Best Smart Smoke and CO Detectors

Prices in this article are typical U.S. retail, checked November 2026. All assessments are research-based and untested.

Section 1: Which alarm still works without Wi-Fi? (Introduction)

The best smart smoke and CO detector for most homes is the First Alert SC5 at $129.99. It pairs a photoelectric smoke sensor with an electrochemical carbon monoxide sensor, adds voice alerts, wireless interconnect and phone notifications, and needs no subscription.

How we chose: we compared four alarms on sensors, power source, interconnect, app and price, then matched each one to a household. One is the replacement for a favorite Google no longer sells, one is built for homes already wired for hardwired alarms, one is deliberately not smart, and one is the budget option. Nothing here was tested hands-on.

A smoke alarm is a sensor and a siren. Everything else — the app, the voice, the notification — is a layer on top, and the layer is only worth paying for if the siren still works when the router does not. That single test eliminates some products and reframes the rest. A $35 alarm that wakes you is worth more than a $130 alarm whose best feature is a push notification you slept through.

This guide is about picking a model. The mechanics — ionization versus photoelectric sensing, the UL 217 editions, where the alarms go — are covered in Smoke and CO Alarms 101, which is the place to start if you are still deciding how many alarms you need. Here the question is narrower: once you know you need them, which one do you buy?

Section 2: What does the smart layer actually add?

The smart layer is about information, not detection. A connected alarm does not sense smoke earlier than a good conventional one; the sensor and the listing standard decide that. What the radio adds is a message: smoke in the hallway, carbon monoxide in the basement, the battery is low, the unit has reached its end of life. If you are at work when the alarm goes off, that message is the entire value.

The useful features are consistent across the four alarms here:

• Away alerts. A push notification when the alarm sounds, which matters most for pets, older relatives and anyone who leaves the house empty.
• Voice location. A spoken “fire in the hallway” instead of a bare horn, which shortens the pause between waking and moving.
• Remote test and silence. Testing from the app is easier than reaching a ceiling, and silencing a cooking false alarm from the couch is the feature people actually use.
• Low-battery and end-of-life notices. The 2 a.m. chirp becomes a message you can act on during the day.

What the smart layer does not add is equally important. It does not call the fire department unless you pay for a monitoring subscription, and even then the alarm has to be verified first. It does not make a cheap sensor accurate. It does not survive a power cut on a hardwired-only unit, and it does not work at all if the internet is down — the local siren still sounds, but the phone stays quiet.

That last point is the reason to read the power spec before the feature list. A battery-powered alarm with a radio keeps detecting and keeps sounding during an outage. A hardwired alarm with a backup battery does the same. A hardwired alarm whose backup cells are dead is a plastic disc on the ceiling, and no app will tell you that if the router is also down.

Interconnect is the feature that saves lives, and it is not the same as smart. When one alarm sounds, interconnected alarms sound too, so a fire in the basement wakes someone on the second floor. Wired interconnect uses a dedicated traveler wire; wireless interconnect uses a radio link between units of the same family. Mixing brands usually breaks the link, which is why the first question is not “which app” but “what is already on my ceiling.”

Section 3: How do the four alarms compare?

First Alert SC5

The SC5 is the most complete alarm here and the only one sold in both battery and hardwired versions under the same name. The battery model uses six CR123A cells, interconnects wirelessly and lists 85 dB at 10 feet; the hardwired model keeps a lithium backup, uses a quick-connect plug and interconnects over the existing wiring. Both conform to UL 217 10th edition, both carry a 10-year limited warranty, and both report through the Resideo Connect app with voice alerts that name the type and location of the hazard.

The reason it leads is not a feature list. It is that Google’s store no longer lists Nest Protect, and the SC5 is the alarm First Alert sells as its replacement, down to a “replaces Google’s Nest Protect” line on the product page. If you already have wired Nest Protect bases on the ceiling, that matters more than any app comparison: the hardwired SC5 is the least disruptive way off a product that is no longer sold.

The trade-off is price and app. At $129.99 it costs $55 more than the Kidde Ring and almost four times the X-Sense. The app is Resideo’s, not Google Home, so the alarm lives in a second app next to the rest of the house. Six batteries is also a lot to change at once when the low-battery notice finally arrives. Buy it if you want the safest default and the cleanest Nest Protect exit. Skip it if the budget is the deciding factor.

Kidde Ring Smart Smoke + CO

The Kidde Ring alarm is the value pick for a house that already has hardwired alarms. At $74.97 it is the cheapest way here to get a smart smoke and CO unit on existing wiring, and Kidde’s pitch is precise: add one smart alarm to an interconnected hardwired network and the whole network reports to the Ring app. You do not have to replace every alarm to get notifications for every alarm.

