Best Smoke and Carbon Monoxide Alarms for First-Time Homeowners (2026)

Smoke and carbon monoxide alarms are the cheapest life insurance a house has, and the first thing to check when you get the keys. The alarms that came with your home may be past their ten-year life, missing from bedrooms, or the wrong type for the room. The good news is that the fix costs less than a dinner out and takes an afternoon.

This guide explains what the sensor types actually detect, when a 10-year sealed alarm beats a hardwired one (and when code says you cannot swap), where every alarm goes according to the NFPA, and compares four alarms from the two brands that make most of what is on the shelf. If you are working through your first month in the house, this pairs with the alarm check in our first-30-days guide.

What the Sensor Types Actually Detect

Ionization smoke alarms respond faster to fast, flaming fires (a grease fire, a wastebasket). Photoelectric alarms respond faster to slow, smoldering fires (a cigarette in a couch, an overheated wire in a wall), which produce more smoke before flames and are the fires that kill people in their sleep. Photoelectric units also nuisance-trip less near kitchens and bathrooms. The NFPA recommends having both technologies in the home, either as separate alarms or as dual-sensor units.

Carbon monoxide alarms use an electrochemical sensor to detect the colorless, odorless gas produced by anything that burns fuel: a furnace with a cracked heat exchanger, a water heater with a blocked flue, a car idling in an attached garage, a generator run too close to the house. A combination unit puts a smoke sensor and a CO sensor in one housing, which simplifies placement in hallways and outside bedrooms.

Sealed 10-Year vs. Replaceable Battery vs. Hardwired

10-year sealed battery alarms have a lithium cell that lasts the life of the alarm; there is nothing to change, no chirping at 3 a.m., and when the unit reaches end of life it tells you to replace the whole thing. They are the right choice anywhere there is no existing wiring.

Hardwired with battery backup alarms run on house power, interconnect so that every alarm sounds when one does, and keep working through an outage on their backup battery. Homes built or renovated in the last few decades usually have them, and if yours does, code generally requires that replacements stay hardwired and interconnected. Swapping a hardwired alarm for a new one from the same brand family is a careful-DIY job: turn off the breaker, unplug the old harness, plug in the new one. Running new wiring to add locations is an electrician's job.

Wireless interconnect alarms talk to each other by radio, so a battery unit in a far bedroom can trigger the rest without wiring. Smart alarms add phone alerts, which matter most when nobody is home.

Where Every Alarm Goes

• Smoke alarms: inside every bedroom, outside each sleeping area (the hallway), and on every level including the basement, per the NFPA. Mount on the ceiling or high on a wall. Keep them out of kitchens and bathrooms, where cooking and steam cause nuisance alarms; most makers say at least ten feet from a cooking appliance.
• CO alarms: on every level and outside each sleeping area, and anywhere the manufacturer specifies for homes with fuel-burning appliances or an attached garage. Follow the maker's minimum distance from furnaces, water heaters, and stoves so normal startup exhaust does not trip them.
• Test monthly, replace on time: smoke alarms at ten years from the manufacture date printed on the back, CO alarms at the replace-by date on the unit (five to ten years depending on model). Vacuum the vents when you test.

At a Glance

Picks Worth Buying

1. Best combination alarm — the sealed 10-year workhorse

For the hallway outside the bedrooms, a combination unit covers both hazards with one screw hole. The P3010CU uses a photoelectric smoke sensor (the one that catches smoldering fires early and nuisance-trips less) and an electrochemical CO sensor, announces which hazard it detected by voice, and runs for a decade on its sealed lithium battery before telling you it is time to replace it. A hush button silences a cooking false alarm for several minutes. Pros: no battery changes, voice tells you smoke vs. CO, simple mounting. Cons: no interconnect, so a fire in the basement does not sound it upstairs; photoelectric only, so pair it with a dual-sensor or ionization unit elsewhere. Best for: any home without hardwired alarms, and the CO layer in homes that have smoke-only hardwired units.

