to install alarm sounders in each room, or use detectors with integral piezos that will sound when any detector goes into alarm.  I recommend using a separate security system panel for this, just for the sake of clarity, rather than trying to cram a complete security and fire system into one panel, calculate power requirements, etc.  This will allow you to separate fire and burglary devices.  You could also add heat detectors in the kitchen, attic, and garage.  I recommend System Sensor or ESL for system-connected smokes, and Gentex for interconnected residential smoke alarms.  All Gentex smoke detectors are photoelectric, and are among the finest.

If you keep your AC smoke detectors in place, without trying to wire them to a security system, this is all you need to worry about:

   - Replace the batteries every six months.
   - Twice a year, functionally test them with aerosol smoke.  Smoke Check works best for ionization smoke detectors.
   - At least twice a year, clean each detector with a vacuum cleaner or canned "air".
   - Replace them every 10 years!

If you want to know more about protecting your family from fire, you can always get a wealth of information from the NFPA's website,

« Last Edit: October 12, 2009, 07:30:25 PM by admin »
Specializing in Ademco, DSC, DMP, Moose, Napco, and GE Concord and NetworX.
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Interesting that I saw your post right after I submitted my question about just this topic. 

If I could find a way to bridge the two systems wirelessly so that the 120vac and the 12vdc would never be anywhere near each other, is that the biggest concern or is it still your concern that the cheaper units would be more prone to false alarms and be more trouble than they are worth?  In the 6 years we have been in the house, they have never gone off without good reason.


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I have a Customer that need Smoke Detectors. I am a Security/Electrical Contractor, I can get Power to 6 Smoke Detectors, would need to be Wireless interconnected due to size of house and cost difference If I was to get a Interconnect wire through out.
Came up with one way is the DSC Neo HS2016 Panel, An HS2lcdRF Keypad, and 6 Smokes(PG9916), it is AC Powered, Battery Backup, and Interconnected, If 1 Sounds, They All sound.
Looking for any Input or other Suggestions. Thanks


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You cannot "interconnect" DSC wireless smoke sensors.  They are 1-way transmitters, not 2-way transceivers.  Not sure where you are going with this.

If you want interconnected, you will have to go with line-volt smokes with a trigger wire.  If your local AHJ will approve it, you can run low-volt smokes with a polarity reversing module (PRM) which will work.
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DSC is into the thinking, and the way I heard it, Was the Power G is 2 way, they each have built in Sounders. If One goes off, they all go off.
This way it would be about $700 in Materials for the Panel, Receiver, 6 Smokes. Figure 3 Hours to get Power to the Panel, Program, and Mount the Detectors.

Was always trying to find a Fast way to provide Immediate Protection,

This is a 2 Level House, 3 smokes up, 3 smokes down stairs,

Although it is about a $1000 for the above method, 2 other methods may cost more?. Lot more work, we are recovering from Hurricane Irma, and hard to spend a lot of time on each job.

Wireless interconnected Smokes still need Power to each, And Regular Smokes with the Interconnect would be a nightmare pulling through Fan, light boxes, switch boxes and receptacles, as well as through the Main Panel.


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