It requires 120V AC power — the AA cells are backup only, and Kidde is specific that they should be alkaline, not lithium. Setup runs through the Ring app, and no other Ring device is needed. The optional piece is monitoring: a compatible Ring subscription adds 24/7 smoke and CO monitoring with emergency response, which is the only path to a professional call in this group. Kidde lists the alarm as ETL certified.

The trade-off is the wiring and the account. There is no battery-only version, so it is a poor fit for a rental with no ceiling wiring or a flat where the landlord owns the alarms. It also puts a life-safety device inside the Ring ecosystem, which is fine if you already use Ring and one more login if you do not. Buy it if you have hardwired alarms and a Ring account. Skip it if you need a unit you can move.

Kidde DETECT Combo 10-Year

This one is here to make a point. The DETECT combo is a smoke and CO alarm with a sealed 10-year battery, voice alerts and no radio at all, for $74.97. It cannot send a notification, it cannot be silenced from a phone, and it will never appear in an app. It also has no cloud account, no firmware to update and nothing to configure.

For a bedroom, a rental, or a second home where the Wi-Fi is unreliable, that is not a limitation. It is the correct design. The alarm does the one job that matters, and the ten-year battery removes the annual ladder ritual that Ready.gov still recommends for replaceable cells.

The trade-off is obvious: no away alerts. If the reason you are shopping is to know when the dog is alone with a smoke alarm, this is not the product. Buy it as the alarm you do not have to think about, or as the bedroom unit in a mixed system. Skip it if remote notification is the whole point.

X-Sense SC07-WX

The SC07-WX is the budget entry at $36.54 for one, $42.99 list, or $95.99 for three. It is a 2-in-1 photoelectric smoke and electrochemical CO alarm with an LCD, a replaceable CR123A cell rated for about a year, and a 2.4 GHz Wi-Fi radio that talks to the X-Sense Home Security app. The app handles alerts, remote silence, battery and fault status, and sharing with up to 12 family members. X-Sense also sells an optional fire dispatch service that sends trucks after you confirm an emergency in the app.

The spec sheet is unusually complete for the price: 85 dB at 10 feet, a stated CO sensitivity curve, an operating range of 40–100°F, and a 10-year maximum service life. One account can hold five houses with up to 49 Wi-Fi devices each, so a large installation is possible without a hub.

The trade-offs are the battery and the paperwork. One year per cell is the shortest interval here, and the product page does not state a certification standard, so check the label on the unit before you trust it. It is also 2.4 GHz only; X-Sense says plainly that it will not join a 5 GHz network. Buy it if you rent, you want phone alerts, and $130 for the first alarm is not happening. Skip it if you want a ten-year sealed battery or a documented listing.

Section 4: Where should alarms go, and when do you replace them?

Placement comes before product. Ready.gov is blunt about the minimum: an alarm on every level of the home, including the basement, with batteries replaced twice a year unless the unit has a 10-year lithium cell. Add one inside each bedroom and one in the hallway outside them, and keep the bedroom door closed at night. The alarm in the hall does not substitute for the one in the room.

Replace the whole unit every ten years, not just the battery. Ready.gov says to replace the alarm itself on that schedule or per the manufacturer, and every unit here states a 10-year service life or warranty. The manufacture date is on the back of the alarm; if you cannot find it, the unit is old enough to replace. This is the single most common failure in a home that “has smoke alarms.”

Reuse the wiring, not the assumption. A hardwired alarm usually sits on a three-wire circuit: line, neutral and a traveler for interconnect. Replacing a unit is often a plug swap — the SC5 hardwired model uses a quick-connect adapter for exactly this — but the adapter has to match the base. Check what is behind the old alarm before ordering, and if the house has no interconnect wire, wireless interconnect is the practical alternative.

Interconnect is brand-shaped. Kidde’s whole-network alerting works because the smart unit sits on a Kidde hardwired network. First Alert’s wireless interconnect links First Alert units. If you mix families, expect two networks and two apps, and do not expect one brand’s alarm to trigger the other’s siren. The safe pattern is to standardize on one family per home, or to treat a new alarm as a standalone unit.

The Nest Protect transition is the live edge case. Google’s store no longer carries Nest Protect, and the First Alert SC5 is the unit sold as its replacement. Existing wired bases and the Google Home history are the two things people ask about, and only the first is a hardware question: confirm the adapter and the traveler wiring before you buy, and plan to move the app side to Resideo. The Security and Privacy hub has the wider picture on which devices belong in that stack.

Section 5: What does a full house of alarms really cost?

Four alarms is the realistic number for a small house, and the price difference multiplies. Four SC5 units at $129.99 is $519.96. Four Kidde Ring units at $74.97 is $299.88, though one smart unit on an existing network may be enough. Four X-Sense units at $36.54 is $146.16. Four DETECT units at $74.97 is $299.88. Then the same purchase happens again in ten years, because the units expire.

