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First Alert SA521CN ONELINK Hardwire Wireless Smoke Alarm with Battery Backup (4 Pack)

4.0 out of 5 stars 145 customer reviews
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Price: $147.99 ($37.00 / Package) & FREE Shipping
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4 Pack
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  • Hardwire, interconnected wireless smoke alarm with battery backup
  • Uses photoelectric sensing technology which is environmentally friendlier
  • Low battery warning; simple to use silence/test button; pullout battery drawer
  • Add additional battery operated First Alert ONELINK alarms to expand without wires
  • 120-volt with 2 AA battery backup
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$147.99 ($37.00 / Package) & FREE Shipping In Stock. Ships from and sold by OJ Basics.

Frequently Bought Together

  • First Alert SA521CN ONELINK Hardwire Wireless Smoke Alarm with Battery Backup (4 Pack)
  • +
  • First Alert SA511CN2-3ST Interconnected Wireless Battery Operated Smoke Alarm with Voice Location, 2-Pack
Total price: $208.98
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Size Name: 4 Pack

Product Description

Size Name: 4 Pack

In the event of a fire every second counts. If a fire were to begin in your basement and the alarm there sounded, you may not hear it right away if you're sound asleep two floors up. With traditional battery-operated alarms, you lose valuable time waiting for the smoke to rise to another alarm, on another level, before the alarm on your floor sounds. But with First Alert ONELINK alarms, if one detects danger, they all sound. This can alert you sooner, and it increases your chances of getting out safely. Until now, alarms that could signal each other were only available in homes with hardwired alarms requiring installation from an electrician. But First Alert ONELINK alarms communicate without wires. The First Alert ONELINK SA501CN alarm uses photoelectric sensing technology which is environmentally friendlier and which reduces nuisance alarms, making it ideal for cooking areas. It is also great for sensing slow smoldering fires quickly. The alarm includes a low battery warning and a super bright escape light. You can easily expand an existing interconnected 120-volt AC system by replacing one alarm in the series with this alarm. The test/silence button both silences a non-threatening alarm or low battery warning and allows you to test the unit's functionality. The AA batteries are easy to install in the convenient pullout battery drawer. Two AA batteries included.--Bree Norlander

Product Information

Size Name:4 Pack

Technical Details

Part Number SA521CN-4
Item Weight 1 pounds
Product Dimensions 5 x 2 x 5 inches
Item model number SA521CN-4
Size 4 Pack
Color White
Power Source battery-powered

Additional Information

ASIN B00FZSYJT0
Customer Reviews
4.0 out of 5 stars 145 customer reviews

4.0 out of 5 stars
Best Sellers Rank #145,179 in Home Improvements (See top 100)
#278 in Home Improvement > Safety & Security > Fire Safety > Smoke Detectors & Fire Alarms
#716 in Home Improvement > Safety & Security > Household Sensors & Alarms
Shipping Weight 3.4 pounds (View shipping rates and policies)
Date First Available October 18, 2013

Warranty & Support

Product Warranty: For warranty information about this product, please click here

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Customer Questions & Answers

Customer Reviews

Top Customer Reviews

By JHN on November 26, 2009
Size: 1 Pack
I have 7 of the First Alert SA520 Onelink units that were installed in my home 2.5 yrs ago. They were easy to set up although getting power to each unit in an older, 3-story home was the biggest challenge. They link to each other well. You may wish to install them in a particular sequence if using many of them on different floors. That way when the first one goes off, you might have a fighting chance of knowing which one it is since that one sounds off first. The reason I say this is because ours have begun going off in the middle of the night for no apparent reason. It has happened 4 times in about 4 months, and we never know when it will happen. They start sounding the alarm but shut off after about 20 secs, by themselves. We thought we had one defective unit but after removing it several days ago, they went off again tonight (6pm). All the other times have been when we were dead asleep, and boy is that a shock! These units are LOUD!
Gave it four stars for loudness and ease of setup. Minus one star for this bad habit I've described. Anyone know what might be causing this? Can they be tripped by RF interference? Says on the back that they "have to accept interference even if detrimental." (Who thought that one up?)

FOLLOW UP LATER.....
I discovered that the unit that was giving me trouble was near a ceiling fan on a dimmer switch that must have been sending dirty power into the circuit while it was running (as motors can do). when the fan is not running, i haven't had the problem with the false alarm. my suggestion is to narrow the search to find the one sending the alarm and take it off of the line power and run it only on the batteries to see if a dimmer or motor is causing the problem.
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Size: 1 Pack Verified Purchase
I bought 5 of these alarms (NYS requires one for each bedroom plus one on each level of house). Three worked fine out of the box. Two had to be returned, they sounded the 3 chirp defective unit code and wouldn't stop. This code is different than the weak battery warning. An google search found other reviews with the same problem; units don't pass the self diagnostic (3 chirp code) after a short period of use.
On the plus side they have a 10 year warrenty and First Alert doesn't seem to hassle about returns.
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Size: 1 Pack Verified Purchase
Our home was built in the late-70's so we had a hardwired smoke alarm on each floor but they were not interconnected, not to mention they didn't have a battery backup. If we were sleeping, could we expect to hear the alarm on another floor, on the opposite side of the house? That's a bet I'm not willing to take, so we spent the extra money on two of these. We also plan on buying a couple of the battery only ONELINK alarms to expand our safety and coverage without hiring an electrician to install.
To program, you insert batteries and close the battery door on one unit; then you insert batteries on the second unit, press a button on the second unit, then press a button on the first unit. Inserting batteries and programming took under two minutes for two alarms. My (getting better but still not what I'd call "handy") husband had the alarms installed in under five minutes each. So, I'd consider them pretty easy to install.
During testing, they communicated well together and they seem loud. Of course, the real test for any fire alarm is during a real fire but I'm not really into burning my house down for the sake of a review. Therefore, I'm giving it five stars and calling it done!
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Size: 1 Pack Verified Purchase
I got this to allow me to expand an existing wired system without running new wires. Because my existing wired system was BRK (same company as First Alert), it used the same wire connection as the smoke detector I was replacing. Screw holes were same distance apart as the old mounting plate. It took longer to drag the ladder out than it did to install this smoke detector.

Installing the expansion detectors and getting them all to talk to each other was a little more time consuming. One note of advice: If you install this one first, leave your ladder out under this one until you have all the new expansion detectors programmed because you have to be able to push the button on the this one to during the programming of the new expansion ones.

The only reason to buy this unit is if you have a wired system and you want to add wireless ONELINK units in new locations and have everything alarm together if one system detects smoke/CO. You should only need only one of this model and then combine it with unwired ONELINK units in your new locations. Worked perfectly for me and only took a little over an hour to add 3 new alarm locations.
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Size: 1 Pack Verified Purchase
With the new requirements to retro fit existing residences with interconnected 110volt smokes, this particular smoke detector is helpful. I had an existing system where one detector was hardwired (on a 2 wire system with no interconnection) on each of 3 floors. We added ceiling boxes and 110volt 2 wire to each bedroom then programmed 6 of these together and installed 3 new and changed the 3 existing. When we we're done and tested anyone of the 6 they all sounded.
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Size: 1 Pack Verified Purchase
We installed the hard wired detectors and now about every 10 days one of them goes off and beeps for about a minute. They do not go off together. A different one each time. Today, the basement one went off and beeped for about a minute and then shut off. About 10 minutes later the one by the master bedroom went off and beeped for about a minute. What can we do to stop this from happening or is it suppose to check itself every now and then? Really scary when it does it in the middle of the night. Or are they defective????
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