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Review of First Alert SC9120B Hardwire Combination Smoke/Carbon Monoxide Alarm with Battery Backup
3.90 stars Good Form 257 Reviews
Reviewed on: July 11, 2013 Author:
Price Range


Pros & Cons


  • Quick plug-in power connector with 9 volt battery
  • Smoke and CO detector in single unit
  • Separate visual and audible signal for smoke and CO
  • Inter-connectable and compatible with other smoke detectors
  • Microprocessor controlled for non-emergency alarms
  • Test and silence buttons with low battery indicator
  • Universal mounting brackets and tamper resistant


  • Chirps false alarms after battery replacement
Review Summary

This is the summary of consumer reviews for First Alert SC9120B Hardwire Combination Smoke/Carbon Monoxide Alarm with Battery Backup. The BoolPool rating for this product is Good, with 3.90 stars, and the price range is $33.42-$47.97.

The First Alert Hardwired combination smoke and carbon monoxide alarm, wires directly into your home's electrical system and the 9 volt battery backup ensures your safety even during power outages. This unit is equipped with ionization technology and an electro-mechanical sensor which is more effective at detecting flaming fires for comprehensive protection. It has separate visual and audible warnings for smoke and carbon monoxide detection. This device is microprocessor controlled for fewer nuisance or non-emergency alarms. This unit can be interconnected with up to 18 other compatible smoke alarms and 6 compatible devices like bells, horns, door closers etc. It is advised to place at least one Combo unit in hall ways or open areas by interconnecting it with smoke detectors in bedrooms for additional safety. The latching alarm indicator remembers which unit initiated an alarm. This device also features test and silence button to cease a non-threatening alarm or low-battery warning which allows testing the unit’s operation. The low battery chirp and missing battery tab alerts the need of a new power source. The battery door and mounting bracket lock system makes it tamper resistance and it backs itself with a 7 years limited warranty.

However, some common concerns from this unit are – chirps low battery or false alarms especially after battery replacement, some malfunctioned units failed to detect the CO levels where as testing them ensured they were functioning properly.

Overall, the First Alert Hardwired Combination Smoke and CO alarm with Battery Backup is a good device for smoke detection and protection from CO in your home. This combo unit is recommended for its reasonable price backed by its easy to use safety features.


  • Product Dimensions: 5.6 x 2.2 x 5.7 inches
  • Product Weight: 10.4 ounces
User Voice
First Alert SC9120B Hardwire Combination Smoke/Carbon Monoxide Alarm with Battery Backup
Easy to install Requires hardwire installation Instructions are a bit difficult to understand Sometimes it did not detect the smoke but test button confirmed it was working Some units started chirping false alarms
Review Group #: 1 Rating :
4.00 stars
9 Reviews See Details
Easy to hook up and install Looks good Dual smoke and CO detectors Some users complained about installation problems
Review Group #: 2 Rating :
4.50 stars
11 Reviews See Details
Easily fits into an existing hardwired fire alarm system Installing by changing the ceiling ring is also easy Hardwired combo CO detectors are more prone to false alarms
Review Group #: 3 Rating :
2.50 stars
14 Reviews See Details
Requires hardwire plugin Easy replacement of battery without removing the unit Some malfunctioning units chirp after battery replacement Low battery indicator alarm
Review Group #: 4 Rating :
3.50 stars
15 Reviews See Details
Reliable Smoke and CO detector alarms are good replacement for just smoke alarms Advisable to place CO detectors in hallways or open areas and inter-connect with smoke detectors in bedrooms
Review Group #: 5 Rating :
2.50 stars
15 Reviews See Details
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By Anonymous on October 23, 2014

My property owner has replaced this model three times so far this year (2014). It goes thru batteries about every two weeks. Absolute garbage.

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