Subscriptions change the maths only at the top end. None of these requires a subscription for alerts. The Kidde Ring unit offers optional 24/7 monitoring, and X-Sense offers an optional fire dispatch service; both are extras, and both are priced by the provider rather than included. If professional monitoring is the reason you are shopping, compare those plans directly rather than assuming the alarm price includes them.

Batteries are a recurring line item. The SC5 uses six CR123A cells, the X-Sense one per unit per year, and the two Kidde units either a sealed ten-year cell or two AA backups. Over a decade, the sealed units are the cheapest to own and the replaceable ones are the easiest to forget. Put the battery date on the unit with a marker.

Every brand is a separate app. Resideo Connect for the SC5, Ring for the Kidde, X-Sense for the budget pick, and nothing at all for the DETECT. A mixed house means three apps and three sets of notifications, which is how people learn to ignore alerts. The notification guide covers the discipline of keeping only the alerts that change what you do.

Wi-Fi is a dependency, not a feature. The X-Sense unit is 2.4 GHz only, and every app feature here stops at the router. If the alarm is in a basement or a detached garage, check the signal before you count on notifications, and keep at least one alarm that does not need the network at all.

Data is small but real. These apps learn alarm events, battery state and device names, and some tie the alarm to a broader security account. That is a modest footprint compared with a camera, but it is another account with another company. The privacy audit is the place to decide whether that trade is worth it.

Section 6: Which alarm should you buy? (Conclusion)

Buy the First Alert SC5 at $129.99 if you want the default answer: both sensors, voice alerts, wired or wireless interconnect, a ten-year warranty and no subscription. Choose the hardwired version if the ceiling already has wiring, especially if it currently holds a Nest Protect. It is the most expensive unit here and the easiest to recommend.

Buy the Kidde Ring Smart Smoke + CO at $74.97 if your home already has hardwired interconnected alarms and you use Ring. One smart unit can bring the whole network into the app, and the optional monitoring is the only professional-response path in this group. Skip it if there is no AC wiring at the ceiling.

Buy the Kidde DETECT Combo 10-Year at $74.97 if you want an alarm that cannot be improved by an app. It is the right bedroom unit, the right rental unit and the right answer for a house where the Wi-Fi is unreliable. It is also the honest choice for anyone who knows they will not maintain another account.

Buy the X-Sense SC07-WX at $36.54 if the budget is real and you still want phone alerts. It is the cheapest way here to get smoke, CO, an LCD and app control, and the three-pack at $95.99 is the sensible buy for a flat. Replace the cell yearly and check the listing on the box.

Skip all four if your existing alarms are less than ten years old, interconnected and working. Add one smart unit to the network instead of replacing everything, test it monthly, and spend the difference on the rooms that have no alarm at all. A DIY security system is the next layer after the alarms, not a replacement for them, and the maintenance guide is what keeps the whole set alive past year three.

Where to buy

Related reading

• Smoke and CO Alarms 101: The Safety Layer — sensors, the UL 217 editions and where the alarms go.
• Home Security Systems 101: What Calls for Help — monitoring, response times and what a subscription buys.
• Best DIY Home Security Systems for a House — the layer after the alarms.
• The Smart Home Maintenance Guide — the seasonal checklist that keeps alarms and batteries current.

Sources

• First Alert SC5 Battery Smart Smoke and Carbon Monoxide Alarm — $129.99, six CR123A cells, wireless interconnect, 85 dB at 10 feet, UL 217 10th edition, 10-year limited warranty, Resideo Connect app, checked November 2026
• First Alert SC5 Hardwire Smart Smoke and Carbon Monoxide Alarm — $129.99, hardwired with lithium backup, quick-connect plug, hardwired interconnect, replaces Nest Protect, checked November 2026
• Kidde Ring Smart Smoke + CO Alarm, Hardwired — $74.97, 120V AC required, two AA backup cells, Ring app, whole-network alerts from one smart unit, optional Ring monitoring, ETL certification, checked November 2026
• Kidde DETECT Combo Smoke + CO Alarm, 10-Year Battery with Voice Alerts — $74.97, sealed ten-year battery, voice alerts, no app, checked November 2026
• X-Sense SC07-WX Smart Smoke and CO Alarm — $36.54 single and $95.99 for three, CR123A cell with about one year of life, 2.4 GHz Wi-Fi only, 85 dB at 10 feet, app alerts and silence, optional fire dispatch, checked November 2026
• Ready.gov: Home Fires — an alarm on every level including the basement, batteries twice a year unless sealed, replace the unit every ten years, checked November 2026
• Google Store: Connected Home — the First Alert SC5 is listed in the smart home lineup and Nest Protect is not, checked November 2026