2. Best hardwired — the like-for-like replacement

If your house already has hardwired alarms with a three-wire harness, this is the straightforward upgrade: it plugs into the existing wiring, interconnects with other First Alert and BRK hardwired units so they all sound together, and keeps working through a power outage on two AA backup batteries. The photoelectric sensor is paired with an electrochemical CO sensor, and the horn uses a tone designed to be heard by people with age-related hearing loss. It is sold as a single unit or in packs for a whole house. Pros: interconnected, battery backup, drop-in replacement for many existing First Alert and BRK installs. Cons: backup batteries need changing (the unit chirps when they are low); mixing brands on one circuit usually breaks interconnect, so buy a full set from one family. Best for: replacing expired hardwired alarms throughout a house.

3. Best smart — the Nest Protect successor

Google discontinued its Nest Protect in 2025 and named First Alert's SC5 as the alarm that works alongside existing Protects in the Google Home app, which makes this the default smart pick. It sends a notification to your phone when smoke or CO is detected, tells you which alarm and which hazard by voice, interconnects wirelessly with other SC5 alarms, and lets you check, silence, or test from the app. A hardwired version fits existing Nest Protect wiring. Pros: alerts when nobody is home, wireless interconnect, Google Home integration. Cons: the most expensive alarm here, and the smart features depend on your Wi-Fi and the app; the sensors still work without them. Best for: households that travel, and anyone replacing expiring Nest Protects.

4. Best smoke-only — the dual-sensor bedroom alarm

Bedrooms need a smoke alarm but not necessarily a CO sensor, and the SA3210 covers both fire types in one unit: the ionization sensor for fast, flaming fires and the photoelectric sensor for the slow, smoky ones. First Alert's sensing is tuned to reduce nuisance alarms, the sealed lithium battery lasts the unit's ten-year life, and the end-of-life signal tells you when to replace it. Pros: both sensor types, no batteries to change, inexpensive enough to put in every bedroom. Cons: no interconnect and no CO sensing. Best for: every bedroom, alongside a combination alarm in the hall.

How We Chose

We compare the category first — what a first-time homeowner actually needs from a smoke or CO alarm — then weigh manufacturer specifications, verified buyer feedback, and how the product type is used in real homes. We don't accept free products or payment for placement, and every pick names its compromise. How we research and review.

Frequently Asked Questions

How many alarms does my house need?

Count the bedrooms, add one for each hallway outside a sleeping area, and add one for each level that has none of those, including the basement. A three-bedroom, two-story house with a basement typically needs six smoke alarms. Add CO detection on every level and outside the sleeping areas, which combination units in the hallways handle.

Can I replace a hardwired alarm with a battery one?

Usually not. If the house was built with hardwired, interconnected alarms, most codes require replacements to be hardwired and interconnected too. Replace like for like from the same brand family so the interconnect keeps working. A battery alarm is fine as an addition, for example inside a bedroom that never had one.

Why does my alarm chirp once a minute?

A single chirp every 30 to 60 seconds is a low-battery warning on a replaceable-battery unit, or an end-of-life warning on a sealed one. Change the battery or replace the alarm; do not just pull it down. Continuous chirping after a fresh battery usually means the unit has reached end of life.

Do I need CO alarms if everything in my house is electric?

If you have no fuel-burning appliances, no fireplace, and no attached garage, the risk is much lower, but many states require CO alarms regardless, and a portable generator or a car left running in an attached garage creates the hazard instantly. Combination alarms cost little more than smoke-only units, so most homes should simply have them.

The Bottom Line

Put a Kidde P3010CU combination alarm in each hallway outside the bedrooms and one on each level without a bedroom, a First Alert SA3210 inside every bedroom, and, if the house is already hardwired, replace the expired units with First Alert SC7010B alarms as a matched set. Choose the First Alert SC5 if you want phone alerts or are replacing Nest Protects. Then write the install date on each one and add the monthly test to your maintenance calendar.

Sources