Google nest protect battery smoke and carbon monoxide detector

Smoke alarms will warn you of danger, but not if you aren’t home to hear them. (They’re also ugly and a pain to shush, and they shriek at burnt toast.) Smart smoke alarms are the cure, and Google Nest Protect is the only one worth buying. It sends speedy alerts of smoke or carbon monoxide to your phone, has great design, is simple to mute, and has a “heads-up” warning before triggering its siren. The Protect also self-tests and has a motion-activated path light.

Our pick

The Google Nest Protect has all the features we recommend in a smart smoke alarm. It can detect both slow- and fast-burning fires, and it wirelessly interconnects with other Protects. The Protect uses voice alerts to warn you which room the danger is in and gives you time to check before it triggers the siren. It’s the easiest alarm to silence, through the polished and friendly Nest app or by pressing its mute button. And the Protect is the only model that tests itself—which cuts down on annoying low-battery chirps in favor of useful smartphone notifications when something is wrong. The Protect is available in both hardwired and battery-powered versions, and therefore it should work in any home.

The research

• Why you should trust us
• Who this is for
• How we picked
• How we tested
• Our pick: Google Nest Protect
• Flaws but not dealbreakers
• Smoke alarm monitors
• What to look forward to
• The competition
• Sources

Why you should trust us

To compile this guide, we read multiple research papers and spoke with three experts at UL (formerly Underwriters Laboratories), which literally writes the book on testing smoke alarms: John Drengenberg, Consumer Safety director; Dwayne Sloan, technical director of Building and Life Safety Technologies; and David Mills, principal engineer Initiating/Indicating Devices. We also scoured hundreds of customer reviews from sites like Amazon, Home Depot, and Lowe’s. Since we first started testing smart smoke alarms, in 2016, we’ve lived with our top picks as part of our long-term testing process.

I’ve been a journalist for more than two decades and have spent countless hours testing smart-home devices for Wirecutter, including smart thermostats, smart garage-door controllers, smart sensors, and smart sprinkler controllers. I also cover smart-home technology for Dwell and The Ambient, among others. In addition, I spent six years working as a volunteer firefighter and live with a professional firefighter who has over two decades of experience in the field.

Who this is for

We believe a smart smoke and carbon monoxide (CO) alarm is a crucial safety device for your home, one that even smart-home skeptics should consider. A traditional smoke and CO alarm alerts you to potential danger, but a smart alarm does this even when you’re not home, or when the alarm is at a rental property or maybe a family member’s home.

Crucially, smart alarms address one of the biggest problems with regular alarms—that an alarm can stop working, either because you knocked it off the ceiling with a broom or because you never replaced the batteries. According to a 2021 report, almost three of every five home-fire deaths in the US resulted from fires in homes with no smoke alarms or with nonfunctioning ones. Nearly half of the alarms that failed had missing or disconnected batteries, and a quarter had dead batteries. Most smart alarms alert you to their battery level in their apps and let you silence “nuisance” alarms on your phone or via a button on the device (rather than pulling the battery out), so you won’t end up with a nonworking alarm in your home.

Because a smart alarm can be silenced from your phone, instead of requiring you to climb a ladder or wave around a broom, it reduces the chance you’ll disable it in a fit of annoyance when you’ve burnt the popcorn. Plus, most smart smoke alarms wirelessly interconnect with one another—if one triggers, they all do—which is an excellent safety feature if your home doesn’t have an existing hardwired interconnection.

If you need to replace your smoke alarms, we think the price premium of around $80 for going smart is a small one to pay for the added peace of mind and safety features.

If you have a professionally monitored security system installed, you may be better off adding a compatible smoke and carbon monoxide detector, which will alert the fire department on your behalf if you don’t turn off the alarm within a designated period. (Because those alarms are part of a security system, we didn’t include any in our testing or coverage.)

Bear in mind that some smoke alarms reach their “end of life” 10 years from the date of manufacture—and not the date of install. That is a UL and NFPA requirement for any alarm with a 10-year sealed battery as its primary power source. Additionally, sensors in some alarms may expire sooner than that, so before buying, always check on the device for its expiration date.

An oft-touted feature of smart alarms is that they eliminate the nightmare of the 2 a.m. mystery battery chirps. This is true, but only if you pay attention to the alerts in the app and change the batteries when directed. If you don’t, the alarm will eventually beep at you (another UL safety requirement).

