While experts have different recommendations on where to install smoke detectors and CO alarms (see our report on CO detectors to learn more about where these should be placed for maximum effectiveness), there are many instances where having both in close proximity is recommended for complete safety -- for example in common areas, such as hallways that are adjacent to bedrooms. For those situations, opting for a combination smoke and carbon monoxide detector can save money and reduce clutter.
Among combination smoke and carbon monoxide alarms, the First Alert SCO501CN-3ST (Est. $50) looks like a top choice. Like the First Alert SA501CN-3ST (Est. $55) discussed in the section on the best smoke detectors, the SCO501CN-3ST uses radio technology to set up a wireless network with other First Alert interconnectable alarms -- smoke (such as the SA501CN-3ST), CO or combination -- without the complications that a hardwired interconnected alarm system would entail, though you might need to experiment a little with alarm location to make sure that each unit is within range of at least one other interconnected alarm.
The SCO501CN-3ST includes a few other noteworthy extras as well. This is a "talking" alarm, and you can program all of your alarms to relay exactly which of the interconnected units is sounding an alert. The built-in smoke detector uses photoelectric technology.
Reviews of the First Alert SCO501CN-3ST are mostly strong. It receives a Best Buy recommendation in one well-regarded expert review for its performance as a CO detector, but only a middling rating as a smoke detector as it only has photoelectric sensors for that function (that review site reserves top grades for smoke detectors with both types of sensors). Still, the SC501CN's performance earns the highest possible grade both when it comes to smoldering fires, and as a CO detector. It receives top ratings across the board for detecting high concentrations of CO, low concentrations of CO and the quality of its alerts.
While user reviews aren't extensive, most owners seem pleased and we don't see the complaints about false alarms that are so with battery operated smoke detectors. At Lowes.com, it earns a 4.5 star rating based on nearly 30 reviews, while at Amazon.com, where this model is called the SCO500B, it earns a 4.7 star rating based on more than 40 reviews.
The Kidde KN-COSM-BA (Est. $25) is also worth considering. This combination smoke and CO alarm has been around for a number of years, and was previously known as the Kidde KN-COSM-B. It has two key differences compared to the SCO501CN-3ST: It is a stand-alone detector that does not communicate with other alarms and the smoke detector uses ionization, not photoelectric technology. It has a voice annunciator that will identify the type of hazard (fire or CO), but can't relay that to other detectors. It's not been professionally reviewed, but there's a large body of mostly satisfied users that post feedback at Amazon.com, where its overall score is 4.5 stars following nearly 600 reviews.
The Kidde KN-COSM-IBA (Est. $40), previously known (and sometimes still available) as the Kidde KN-COSM-IB, is the hardwired version of the Kidde KN-COSM-BA -- but one with a significant difference: It can interconnect with up to 18 Kidde alarms (plus an additional six sounding devices). However, it relies on a hardwired connection to do so, making it best for new construction or a major renovation project. Like the KN-COSM-BA, it has a voice alert to announce whether the hazard is smoke or carbon monoxide, but that and location information is not passed to other alarms in the system. It has been professionally reviewed (under its old model number), but did not do particularly well. Users are mostly pleased, though some complain of false alarms or of receiving the wrong/different unit compared to the model listed and pictured. At Amazon.com, it earns a 4.2 star rating following more than 865 reviews.
As is the case with smoke detectors, some states and cities require combination smoke and CO detectors to use a non-replaceable battery with a 10-year lifespan. If that applies to you, the Kidde P3010CU (Est. $60) is a model to consider. It's a stand-alone alarm with voice warnings to signal the nature of the threat. The smoke detector, like the alarms above, uses a photoelectric sensor. User feedback isn't extensive, but it earns a 4.4 star rating at HomeDepot.com based on nearly 40 reviews.
Having devices that connect to the Internet is all the rage. Everything from "smart" appliances to thermostats to, yes, combination smoke and CO detectors are now available. Among the latter the best known of these is the Nest Protect (Est. $100). It debuted to rave reviews from technology sites and publications, such as CNET.com, and is recommended as a CO detector by the one expert reviewer that evaluates traditional units as well.
The Protect's history is a little checkered, however. One of its key features at launch was the ability to silence the alarm with the wave of a hand. Unfortunately, that feature also detected the activity often related to the discovery of the fire -- like running out of the room -- and interpreted it as a command to silence the alarm, compromising effectiveness. The Nest Protect was the subject of a Consumer Product Safety Commission recall over that. It's since returned to market, with the feature removed, and at a lower price.
Like the First Alert SCO501CN-3ST, the Nest Protect is a combination CO detector and photoelectric smoke detector. It also has additional sensors to detect motion and moisture; possible applications, reviewers note, include interacting with a Nest thermostat to let it know when rooms are unoccupied. It forms a network with other Nest Protect units, and can pass information on regarding the location and nature of a detected hazard. It performs well as a CO detector, though it's more effective in sounding an alarm when it spots high CO concentrations than low. As a smoke detector, it's great against smoldering fires, but not so hot when it comes to flaming ones -- exactly the profile one would expect from a photoelectric-only smoke detector. User reviews, once mixed, are now much stronger as the product has been updated and tweaked since release. It currently earns a 4.6 star rating following more than 2,100 reviews at Amazon.com.
The Nest Protect's other noteworthy feature is its ability to connect to the Internet. As long as you are within reach of an Internet connection, the Protect can send a message to your cellphone or tablet alerting you of trouble at home. The associated app has features such as setting up a home emergency plan and an overview of events in your home over the last 10 days. The Nest Protect comes in both battery and hardwired versions.