If you already have a house full of working smoke alarms, or you simply don’t want to pay more for a smart alarm, a smoke alarm monitor may be an option (see Smoke alarm monitors for more info). These devices or services listen for the sound of a regular smoke/CO detector and then send you an alert on your smartphone. For instance, Alexa Guard, a free feature for all Amazon Echo–branded devices, sends a notification if it hears an alarm, along with an audio recording, and you can even “drop in” on the device to hear what’s going on in real time. These devices don’t allow you to mute your alarm, but at least they notify you quickly if something is awry at home.

How we picked

Following our research, we determined that to justify the price premium and provide the best functionality, a smart alarm should:

• offer both smoke and CO detection
• be easy to install
• send an alert to your smartphone when it senses danger
• use voice alerts as well as a siren (proven to be more effective, especially for children)
• tell you which room the danger is in
• have a silencing feature that doesn’t involve waving a dish towel around
• provide intelligent low-battery alerts and warn you if it is malfunctioning
• notify designated emergency contacts automatically if you don’t respond
• integrate with smart-home systems, to do things like turn lights on and unlock doors to help in an emergency
• be interconnectable, so that when one alarm sounds, others do too

This last aspect is worth emphasizing, because interconnected alarms can save you precious seconds if you have to evacuate your home. In fact, many states now require interconnected alarms for new construction and some remodels (this can be a wired or wireless communication, as long as it’s not dependent on a working Internet connection).

We tested every standalone, smart smoke alarm currently available that is UL Listed (or an equivalent). All but one alarm we tested used photoelectric sensors, which are faster at detecting smoldering fires (the more common type in homes). Only the Kidde alarm uses ionization sensors, which respond slightly more quickly to fast-burning fires but are prone to nuisance alarms, making them more likely to be disabled.

Our pick, the Google Nest Protect, uses a variant of the photoelectric sensor that it calls a Split-Spectrum Sensor; the company claims that this detects fast-burning fires more quickly than traditional photoelectric sensors do, without as much risk of false alarms. The new UL standard, 217v8, which goes into effect in 2022, essentially does away with photoelectric and ionization distinctions and instead requires alarms to have multi-criteria sensors, capable of detecting both. An updated compliant version of the Nest Protect will have a UL 217v8 label on the box.

There are a number of smoke detectors that work with smart security systems. A major benefit to this is that they can be professionally monitored. For this guide, we didn’t test them, because they aren’t intended to be used on their own. For more on those, read our review of the best home security systems. And though we believe in the importance of remote notifications, self-testing, and early alerts for low batteries, if you’re simply not interested in those smart features, or the cost of smart alarms is too much for you, we also recommend these standard smoke alarms.

How we tested

To test these devices, we put them in a two-story, detached home for a minimum of three months and up to four years. We used real smoke scenarios, including cigar smoke, smoke from extra-long matches, and smoldering paper (placed next to a big bucket of water), to generate heat and smoke to trigger the alarms. Then we monitored smart alerts, wireless interconnectivity capabilities, and any smart-home connections. In assessing each device’s companion app, we focused on ease of use and installation guidance, how well and how quickly notifications were sent and received, and the speed and ease of silencing alarms.

All of the devices responded appropriately to the presence of smoke, and only one gave any false alerts while installed in our test house. Because your safety is at stake, for us to even consider a smoke alarm for testing, it had to meet UL standards for smoke and carbon monoxide (CO) detection. UL tests all smoke alarms, smart or not, by the same standard, simulating both fast-burning and smoldering fires. Every alarm we tested is UL Listed, apart from the First Alert brands, which are tested to UL standards by ETL (also a Nationally Recognized Testing Laboratory).

Because we tested only devices that had already been UL lab–certified, we focused our review on ease of installation and use, as well as on the “smarts” each device offered. But they had to do their primary job well: detecting potentially life-threatening situations in your home.

Our pick: Google Nest Protect

Our pick

The second-generation Google Nest Protect smoke and carbon monoxide alarm is the best smart smoke alarm for everyone because it reliably and calmly alerts you to potential danger—whether you’re home or away—using a combination of voice, siren, and smartphone alerts. If you have more than one Protect, they interconnect wirelessly, and the Protect also self-tests and informs you of device malfunctions or low batteries via smartphone alerts—without those annoying chirps. You can silence nuisance alarms through the Protect’s app, so you’ll be less likely to disable it in a fit of annoyance (as many people do when an alarm starts chirping). The Protect is also the simplest smart alarm to install and control. Note that Nest is releasing an updated version of this model that will be compliant with newer UL standards (specifically UL217v8). To confirm you are buying the newest version, look for a UL217v8 label on the box, or consult the product description for the phrase “Helps reduce cooking nuisance alarms.”

Nest Protect uses a photoelectric Split-Spectrum Sensor, which the company claims detects fast-burning fires more quickly than traditional photoelectric sensors—more in line with the performance of ionization sensors. The Protect accomplishes this with a second LED that can pick up the finer particles of fast-burning fires but is less likely to suffer from nuisance alarms. This was confirmed in our testing—we haven’t experienced any nuisance or false alarms in six years.

In testing, the Protect went off only when there was a significant smoke event, and not when we were cooking or showering (it’s situated far away from the kitchen and bathroom). And it responded significantly faster than the First Alert Onelink Safe & Sound and Onelink Smoke & Carbon Monoxide Detector we tested.

One of our favorite Protect features, from a livability perspective, is the Heads-Up warning, which gives you a chance to address a situation before a loud siren sends every person and animal in the house into a panic: When it’s triggered, the Protect’s gentle chime is followed by an announcement: “Smoke is detected in the hallway, an alarm will sound soon, the alarm is loud.” We think this is ideal, since it alerts without causing panic. And any nuisance alarms were simple to silence with a gentle press on the Protect’s large, easily accessible button or through the app.

The Nest app is also the easiest to use for silencing alarms from the phone. Alerts popped up instantly and gave clear indications as to why we couldn’t silence the alarm, if the smoke levels were too high; this was not the case with the other models we tested that offered this feature.

The Protect is packed with sensors, including ones for humidity, room occupancy, and ambient light, on top of carbon monoxide, smoke, and heat. And so its home-automation capabilities are impressive—but, notably, only if you use Google Nest smart-home devices. For instance, if you have a Nest Cam, it will start recording video if an alarm sounds (even without a Nest Aware subscription). A Nest Thermostat will shut down the HVAC system in an attempt to stop a fire from spreading (one of the first things firefighters do when they arrive at a house fire is shut down the heating and ventilation system). Protect’s occupancy detectors also feed data about whether you’re home to a Nest thermostat, and it can adjust the temp accordingly. We particularly liked the Pathlight feature—a smart night-light that activates when a room is dark and motion is sensed. During an emergency, it will turn red to help you find a way to safety.

You can also use those sensors to make your home a bit smarter. Hardwired Protect alarms can be added to the Presence Sensing feature in the updated Google Home app (to access it you’ll have to connect your Nest app to the Google Home app, if you haven’t already). Presence Sensing uses the Protect’s motion sensor, as well as other compatible devices you may enable (including your smartphone, any other Nest thermostats, Nest smart locks, or Nest Secure sensors), to tell if anyone is home, and then it decides whether to enable the Home or Away Routines. These Routines can be customized in the Google Home app to, perhaps, tell lights to turn on or off, set cameras to record or shut off, or adjust the your thermostat. Disappointingly, there are no options to use the Protect as a trigger, so you can’t set up your smart home to react to fire, smoke, or CO by turning lights on and unlocking doors (as you used to be able to before Google shut down the Works with Nest program).

The Protect is the only alarm we tested that self-tests its sensors (it does this every 200 seconds). It also tests its speaker and alarm once a month during a window of time that you select (and that you can override if it happens to coincide with nap time). A color-changing status light on the Protect lets you know the device is working properly: If it’s glowing yellow, press it, and the Protect will say aloud what’s wrong (or you can look at the app). This kind of peace of mind is worth a lot, in our opinion.

Installing the Nest Protect is very straightforward. We tested two units, one battery-powered, one hardwired. We used the app (iOS and Android) to scan each device, pair each one with our Nest account, and then assign each Protect to a room. The app and the Protect alarms then performed a mutual test, and the alarms were ready to go on the ceiling (or wall—your preference). For the hardwired alarm, using your existing interconnect wire isn’t necessary since Nest uses its own wireless mesh network to connect all your Protect alarms together.

In addition to the standard username and password system for logging in, the Nest app also offers the far more secure two-factor authentication (2FA), which requires you to input a special one-time-use code, received via email or text, to access your camera and recordings. (For more tips on securing your devices, see our post How to Protect Your Smart Home From Hackers.)

Flaws but not dealbreakers

The Nest Protect is considerably more expensive than a standard smoke and CO alarm: To properly outfit a two- or three-bedroom home, you’re looking at around $600 for a collection of devices that, like all smoke alarms, you’ll have to replace in a decade. (Nest does offer a discount on multiple devices through its website.)

Unlike other Nest devices, the Protect isn’t especially integrated into the Google Home ecosystem. That means you have to use the Nest app to set up and control it—not the Google Home app as with other Nest devices. This may be confusing for anyone looking to set up a smart home centered on Google Nest products.

Complaints about battery-powered Protect alarms chewing through batteries in a matter of weeks are somewhat common, probably from people who put standard alkaline AA batteries in them, rather than the more expensive lithium ones that Google Nest recommends. The battery-powered Protect uses its batteries for the Pathlight feature, so if you have the Protect in a hallway, frequent activation will drain the batteries more quickly. You can disable Pathlight or set it to the lowest brightness level to conserve battery life. In our test home, we’ve replaced the batteries just once in three years, and that device is installed in a main hallway.

Nest offers only a two-year limited warranty for the Protect, compared with the Onelink Safe & Sound’s three-year warranty and the Onelink Smoke & Carbon Monoxide Detector’s five-year warranty.

Smoke alarm monitors

If you already have a house full of smoke alarms that you don’t want to replace, or you’re in a rental and not allowed to swap out your smoke alarms, you have another option for getting notifications when your alarms go off: smoke alarm monitors.

These are plug-in devices with microphones that listen for the standard smoke and CO alarms you already have. When they hear one, they send a notification to your smartphone. As a bonus, you may already have a device in your home that can do this.

Amazon Alexa Guard
All Amazon Echo–branded speakers have a free, built-in feature called Alexa Guard. When enabled through the Alexa app and then armed by a voice command, your Echo listens for your home's various smoke alarms and sends an alert to your smartphone if they activate. You receive a push notification from the device that heard the noise (if you have multiple speakers, you may get multiple alerts), saying, “Possible smoke or CO detected in [room your Alexa is in].” You can then tap the notification on your phone to listen to the sound or Drop In on the device to see or hear what’s going on. You will then get an alert when the alarm stops.

In our testing this worked reliably, hearing all the various alarms in our test house. However, you have to remember to enable the Guard feature when you leave—you can do that in the app or by saying “Alexa, I’m leaving.” You also need one Echo device in each room for it to work reliably. As a bonus, Alexa will also listen for sounds of glass breaking, as well as integrate with your Ring or ADT alarm system. (Note there is also a subscription version called Alexa Guard Plus, but we don't think it’s worth the investment.)

If you already have a Ring Security System, the $35 Ring Alarm Smoke & CO Listener is an inexpensive way to turn your existing smoke alarms smart. The Listener triggers when it hears another smoke or CO alarm. In testing it worked very well, and we liked that it not only sends an alert to your phone but it also triggers the Ring Alarm Base station siren, which is a loud 104dB—louder than most smoke alarms and much louder than any other monitor we tested. But in order for it to work, you need to mount a Listener by each of your smoke detectors, which can add up quickly in a large home. Also, if you have professional monitoring the fire department will be automatically dispatched when the Listener detects a smoke or CO alarm—and you can’t cancel the alarm. Pro tip: Be very careful about burning the toast.

What to look forward to

We are in the process of testing two new Kidde models and will update this guide with results in fall 2022.

The Kidde Smoke + Carbon Monoxide Alarm  has voice alerts—saying “Fire!” when smoke is detected, or “Replace Alarm” when the battery is near the end of its life—and a Smart Hush feature in the app lets you temporarily silence false alarms. It also has an option to notify friends or family when smoke or carbon monoxide is detected, and is compatible with Amazon Alexa and Google Assistant. A similar model, the Kidde Smoke + Carbon Monoxide with Indoor Air Quality, adds sensors for humidity, temperature and volatile organic compounds.

The competition

Although we strongly recommend our picks for most people, if you’re looking for a HomeKit-compatible alarm, or you really don’t want to buy into the Google Nest ecosystem, First Alert’s Onelink brand is currently your best option. There are two models. First Alert’s Onelink Smoke & Carbon Monoxide Detector (second-gen) and First Alert’s Onelink Safe & Sound. Both work with HomeKit and can trigger automations (Onelink advertises Alexa compatibility but the alarms can’t trigger Routines in Alexa). Neither Onelink alarm has an early warning—just a siren followed by a voice and location warning after about 30 seconds. You can silence certain alarms from the app or a button on top of the device. There’s no self-testing feature, but you can test it through the app, rather than having to climb on a ladder. When we first started testing Onelink alarms, the app was frustrating to use and constantly logged us out (therefore not sending alerts when the alarms went off). But in our long-term testing, we’ve seen a significant improvement in reliability in the app.

First Alert’s Onelink Safe & Sound is our favorite model of the two because it triggers automations more quickly than the other one (waiting just eight to 12 seconds to send an alert or activate smart-home integrations instead of 90). Additionally, in our long-term testing, it has never triggered a false alarm (unlike its sibling). And even though, at $250, it’s a very expensive smoke and CO detector, the fact that it has a built-in Alexa voice-controllable speaker that doubles as an AirPlay 2 speaker helps cushion the sticker shock. The speaker produces respectable sound and, when grouped with other AirPlay speakers, is a nice addition to a multi-room audio setup (it can’t be used in Alexa multi-room music groups). This alarm has room-specific voice alerts and a color-changing LED ring that flashes during an emergency. It works as a customizable nightlight, and it can wirelessly interconnect with other Onelink alarms, plus some existing wired interconnect systems. But it’s hard-wired only, so you have to have an electrician install it if you don’t have existing wiring.

First Alert’s Onelink Smoke & Carbon Monoxide Detector (second-gen) is a less expensive, more traditional alarm that comes in hardwired and battery versions. It offers room-specific voice alerts, a color-changing LED ring that flashes during an emergency, and wireless interconnection with other Onelink alarms (the wired version can connect to a variety of other alarms, too). It works with HomeKit automations, but it takes 90 seconds after the alarm starts for the other smart-home devices to trigger (this is a setting determined by the manufacturer). Oddly, voice alerts come after the siren—unlike the gentle warning you get with Nest. In our long-term testing of this unit, we have had one false alarm event that triggered all the units to sound.

If you have a Z-Wave home-automation set-up, the First Alert Z-Wave Smoke & Carbon Monoxide Alarm (ZCOMBO) works with many Z-Wave hubs (including SmartThings and Amazon’s Ring Alarm, but not Wink). It’s battery-powered only (there’s no hardwired mode), it doesn’t offer voice alerts or self-testing, and, unlike Protect, it has no wireless interconnectivity with other alarms.

The $179 Owl Wired is unique among smart smoke detectors, as an all-in-one wired smoke, CO, motion, noise, temp, and humidity detector. It can interconnect with traditional wired alarms but so far does not yet support integration with smart platforms like Amazon Alexa, Apple HomeKit or Google Assistant (Owl says those are coming). More importantly, Owl says that while Wired has passed UL testing, for complex business reasons it does not have official UL certification. We will hold off reviewing it for now.

The $40 X-Sense Wi-Fi Smoke Detector is a connected smoke detector that is set up and managed using the Tuya smart app—it’s not compatible with smart home platforms like Alexa, Google Assistant or HomeKit— and it can send smartphone alerts when an alarm is triggered. It also does automatic self-testing of its battery and power connections, and can be silenced through the app. The X-Sense doesn’t detect CO which we think is a feature that every modern smoke detector should have,  while the company says it conforms to UL standards, it’s not yet UL-listed. We will keep an eye on X-Sense and whether new models may be worth testing.

Sources

1. Dwayne Sloan and David Mills, Technical Director Building and Life Safety Technologies, UL and Principal Engineer Initiating/Indicating Devices, UL, interview, August 1, 2021

2. John Drengenberg, Consumer Safety director, UL, interview, September 1, 2016

3. Dan Kaiser, Smoke Alarms and Smoke Detectors – New and Revised Requirements, UL, December 14, 2020

4. Joan Engebretson, What You Need to Know About the New Smoke Detector Standards, SDM Mag, October 22, 2020

5. David Ludlow, The best smart smoke detector and alarms, The Ambient, May 13, 2021

6. Marty Ahrens, Smoke Alarms in US Home Fires, National Fire Protection Association (NFPA), February 1, 2021

7. Smart Enforcement Workshop (PDF), NFPA, November 19, 2015

8. Planning and Implementing a Successful Smoke Alarm Installation Program (PDF), NFPA